DGCS Volume 3

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

B.1

9.2.3 Incorporating Climate Change .............................................................................................................. 9-4 9.2.4 Suggested Allowance for Climate Change ............................................................................................. 9-5 9.3 REFERENCES.................................................................................................................................................... 9-6

B.2

SEDIMENT CONTINUITY ................................................................................................................... 2

B.3

SEDIMENT PROPERTIES ................................................................................................................... 3

B.4

SEDIMENT TRANSPORT CONCEPTS ................................................................................................ 4

OVERVIEW ......................................................................................................................................... 1

B.4.1 INITIATION OF MOTION .....................................................................................................................................4 B.4.2 MODES OF SEDIMENT TRANSPORT .....................................................................................................................5 B.4.3 EFFECTS OF BED FORMS AT STREAM CROSSINGS ..................................................................................................6 B.4.4 SEDIMENT TRANSPORT EQUATIONS ....................................................................................................................6 B.4.4.1 Meyer-Peter & Muller Equation .............................................................................................................7 B.4.4.2 Einstein Method ......................................................................................................................................9 B.4.4.3 Colby Method ..........................................................................................................................................9 B.5

vi

REFERENCES .................................................................................................................................... 12

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Volumes Volume 1

Introduction and Overview

Volume 2A GeoHazard Assessment

Volume 2B Engineering Surveys

Volume 2C Geological and Geotechnical Investigations Volume 3

Water Engineering Projects

Volume 5

Bridge Design

Volume 4

Volume 6

Highway Design

Public Buildings and Other Related Structures

Annex A B

Estimating Scour

Sediment Transport Concepts

vii

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Tables and Figures Table 3-1

Values of 'c' Recommended for Rational Formula ....................................................................................... 3-8

Table 3-3

Equations for Estimating the Time of Concentration in Urban ........................................................... 3-10

Table 3-2 Table 3-4 Table 3-5 Table 3-6 Table 3-7 Table 3-8 Table 3-9

Table 3-10 Table 3-11 Table 4-1 Table 4-2 Table 4-3 Table 4-4 Table 4-5 Table 4-6 Table 4-7 Table 4-8 Table 4-9

Table 4-10 Table 4-11 Table 4-12 Table 5-1 Table 5-2 Table 5-3 Table 5-4 Table 5-5 Table 5-6 Table 5-7 Table 5-8 Table 5-9

Table 5-10 Table 5-11 Table 5-12

Table 5-13 Table 5-14 viii

Kravens Formula ...................................................................................................................................................... 3-10

Values of Horton's Roughness n* ...................................................................................................................... 3-10

Constant (c) for Regional Specific Discharge Curve ................................................................................. 3-14 Runoff-Volume Models .......................................................................................................................................... 3-18

Direct-Runoff Models ............................................................................................................................................. 3-19 Baseflow Models ....................................................................................................................................................... 3-19

Routing Models ......................................................................................................................................................... 3-19 Information to be Provided on Parameters with Hydrological Models ........................................... 3-20 Minimum Hydrological Reporting Requirements ..................................................................................... 3-21 Stream Types ................................................................................................................................................................ 4-4

Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) - Natural Channels ................. 4-12 Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) - Floodplains ............................. 4-12 Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) – Man-made Channels &

Ditches .......................................................................................................................................................................... 4-13 Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) - Pipes ........................................ 4-13

Local Losses Coefficient (K) .............................................................................................................................. 4-20 Weir Coefficient, μ for Different Weir Shape ............................................................................................. 4-27 GeoHazard Impacts on Hydraulics ................................................................................................................. 4-34

Overview of Different Model Types ............................................................................................................... 4-35 Advantages and Disadvantages of Model Types ...................................................................................... 4-36

Overview of Different Software for Flood and Drainage Analysis ................................................... 4-38 Minimum Hydrological and Hydraulic Reporting Requirements ..................................................... 4-40 Design Flood - Suggested Protection Levels .................................................................................................5-3 Causes of Dike Damage and Potential Countermeasures ........................................................................5-5

Freeboard Allowance for Dikes ..........................................................................................................................5-6 Recommended Crest Widths for Dikes ............................................................................................................5-8

Overview of Different Slope Protection Works & Considerations ................................................... 5-31 Coefficient for Riprap Design ............................................................................................................................ 5-36

Dry Boulder Rip Rap Sizing (D50 in mm) ..................................................................................................... 5-37

Minimum Diameter of Boulder (Riprap Type) ......................................................................................... 5-45 Indicative Velocity Limits for Gabions and Gabion Mattress ............................................................. 5-45 Potential Failure Mechanisms for Revetments ......................................................................................... 5-52

ICOLD Definition of a Large Dam .................................................................................................................... 5-57 Minimum Freeboard for Small Dams ............................................................................................................ 5-60

Overview of Some Typical Outlet Control Structures ............................................................................ 5-67

Different Types of Floodway/ Road Embankment Protection .......................................................... 5-71

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 6-1

Minimum Capacity of Drainage Infrastructure ............................................................................................6-2

Table 6-3

Manning’s Roughness of Rock Lined Channels with Shallow Flow ....................................................6-5

Table 6-2 Table 6-4 Table 6-5 Table 6-6 Table 6-7 Table 6-8 Table 6-9

Table 6-10 Table 6-11 Table 6-12

Table 6-13 Table 7-1 Table 7-2 Table 7-3 Table 7-4 Table 7-5 Table 9-1 Table 9-2 Table 9-3 Table 9-4

Channel Types and Examples ..............................................................................................................................6-3

Manning’s Roughness for Grassed Channels (50–150 mm blade length)* .....................................6-6 Permissible Velocities for Different Channel Linings ...............................................................................6-6 Permissible Velocities .............................................................................................................................................6-8 Recommended Side Slope Material................................................................................................................ 6-11

Typical Transition Losses ................................................................................................................................... 6-13 Recommended Inclusions for Safety ............................................................................................................. 6-14 Blockage Factors to be Applied to Culverts ................................................................................................ 6-16

Example Hydrograph Inputs ............................................................................................................................. 6-31 Worked Example Detention Routing............................................................................................................. 6-32 Basin Freeboard Requirements ....................................................................................................................... 6-33

Protection Levels for Coastal Structures ........................................................................................................7-2

Tidal Terminology ....................................................................................................................................................7-2 Suggested Hudson Coefficient Values ..............................................................................................................7-7

Dimensionless Breaker Parameter ...................................................................................................................7-8 Relationship for Toe Protection ...................................................................................................................... 7-10 Sea Level Rise Predictions (IPCC, 2013) .........................................................................................................9-2

Overview of Different Impacts of Climate Change .....................................................................................9-3

Suggested Approach for Incorporating Changes to Extreme Rainfall ...............................................9-5 Suggested Approach for Incorporating Sea Level Rise ............................................................................9-6

Figure 3-1

Typical Catchment Configuration ......................................................................................................................3-2

Figure 3-3

Unit Hydrograph Method ................................................................................................................................... 3-12

Figure 3-2 Figure 3-4 Figure 3-5 Figure 4-1 Figure 4-2 Figure 4-3 Figure 4-4 Figure 4-5 Figure 4-6 Figure 4-7 Figure 4-8 Figure 4-9

Figure 4-10 Figure 4-11 Figure 4-12

Overview of Rational Formula Applicability* ..............................................................................................3-7 Specific Discharge Curve ..................................................................................................................................... 3-15

HEC-HMS Watershed Runoff Processes ....................................................................................................... 3-18

Drainage Basin Zones ..............................................................................................................................................4-2

Typical Longitudinal River Profile ....................................................................................................................4-2 Sinuosity........................................................................................................................................................................4-4

Meandering Stream Processes (Source: Ohio DNR, undated)...............................................................4-5 Energy Grade Line ....................................................................................................................................................4-7 Specific Energy Diagram ........................................................................................................................................4-9

Hydraulic Jump Diagram .................................................................................................................................... 4-10 Non-Uniform Flow Profiles ................................................................................................................................ 4-15 Part-Full Flow Relationship for Circular Pipes ......................................................................................... 4-16

Hydraulic Gradeline and Energy Grade Line for Piped Drainage Systems ................................... 4-17 Commonly Used Culvert Shapes ...................................................................................................................... 4-22

Standard Inlet Types (Schematic) .................................................................................................................. 4-22 ix

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 4-14

Weir Coefficient with Tailwater Submergence ......................................................................................... 4-27

Figure 4-16

Profile of Rectangular Sluiceway .................................................................................................................... 4-28

Figure 4-15 Figure 4-17 Figure 4-18 Figure 4-19 Figure 5-1 Figure 5-2 Figure 5-3 Figure 5-4 Figure 5-5 Figure 5-6 Figure 5-7 Figure 5-8 Figure 5-9

Figure 5-10 Figure 5-11 Figure 5-12 Figure 5-13

Figure 5-14 Figure 5-15 Figure 5-16 Figure 5-17 Figure 5-18 Figure 5-19 Figure 5-20 Figure 5-21 Figure 5-22 Figure 5-23 Figure 5-24 Figure 5-25 Figure 5-26 Figure 5-27 Figure 5-28 Figure 5-29 Figure 5-30 Figure 5-31 Figure 5-32 Figure 5-33 Figure 5-34 x

Tailwater Conditions for Submerged Overfall .......................................................................................... 4-28 Sluiceway Discharge Coefficient as a Function of h/a & hu/a ............................................................ 4-29

Free Discharge (Top) and Submerged Discharge (Bottom) ............................................................... 4-30 Limit between Free & Submerged Discharge ............................................................................................ 4-30

Example Countermeasure against Seepage ..................................................................................................5-6 Key Components of a Dike ....................................................................................................................................5-6 Dike Height ..................................................................................................................................................................5-7 Freeboard due to Backwater Effects ................................................................................................................5-7

Plan and Perspective of Dike Showing the Location of Access Road .................................................5-8 Example of Crib-Wall used with Restricted Space .................................................................................. 5-10

Arrangement of Berm .......................................................................................................................................... 5-10 Incorporating Settlement into Design of Levee ........................................................................................ 5-11 Self-Standing Retaining Wall (Example) ..................................................................................................... 5-15 Parapet Wall (Example) ...................................................................................................................................... 5-16

Illustrative Example of Overflow Dike .......................................................................................................... 5-17

Widening and Increasing the Height of Dike ............................................................................................. 5-17

Example of Spur dikes used to protect outer River Bank .................................................................... 5-18 Example of Spur dikes used with Bridge Design...................................................................................... 5-19 Example Permeable Spur Dike ......................................................................................................................... 5-20 Dimensions of Spur Dike – Impermeable Overflow Type .................................................................... 5-22 Effective Length of a Spur Dike ........................................................................................................................ 5-23

Scour Adjustment for Spur Orientation ....................................................................................................... 5-24

Toe Protection Works for Spur Dike ............................................................................................................. 5-25

Shape of Spur Dike................................................................................................................................................. 5-25 Location of Revetment at River Bend ........................................................................................................... 5-26 Components of a Revetment ............................................................................................................................. 5-27

Components of a Revetment Cross-Section ............................................................................................... 5-28 Velocity Adjustment Factor ............................................................................................................................... 5-30

Sodding with Grass or Some Other Plans (Natural Type) .................................................................... 5-31 Wooden Pile Fence ................................................................................................................................................ 5-32

Dry Boulder Rip Rap ............................................................................................................................................. 5-32

Grouted Rip Rap, Spread Type ......................................................................................................................... 5-32 Grouted Riprap, Wall Type ................................................................................................................................ 5-33 Gabion Mattress, Spread Type ......................................................................................................................... 5-33 Gabion Mattress (Gabion Wall), Pile-up Type ........................................................................................... 5-33

Rubble Concrete, Spread Type ......................................................................................................................... 5-34 Rubble Concrete, Wall Type .............................................................................................................................. 5-34

Reinforced Concrete ............................................................................................................................................. 5-35

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-35

Gravity Wall .............................................................................................................................................................. 5-35

Figure 5-37

Typical Forces Acting on a Gabion Wall ....................................................................................................... 5-38

Figure 5-36 Figure 5-38 Figure 5-39 Figure 5-40 Figure 5-41 Figure 5-42 Figure 5-43 Figure 5-44 Figure 5-45 Figure 5-46 Figure 5-47 Figure 5-48 Figure 5-49 Figure 5-50 Figure 5-51 Figure 5-52 Figure 5-53

Figure 5-54 Figure 5-55 Figure 5-56 Figure 5-57

Sheet Pile and Reinforced Concrete (Segment Combination) ............................................................ 5-35 Example Vegetated Bank Protection ............................................................................................................. 5-40 Height of Revetment ............................................................................................................................................. 5-40

Provision of a Berm in a Revetment .............................................................................................................. 5-41 Depth of Foundation ............................................................................................................................................. 5-42 Foundation Work ................................................................................................................................................... 5-43 Types of Foot Protection Works ...................................................................................................................... 5-44

Concrete Block Type - Orderly Pile Up - Single Unit............................................................................... 5-46 Concrete Block Type –Orderly and Random Types ................................................................................ 5-46

Weight of Concrete Block ................................................................................................................................... 5-46 Width of Foot Protection Required ................................................................................................................ 5-47

Riprap Revetment with Mounded Toe Approach .................................................................................... 5-48 End Protection Works .......................................................................................................................................... 5-49 Crest Protection ...................................................................................................................................................... 5-49

Development of Residual Hydraulic Pressure without Drainage Pipes/ Weep Holes ............ 5-50 The Need for Filter Cloth/ Gravel ................................................................................................................... 5-50 Typical Groundsill Layout .................................................................................................................................. 5-55

Groundsill Locations ............................................................................................................................................. 5-57

Sluiceway for Drainage ........................................................................................................................................ 5-64 Typical Detail for Overtopping at Bridge Approach/ Floodway ....................................................... 5-70 Typical Types of Roadway Embankment Protection ............................................................................. 5-72

Figure 5-58

Typical Types of Roadway Embankment Protection ............................................................................. 5-73

Figure 6-2

Open Channels and Freeboard (Source: QUDM, 2013) ......................................................................... 6-11

Figure 6-1 Figure 6-3 Figure 6-4 Figure 6-5 Figure 6-6 Figure 6-7 Figure 6-8 Figure 6-9

Figure 6-10 Figure 6-11 Figure 6-12 Figure 6-13 Figure 6-14 Figure 6-15 Figure 6-16

Turf Reinforcement Matting Profile .................................................................................................................6-9

Example Low Flow Channel for Dry Weather Flows ............................................................................. 6-12 Maximum Rate of Expansion ............................................................................................................................ 6-12 Debris Deflector Walls ......................................................................................................................................... 6-17 Typical Inlet Structures ....................................................................................................................................... 6-17

Dry Boulder (Riprap) Outlet ............................................................................................................................. 6-18 Sizing of Dry Boulder Outlet Structures for Single Pipe or Box Culverts ...................................... 6-19

Sizing of Dry Boulder Outlet Structures for Multiple Pipe or Box Culverts ................................. 6-20 Typical Rock Pad Outlet Configuration ........................................................................................................ 6-20 Typical Orientation and Set-Back of Outlet ................................................................................................ 6-21 Grated Pit (in depression) Inflow Rating Curves ..................................................................................... 6-23

Side Opening Pit (in kerb or gutter) Inflow Rating Curves ................................................................. 6-24 Inlet Weir Flow Behavior ................................................................................................................................... 6-25 Inlet Orifice Flow Behavior ................................................................................................................................ 6-26 Typical Schematic of Detention Basin........................................................................................................... 6-28 xi

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-17

Example of Above Ground Detention System after Heavy Rain ........................................................ 6-28

Figure 6-19

Example Underground Detention System using Permeable Pipes .................................................. 6-29

Figure 6-18 Figure 6-20 Figure 6-21

Example Underground Storage System ....................................................................................................... 6-29 Example Underground Detention System ................................................................................................. 6-30 Basin Volume Estimation .................................................................................................................................. 6-33

Figure 6-22

Typical Spillway Design ...................................................................................................................................... 6-35

Figure 6-24

Positive Displacement Pump ............................................................................................................................ 6-39

Figure 6-23 Figure 6-25 Figure 6-26

Figure 6-27 Figure 7-1 Figure 7-2 Figure 7-3 Figure 7-4 Figure 7-5 Figure 7-6 Figure 7-7 Figure 7-8 Figure 7-9

Figure 7-10

Centrifugal Pump ................................................................................................................................................... 6-38 Estimated Required Pump Storage from Inflow Hydrograph ............................................................ 6-40 Typical Wet-Pit Pumping Station .................................................................................................................... 6-41

Typical Dry-Pit Configuration .......................................................................................................................... 6-42 Example of Sea Wall .................................................................................................................................................7-4 Example of Rock Sea Wall/ Revetment ...........................................................................................................7-4

Typical Revetment Section ...................................................................................................................................7-5 Overview of Parameters for Wave Runup .....................................................................................................7-8

Types of Waves ..........................................................................................................................................................7-8

Example of Toe Protection Options ............................................................................................................... 7-11 Example Scour Protection using Toe extending to Depth of Anticipated Scour in Moderate

Scour Environments ............................................................................................................................................. 7-12 Example Scour Protection using Toe extending to Depth of Anticipated Scour in Severe Scour

Environments .......................................................................................................................................................... 7-12

Example Sea Wall - Constructed to appear like a natural bluff ......................................................... 7-13 Example Detached Breakwaters .................................................................................................................... 7-13

Figure 7-11

Example of Groynes as Shoreline Protection ............................................................................................. 7-14

Figure 8-2

Distribution System Classification ....................................................................................................................8-8

Figure 8-1

Figure 4-13

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Rainfall Distribution in the Philippines ...........................................................................................................8-2

Definition Sketch - Triangular Section.......................................................................................................... 4-24

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Abbreviations Abbreviation

Definition

ALS

Airborne Laser Survey

AMWS

Association of Massachusetts Wetland Scientists

BF

Blockage factor

BMPs

Best Management Practices

CAD

Computer Aided Design

CETMEF

Centre d'Études Techniques Maritimes Et Fluviales

CIRIA

Construction Industry Research and Information Association

CUR

the Netherlands Centre for Civil Engineering Research and Codes

DEM

Digital Elevation Model

DENR

Department of Environment and Natural Resources

DFL

Design Flood Level

DGCS

Design Guidelines Criteria and Standards

DID

Department of Irrigation and Drainage (Malaysia)

DNR

Department of Natural Resources (Ohio)

DPWH

Department of Public Works and Highways

DTMR

Department of Transport and Main Roads (Queensland)

EGL

Energy Grade Line

EO

Executive Order

FCSEC

Flood Control and Sabo Engineering Centre

FHWA/FHA

Federal Highway Administration

GIS

Geographic Information System

GPS

Global Positioning System

GPTs

Gross Pollutant Traps

HEC23

Hydrologic Engineering Centre Circular 23

HEC-HMS

Hydrologic Engineering Center-Hydrologic Modelling System

HEC-RAS

Hydrologic Engineering Center – River Analysis System

HGL

Hydraulic Grade Line

HWL

High Water Level

ICOLD

International Commission on Large Dams

IPCC

Intergovernmental Panel on Climate Change

IRR

Implementing Rules and Regulations

LiDAR

Light Detection and Ranging

LLDA

Laguna Lake Development Authority

LWUA

Local Water Utilities Administration

MC

Memorandum Circular

MMDA

Metropolitan Manila Development Authority

MO/DO

Ministry Order/Department Order

MSMA

Manual Saliran Mesra Alam (Urban Stormwater Management Manual for Malaysia)

MWSS

Metropolitan Waterworks and Sewerage System

N/A

Not Applicable

NAMRIA

National Mapping and Resource Information Administration

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Abbreviation

xiv

Definition

NDCC

National Disaster Coordination Council

NJDEC

New Jersey Department of Environmental Conservation

NRE

Department of Natural Resources and Environment

OWL

Ordinary Water Level

PD

Presidential Decree

PAGASA

Philippine Atmospheric, Geophysical and Astronomical Services Administration

PNG DoW

Papua New Guinea Department of Public Works

PPA

Philippine Ports Authority

Project ENCA

Enhancement of Capabilities in Flood Control and Sabo Engineering

PUB

Public Utilities Board (Singapore’s national water agency)

QUDM

Queensland Urban Drainage Manual

Rep.

Representative

RIDF

Rainfall Intensity-Duration-Frequency

SCS

Soil Conservation Service

SMA

Soil Moisture Accounting

SUDS

Sustainable Urban Drainage Systems

TRM

Turf Reinforcement Matting

UDFCD

Urban Drainage and Flood Control District

UH

Unit Hydrograph

USACE

United States Army Corps of Engineers

USBR

United States Bureau of Reclamation

USDA

United States Department of Agriculture

WSUD

Water Sensitive Urban Design

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Glossary Acronym

Definition

Abutment

Structure at the two ends of a bridge used for transferring the loads from the bridge superstructure to the foundation bed and giving lateral support to the embankment.

Afflux

The upstream rise of water level above the normal surface of water in a channel caused by an obstruction in the waterway, such as a bridge or weir or by regulation. The increased amount of water which occurs upstream from a structure (dam) or obstruction in a stream channel, due to the existence of such obstruction and the raising of the water level to considerable distance upstream.

Alluvial

Soil or earth material which has been deposited by running water.

Alluvial Fan (alias Gravel Wash)

A fan shaped deposit formed where a stream emerges from an entrenched valley into a plain or flat.

Alluvial Stream

Stream flowing mainly in self-transported alluvial deposits.

Annual Risk of Exceedance

The chance or probability of a natural hazard event (usually a rainfall or flooding event) occurring annually and is usually expressed as a percentage.

Apron

A floor or lining of concrete, timber, or other resistant material at the toe of a dam, bottom of a spillway, chute, etc. to protect the foundation from erosion and falling water or turbulent flow.

As-Built Plan

A scaled drawing that shows a project and infrastructure components after completion of construction

Avulsion

A sudden cutting off of land by floods, currents, or change in course of a body of water.

Backwater

The rise of water level that occurs immediately upstream from a structure (eg.dam) or obstructions in a river to a considerable distance brought about by the presence of structure.

Bed Load

Material moving on or near the stream bed by rolling, sliding, and sometimes making brief excursions into the flow of new diameters above the bed.

Bed Material

The material of which the riverbed is composed.

Berm

A horizontal strip or shelf built into an embankment or cut, to break the continuity of an otherwise long slope.

Bioengineering

The use of mechanical elements in combination with biological elements (e.g.plants) particularly for control of erosion and prevention of slope failures.

Borrow Site

An excavation source ouside the project area that is used to supply soils for earthwork construction (i.e. gravel pit).

Borrow Materials

Filling materials acquired from a Borrow Site.

Bridge

A structure carrying a road over a road, waterway or other feature, with a clear span over 3.0 meters along the centreline between the inside faces of supports. A bridge may have an independent deck supported on separate piers and abutments, or may have a deck constructed integral with supports.

Catchment Area (alias Catchment Basin, Watershed, Drainage Area, Drainage Basin, River Basin)

The area from which a lake, stream or waterway receives surface water which originates as precipitation.

Climate Change

A long-term change in the statistical distribution of weather patterns over periods of time that range from decades to millions of years.

Coarse-grained Soils

Soils with more than 50% by weight of grains retained on the number 200 sieve (0.075 mm).

Cohesionless Soils

Granular soils (sand and gravel type) with values of cohesion close to zero.

Cohesive Soils

Clay type soils with angles of internal friction close to zero.

Concrete

A mixture of cement, fine aggregate, coarse aggregate and water.

Cross Section (alias Cross Section Plan)

View generated by slicing an object at an angle perpendicular to its longer axis.

Culvert

A structure in the form of a pipe or box, below road level, for conveying storm water runoff .

Cutoff

A wall or diaphragm of concrete or steel, or a trench filled with puddled clay or impervious earth.

Debris

Any uprooted trees and other materials carried by the water in the creek or river.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Density

The ratio of the total mass to the total volume of material.

Design Life

Period assumed in the design for which the infrastructure is required to perform its function without replacement or major structural repair.

Detached Breakwaters

A structure parallel, or close to parallel, to the coast, build inside or outside the surf zone.

Digital Photogrammetry (alias Photogrammetry)

The art of using computers to obtain the measurements of objects in a photograph. It typically involves analyzing one or more existing photographs or videos with photogrammetric software to determine spatial relationships.

Ditch

An artificial open channel or waterway constructed through earth or rock, for the purpose of carrying water.

Drawdown

The magnitude of the lowering of a water table, usually near a well being pumped.

Dredging

Removal from beneath water and raising through water of soil rock and debris.

Embankment

A raised structure of soil aggregate, rock or a combination of the three.

Energy Grade Line

A line joining the elevation of energy heads of a stream; a line drawn above the hydraulic grade line a distance equivalent to the velocity head of the flowing water at each cross section along a stream or channel reach or through a conduit.

Factor of Safety

The ratio of a limiting value of a quantity or quality to the design value of that quantity or quality.

Flood Control

Detention or diversion of water for the purpose of reducing discharge for downstream inundation.

Flood Plain

Flat land bordering a river and subject to flooding

Force

A push or a pull in a given direction on a body that changes or tends to change its state or rest. (or its state of motion).

Free Water (alias Phreatic Water, Gravitational Water)

Water that is free to move underground through a soil mass under the influence of gravity.

Gabion

A basket or cage filled with earth or rocks and used especially in building a support or abutment.

Grain Size Distribution Curve

A curve drawn on a log scale to represent the distribution of particle sizes in a soil.

Gravity Walls

Retaining walls which depend upon their selfweight to provide stability against overturning and sliding; usually made of a high bulk structure

Grouted Riprap

When the stones in the rip-rap are fastened together by grout of mortar.

Groin (alias Groyne)

A wall, crib, row of piles, stone, jetty or other barrier projecting outward from the shore or bank into a stream or other body of water, for the purpose of protecting the shores or bank from erosion, arresting sand movement along the shore, concentrating the low flow of a stream into a smaller channel, etc.

Hydraulic Grade Line

Line connecting the points to which the liquid would rise in piezometer tubes if inserted at various places along any pipe. It is the measure of the pressure head plus the elevation of the pipe at these various points.

Hydrofracturing

A well stimulation process used to maximize the extraction of underground resources.

In-situ

Undisturbed, existing field conditions.

Land-use Map

Maps that reflect the land resources and types of land use in the national economy.

Levee (alias ‘Dike’)

An embankment, generally constructed on or parallel to the banks of a stream, lake or other body of water for the purpose of protecting the land side from inundation by flood water, or to confine the stream flow to its regular channel.

Light Detection and Ranging (LiDAR)

A remote sensing technology that measures distance by illuminating a target with a laser and analyzing the reflected light. Although thought by some to be an acronym of Light Detection And Ranging, the term lidar was actually created as a portmanteau of "light" and "radar".

Lining

A protective covering over all, or over a portion of the perimeter of a conduit, canal, or reservoir, to prevent seepage losses, to withstand pressure, or to resist erosion.

Longitudinal Section

View generated by slicing an object at an angle parallel to its longer axis

Manhole

An opening through which a person may enter or leave a sewer, conduit, or other closed structure for inspection cleaning, and other maintenance operations, closed by a removable cover.

Matchline

A line on a design drawing that projects a location or distance from one portion of the drawing to another portion of the drawing.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Maximum Flood Level

The highest recorded flood level.

Mean Sea Level

The average height of the sea for all stages of the tide. Mean sea level is obtained by averaging observed hourly heights of the sea on the open coast or in adjacent waters having free access to the sea, the average being taken over a considerable period of time.

Navigational

Pertaining to, or used in, conducting ships or other vessels on the water from one place to another.

Open Channel

Any conduit in which water flows with a free surface. Channel in which the stream is not completely enclosed by solid boundaries and therefore has a free surface subjected only to atmospheric pressure.

Ordinary Water Level

The height of water in the river under normal condition.

Parcellary Survey

A survey to determine and establish the legal boundary of real properties.

Pier

A structure usually of concrete or stone masonry, which is used to transmit loads from the bridge superstructure to the foundation soil and provide intermediate supports between the abutments.

Pile

A slender member that is driven (hammered), drilled or jetted into the ground. Piles are usually constructed of timber, steel or pre-stressed reinforced concrete.

Piping

The movement of soil particles as a result of unbalanced seepage forces produced by percolating water.

Profile

Series of elevation along a line.

Reinforced Concrete

A composite material which utilizes the concrete in resisting compression forces and some other materials, usually steel bars or wires, to resist the tension forces.

Retaining Wall

A structure usually made of stone masonry, concrete or reinforced concrete that provides lateral support for a mass of soil.

Riprap

Rock or other material used to armor shorelines, streambeds, bridge abutments, piling and other shoreline structures against scour and water erosion.

River Training

A group of engineering works built along a river or a section thereof in order to direct or lead the flow to a prescribed channel, with or without the construction of embankments.

Rubble Concrete

Concrete in which large stones are added to the freshly placed concrete while it is still soft and plastic.

Runoff

Surface water of an area of land.

Sand

Particles that pass through a number 4 sieve (4.75 mm), and retained on a number 200 sieve (0.075 mm).

Scour

Lowering of stream-bed or undermining of foundations by erosive action of flowing water.

Scoured Depth

Total depth of water from surface to a scoured bed level.

Depth of Scour

The depth of materials removed below the set datum.

Settlement

The downward movement of soil, or the downward movement of a foundation.

Sheet Piles

A long vertical earth retention and excavation support, steel, vinyl or reinforced concrete, driven into the ground with interlocking edges to form a continuous wall to resist water or earth pressure.

Stilling Basin

A depression in a channel or reservoir deep enough to reduce the velocity or turbulence of the flow.

Artificial Submerged Reefs

An alternative method of shoreline stabilization and beach erosion control, using a man-made underwater structure to mitigate the wave induced erosion.

Time of Concentration

The period of time for the stormwater or rainwater to flow from the most distant point to the point under consideration.

Topographic Plan

A graphic representation of horizontal and vertical positions of an area which uses contour lines to show mountains, valleys, and plains.

Topographic Survey (alias Ground Survey)

Collection of data to represent horizontal and vertical positions of an area, including features such as roads, bridges and bodies of water with contours, elevations and coordinates.

Tributary

A stream or other body of water, surface or underground, which contributes its water, either continuously or intermittently, to another and larger stream or body of water.

Vertical Alignment

The position or the layout of the highway on the ground which includes level and gradients.

Wave Height

The height of the wave from the wave top, called the wave crest to the bottom of the wave, called the wave trough.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Wave Runup

The maximum vertical extent of wave uprush on a beach or structure above the still water level (SWL).

Weep Hole

An opening provided during construction in retaining walls, aprons, canal linings, foundation, etc., to permit drainage of water collecting behind and beneath such structures to reduce hydrostatic head.

Weir

A low dam built across a river to raise the level of water upstream or regulate its flow.

Wetlands (alias Swamp, Marshes, Bogs)

Those areas that are inundated and saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions.

Wingwall

A vertical wall located at both ends of the coping of the abutment or at both extreme wall of a reinforced concrete box culvert.

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1

General Provisions

1.1

Scope and Application This guideline aims to provide the Department of Public Works and Highways (DPWH) Engineers (including concerned Local Government Units and Government Consultants) with the basic knowledge and essential tools in undertaking design of water engineering projects specifically for flood control, water supply, coastal facilities and drainage infrastructures.

This guideline provides an overview of some of the key issues, considerations and items to be incorporated into design. As with the Guide, this is not meant to be an exclusive list of design criteria or a manual for the design of these infrastructures. Therefore, it is important that the designs of these infrastructures are undertaken by suitably qualified engineers with experience in undertaking this work.

1.2

The design of Sabo Engineering structures is not covered by this Guide. For the design of Sabo Engineering structures, reference should be made to the Flood Control and Sabo Engineering Center (FCSEC) Guideline.

Governing Laws, Codes, Memoranda, Circular and Department Orders

Water engineering projects are indispensable in the socio-economic development and the protection of lives, infrastructures, agricultural, and other resources of the country. To promote water engineering activities, laws, codes and department orders governing were formulated and executed, which include the following: 



 

 



PD 1067. Water Code of the Philippines, thereby revising and consolidating the laws governing the ownership, appropriation, utilization, exploitation, development conservation and protection of water resources.

PD 296. Directing all persons, natural or juridical, to renounce possession and move out of portions of rivers, creeks, esteros, drainage channels and other similar waterways encroached upon by them and prescribing penalty for violation hereof. PD 78 of 1972 creation of The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA).

Letter of Instruction (LOI) No. 19 dated Oct. 2, 1972 directed then Secretary of Public Works and Communications, to remove all illegal construction including buildings on and along esteros and riverbanks, and to relocate, assist in the relocation and determine sites for informal settlers and other persons to be displaced PD No. 772 of 1972, for penalizing informal settlers and other similar act.

PD No. 198. The Provincial Water Utilities Act of 1973, for declaration of a national policy of local water utilities and for creating the Local Water Utilities Administration (LWUA). PD 1149 of 1977 organized the National Flood Forecasting Office as one of the major organization units of the PAGASA. This P.D. amends certain sections of

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects



 



   

       

1-2

P.D. No. 78 otherwise known as “The Atmospheric, Geophysical and Astronomical Science of 1972”. The present PAGASA is attached to the National Science and Technology Authority by Executive Order (EO) No. 128 in 1987. PD 1566 of June 11, 1978 establishment of a National Program on Community Disaster Preparedness. The National Disaster Coordination Council (NDCC) issued the Calamity and Disaster Preparedness Plan in 1988. Flood fighting is undertaken nationwide by virtue of this PD. Ministry Order No. 20, Series of 1982. Guidelines for the Preparation, Evaluation and Ranking of Flood Control and Drainage Projects.

PD 187 as amended by P.D. 748 and Batas Pambansa Blg. 8, An act defining the Metric System and its Units, providing for its implementation and for other purposes; and MPWH Memorandum Circular No.6, dated January 6, 1983, re Metric System (SI) Tables. Under the Local Government Code, a city or municipality may reclassify agricultural lands and provide the manner of their utilization and disposition.

Executive Order No. 192 of 1987 mandates Department of Environment and Natural Resources (DENR) for conservation, management, development and proper use of the country’s environment and natural resources including those in the watershed. Republic Act No. 4850, creating the Laguna Lake Development Authority (LLDA). Republic Act No. 6234, creating the Metropolitan Waterworks and Sewerage System (MWSS).

Executive Order No. 215 and 462, for private sector participation in hydrological endeavors. Republic Act No. 7924 of 1994, for creating the Metropolitan Manila Development Authority (MMDA), defining its powers and functions, providing funding therefore and for other purposes.

Republic Act 9003 - Solid Waste Management Act, overall institutional framework of managing solid wastes including functions and responsibilities

IRR of Republic Act 9003 Section 6 – Creation of Local Solid Waste Management Committee (Creation of Barangay Solid Waste Management Committee) Republic Act 7942 – Philippine Mining Act of 1995

PD No. 825 – Providing Penalty for Improper Disposal of Garbage PD No. 856 – Sanitation Code

DMC 5-97 – Navigation Clearance for Road Bridges [(CGAO/CG-10)-HQ Philippine Coast Guard]

DO 50 of 1987 – Soil Investigation for Design of Foundation of Various Structures DM of 2011 – Upgrades on Flood Control and Road Drainage Standards

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

  

1.3

DO 06 of 2012 – Coconet Bioengineering Solutions

DO 68 of 2012 – Guidelines in Design of Slope Protection Works

Memorandum of 1983 – Quarrying of Construction Aggregates and/ or Materials

Planning Process

The planning process can be undertaken in a number of different ways. A general procedure for planning process identifies three key stages: 

 

Master Plan – a high level strategic plan that assesses existing constraints and issues, and identifies potential solutions at a large scale. For example, for flood control a river basin wide approach may be adopted where analysis is undertaken on the flooding issues and potential flood control alternatives are identified; Feasibility Study – prioritization and selection of projects from the Master Plan; Implementation Plan – a plan that specifies the works selected from the Feasibility Study, including the funds required and the estimated benefits.

The above approach is based on Technical Standards and Guidelines for Planning of Flood Control Structures (FCSEC, 2010), and generalized for water engineering projects in general. Within the context of DPWH, the Master Plan and Feasibility Study represent stages prior to, and during the Concept Development phase of the design process, while the Implementation Plan represents the Design Development and Detailed Design Phase.

This approach should be adopted, rather that targeting a specific problem in isolation.

1.4

Where it is not possible to undertake a master plan approach, Concept Development should still be undertaken to ensure that sufficient constraints are identified and that adequate budgets are allocated, prior to moving to the Design Development and Detailed Design Phase.

Structure of Volume 3

Volume 3 is divided into a number of key sections. An overview of these sections is provided below: 



Section 2 – Data Requirements. This section identifies some of the key input data sources for water engineering projects. The focus of this section is on identifying data sources, providing an overview of these sources and key limitations and constraints, and providing guidance on scoping for collection of these data sources, where required.

Section 3 – Hydrology. This section provides an overview of current hydrological techniques that are typically applied in the Philippines, and the use

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects





of these methods. This chapter is intended on being a key reference chapter for flood control, drainage, highway drainage and bridge design.

Section 4 – Hydraulic Analysis. This section provides an overview of basic hydraulic principles, as well as background on different river processes. As with the hydrological chapter, this chapter is intended on being a key reference chapter for flood control, drainage, highway drainage and bridge design. This section also provides general guidance on likely impacts of various geohazards on hydraulics. Section 5 – Flood Control & Regulating Structures. This section provides guidance on flood control and regulating structures, including:

- Dikes/ Levees - Spur Dikes

- Revetments - Small Dams - Groundsills



- Sluiceway and Conduits for Dikes/ Embankments

Section 6 – Drainage. This section provides guidance on the design of drainage infrastructure, including:

- Open Drains and Channels

- Pipe Networks, Inlet Manholes and Manholes - Culverts

- Detention Basins

- Overland Flowpaths

 

  

1-4

- Pumping Stations

Section 7 – Coastal Structures. This section provides general guidance focusing on shoreline protection. It provides general guidance on revetment design.

Section 8 – Water Supply. This section provides the general guidance in the design of water supply system particularly in small water system or rural development. Section 9 - Climate Change. This section provides a general overview of considerations for climate change when undertaking Water Projects.

Annex A – Estimating Scour. This section provides a description of stream stability and scouring mechanism.

Annex B – Sediment Transport Concepts. This section provides a brief introduction to key sediment transport concepts.

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

1.5

References Flood Control & Sabo Engineering Center, June 2010, Technical Standards and Guidelines for Planning of Flood Control Structures, Japan International Cooperation Agency, Philippines.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

2

Data Requirements

2.1

Survey

2.2

All survey should be collected based on the methods and requirements identified in Volume 2.

Scoping Survey

In defining the scope of the survey that is required, it is important to understand the requirements of the design that is being undertaken and define the area, detail and accuracy of the survey appropriately. Key considerations in scoping of survey for water engineering projects include: 

 





The survey collected should be sufficient to undertake the analysis, while also being sufficient to design any specific infrastructure (such as levees, revetments etc.).

Survey should be collected a sufficient distance upstream and downstream so that the hydraulic behavior of the study area can be adequately understood.

It may be appropriate to have a higher level of resolution in the survey within the immediate vicinity of the proposed works, while a lower resolution upstream and downstream of this area. Consideration for specifying provision of the survey in an electronic format, without the need for drafted plans. This data should be provided as a three dimensions CAD file, which will allow direct interpretation by the designer. This may result in savings in time and cost of preparation of the survey data. Alternative formats, such as GIS, should also be considered where this is appropriate.

It is essential that a clear scope of works is prepared for the surveyors, to ensure that the survey collected meets with the requirements for the project. This scope of works should be prepared by the engineering team who is to undertake the design/ hydraulic analysis etc. This scope of works should include (where applicable):

- Locality of the project site, including key place names, road names and

coordinates where available

- Plans or sketches showing the location of the cross sections to be collected,

along with locations of topographic information required

- A project briefing document, identifying key requirements (e.g. accuracy,

details required etc.)

- It may be appropriate to use photographs and other tools to assist in

identifying location of survey details required, where it may not be clear

- The road network alignment and profile along the distribution system and

transmission mains (i.e. from water source to distribution system)

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

- Location of houses, public building, utility facilities, treatment plant, water

tanks

- Tidal level measurements

2.3

Other Data

2.3.1

Rainfall Data

Rainfall information is a key input to hydrological analysis. There are two key types of rainfall data relevant to the design process: 

 

2.3.2

Recorded rainfall data – this data is recorded by rainfall gauges, and provides information on historical rainfall that has occurred. Historical rainfall can be used to verify design rainfall information or as an input to a hydrological model in order to calibrate it to a historical flood event. This data is typically available as a depth of rainfall over a specified time period. Rainfall gauges may collect at small time increments (e.g. 5 minutes) through to daily time increments. The use of multiple recorded rainfall gauges may assist in understanding the path of a particular storm as well as the areal distribution of the rainfall.

Design rainfall information – this information is the predicted 100 year rainfall, 50 year rainfall etc. that is available from PAGASA.

Evaporation Data

Evaporation data may be required for water supply projects (continuous hydrological modelling, reservoir analysis etc). Evaporation data is typically collected alongside rainfall gauges, but will not be available with every rainfall gauge.

2.3.3

Measured evaporation data is also referred to as “pan evaporation” data, based on how the data is collected. For some continuous hydrological modelling software, evapotranspiration is required, and therefore a conversion factor is needed. This conversion factor should be confirmed based on local conditions – an indicative value of 70% may provide a reasonable preliminary estimate. River or Channel Gauge Data

River or channel gauge data may be available for some flood control and drainage projects. The data may either be recorded water levels, or, where a rating curve is available, observed discharges as well. The following are key considerations:  



Ensure that the datum used for the collection of the data matches the datum of the survey. Where this is not the case, a transformation may be required. Rating curves to determine discharges have inherent inaccuracies, particularly with larger flows. It is best for the designer to understand the limitations and ensure that they are aware of the likely variance in flow estimates.

Changes in river profile over time in the vicinity of the gauge have the potential to impact on the observed water levels or discharge estimates. 2-2

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

2.3.4

Tidal Data

Tidal data can come in two forms: 



2.3.5

Observed or measured data. This data is measured in the field and represents historical tidal measurements. Similarly with river gauges, the datum needs to be confirmed and ensure that this is consistent with the survey being used. A transformation may be required where this is not the case.

Predicted data. Predicted data is based on tidal constants that are available for a number of key ports and harbors. Predicted data does not represent “real” observed data, which may be influenced by factors such as storm surge and other weather at the time. It represents the expected tidal levels where these influences do not occur.

Tidal data is available from NAMRIA (National Mapping Resource Information Administration). Wind Speed Data

Wind speed data is available for a number of locations around the country, such as airports and harbors. The information may include gust speed, average wind speed and direction over specified increments in time that are dependent on the measurement.

Most wind speed measurements are measured a certain height above the ground, and therefore a conversion factor may be required for some coastal modelling software, but this should be confirmed with the software manual.

It is also important to take into consideration the locality of the wind speed measurement in respect to the study area. For example, a gauge at an airport near the coast (with low vegetation) is unlikely to be representative of a lake 20km inland and surrounded by forest. 2.3.6

Wind data is available from PAGASA.

Land-use Mapping

Land-use mapping data can be used to:   2.3.7

Define catchment characteristics, both for existing land-uses and potential future land-uses; Define roughness characteristics for hydraulic analysis.

Aerial Photos

Aerial photos provide useful information on catchment and floodplain characteristics. They may not be available in all study areas. They can be available in a range of scales and resolutions.

If aerial surveys such as LiDAR or photogrammetry are being collected, then it is usually possible to acquire aerial photos at the same time. Some aerial photos are geo-referenced, which means that they can be uploaded into a GIS or CAD based software in the correct coordinated location.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

2.3.8

It is important to understand the date that the aerial photography was taken, as changes may have occurred in the catchment since that time.

Soils Investigation

The stability and performance of a structure such as weir, gate, coastal revetment or dam, etc. founded on soil depend on the subsoil conditions, ground surface features, type of construction, and sometimes the meteorological changes. Subsoil conditions can be explored by drilling and sampling, seismic surveying, excavation of test pits, and by the study of existing data. These techniques are outlined in detail in Volume 2C.

2.3.9

The data required for soil investigation for structures is equivalent to the investigations outlined in Volume 4. It is recommended that data required for this volume is identified for areas where structures are proposed.

Riverbed Material

The riverbed material is important to understand the river characteristics, potential for scour, potential flood control options etc. Key information required is the grain size distribution and classification of the soil. This information is required for almost all flood control based projects, and drainage projects undertaken in combination with natural channels. Information on the riverbed should be collected at representative locations Techniques to collect this information include:

   

Sieve analysis (typically for grain sizes less than 100 mm). This method is outlined in Volume 2.

On-Site measurements (for coarse bed streams and rivers, with grain sizes greater than 10 mm). These methods are outlined in Volume 2. One-dimensional sampling method (for grain size greater than 200 mm) Two-dimensional sampling method (for grain size less than 200 mm)

Samples should be collected based on an inspection of the river, and identification of any significant changes in riverbed material. However, as a guide, it is recommended that riverbed material be collected:

  

Master plan – collection of riverbed material information at minimum spacing as follows:

Minimum of 1 site to be sampled every 2 km, to be taken at same location of surveyed cross section.

Concept Development/ Design Development/ Detailed Design – one site every 200 to 500 m, depending on the riverbed characteristics. Samples should be taken at representative locations of riverbed material for that portion of the river. Samples should be taken at the same location as a surveyed cross section.

Note that at each site for the riverbed sample, a sample should be taken at the centre (where access is possible), left and right banks.

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2.3.9.1

Following a flood, finer sediments may be deposited on the riverbed surface. In order to obtain a representative sample, it may be necessary to extract the sample following removal of the surface material.

One-Dimensional Sampling Method

This on-site testing procedure is outlined below:  

Find a representative sampling spot in the river where a sample of riverbed material is exposed and is representative of the study area or design area.

Within the sampling spot, find the biggest riverbed material and approximately determine its size. Measure 20 evenly spaced sampling point on the ground using a steel tape with interval the same as that of the biggest riverbed material. If the maximum riverbed diameter is 50 cm, then the sampling interval should also be 50 cm as shown in Figure 2-1.

Figure 2-1

One-Dimensional Sampling Method

Source: FCSEC, 2010



2-5

Pick the stones beneath the sampling interval point and arrange it in a straight line (Figure 2-2), from smallest to biggest. Select the stone size from the 12th smallest interval from the arrangement. This is the equivalent 60% of the riverbed material samples and the corresponding representative riverbed diameter (dr).

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 2-2

Representative Grain Size Sample

Source: FCSEC, 2010

This sample can be measured using a ruler, and dr can be computed using the formula: Equation 2-1

𝑑𝑑𝑟𝑟 = (𝑋𝑋1 𝑌𝑌1 𝑍𝑍1 )

1⁄ 3

An overview of the parameters is provided in Figure 2-3. Figure 2-3

Measurement of Sample

Source: FCSEC, 2010

Using these diameters, percent finer (P(di)) corresponding to di (‘i’ is the smallest diameter stone) can be obtained as follows: Equation 2-2

𝑃𝑃(𝑑𝑑𝑖𝑖 ) =

𝑑𝑑13 + 𝑑𝑑23 + ⋯ + 𝑑𝑑𝑖𝑖3 3 . 100 𝑑𝑑13 + 𝑑𝑑23 + ⋯ + 𝑑𝑑20

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

where: 2.3.9.2

d1, d2, di ...

= stone diameter

Two-Dimensional Sampling Method

An improvised screen (Figure 2-4) with equally spaced string on a 1 m square wooden frame is used for sampling.   

Find the best sampling spot in the river where representative sample of riverbed material is exposed. Get a sample riverbed material and approximately determine its size.

Within the sampling spot, find the biggest riverbed material and approximately determine its size. When maximum riverbed diameter D < 10 cm, use a 1.0 m x 1.0 m improvised screen with openings evenly spaced at 10 cm both ways. When maximum riverbed diameter D > 10 cm, use a 1.0 m x 1.0 m improvised screen with openings uniformly spaced at 20 cm both ways at the middle. Note that the any reasonable sized string may be used, as the string size is not important, provided it is strong enough to be strung tightly across the frame.

Figure 2-4 Improvised Screen for Two-Dimensional Sampling Method

Source: FCSEC, 2010

Lay the improvised screen on the exposed ground making sure that representative riverbed materials are contained within the 1 m2 area (Figure 2-5).

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 2-5 Two-Dimensional Sampling Method

Source: FCSEC, 2010

Pick gravels just beneath of each intersection of strings of the improvised screen and arrange it in a straight line (Figure 2-6), from smallest to biggest. Select the 60% smallest sample from the arrangement. Say, the 15th sample in the 20 cm spacing strings (within 5 x 5 = 25 samples) or the 60th sample in 10 cm spacing strings (within 10 x 10 = 100 samples). Figure 2-6 Representative Grain of Sample

Source: FCSEC, 2010

Measure the dimensions of the selected grain and calculate the representative grain diameter of the site. Calculation procedure is same as the One-Dimensional Sampling Method.

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2.3.9.3

2.3.10

Coastal Bed Material

Coastal bed material is an important consideration in the design of coastal protection measures. The techniques, as identified in Section 2.3.8 and 2.3.9 can also be adopted to describe the characteristics of the bed material.

Specific Data for Water Supply Projects

The first step in designing a water system is to determine how much water is needed by the population to be covered. The water to be supplied should be sufficient to cover both the existing and future consumers. It must include provisions for domestic and other types of service connections. In addition to the projected consumptions, an allowance for non-revenue water (NRW) that may be caused by leakages and other losses should be included.

2.3.10.1

Water consumptions served by water utilities are commonly classified into domestic use, commercial use, Institutional use, or Industrial Use. In rural areas, water consumption is generally limited to domestic uses, i.e., drinking, cooking, cleaning, washing and bathing. Domestic consumption is further classified as either Level II consumption (public faucets) or Level III consumption (house connections). Unit Consumptions

Unit consumption for domestic water demand is expressed in per capita consumption per day. The commonly used unit is liters per capita per day (lpcd). If no definitive data are available, the unit consumption assumptions recommended for Level II and Level III domestic usages in rural areas are as follows:  

Level II Public Faucets: 50 - 60 lpcd (Each public faucet should serve 4 - 6 households) Level III House Connections: 80 - 100 lpcd

If there are public schools and health centers in the area, they will be supplied from the start of systems operation and be classified as institutional connections.

Commercial establishments can also be assumed to be served, after consultation with the stakeholders, within the design year. The unit consumptions of institutional and commercial connections are, in terms of daily consumption per connection, usually expressed in cubic meters per day (m3/d). Unless specific information is available on the consumptions of these types of connections, the following unit consumptions for commercial and institutional connections can be used.  

Institutional Connections: 1.0 m3/d Commercial Connections: 0.8 m3/d

The total consumption is the sum of the domestic, institutional and commercial consumptions expressed in m3/d.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

2.3.10.2

Design Population

The design population is the targeted number of people that the project will serve.

The projection of served population and water demand is based on the assumption of design period (say 5 or 10 year) and the design year (or base year). There are two ways of projecting the design population.

1. Estimate the population that can be served by the sources. In this case, the

supply becomes the limiting factor in the service level, unless a good abundant and proximate source is available in the locality.

2. Project the community or barangay population, and determine the potential

service area and the served population.

Population growth to be assumed will need to be determined in consultation with relevant government bodies. The latter projection method is most commonly adopted. First, the annual municipal and barangay growth rates are determined from previous population census as expressed in the following equation: Equation 2-3

P2007 = P2000 (1 + GR)n or

GR = where:

P2007

P2000

1

n

-1

P2007

=

GR

=

population in 2007

n

= =

annual growth rate (multiply by 100 to get percent growth rate)

P2000

population in 2000

number of years between the two census, in this case n = 7

The projected population is then estimated with the same basic population equation on a year to year projection starting from initial year population. After determining the projected population, the next step is to determine the actual population to be served. The primary factors in assessing the served population are socio-economic conditions of potential service area, level of acceptance of residents for proposed water system, availability of and abundance/scarcity of alternative water sources and potential development program in the municipality.

Detailed discussion can be found in the Rural Water Supply Design Manual (WPP, 2012).

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2.3.10.3

Water Quality

Water quality of the source water for supply is an important consideration in water supply projects. Water quality sampling should be undertaken by appropriate qualified personnel, and tested in appropriately certified laboratories.

The chemical, physical and microbiological water quality parameters should be tested as required by the end use. The physical aspects include water turbidity, color, taste, and odor. The chemical aspects are the hardness, alkalinity and acidity, carbon dioxide, dissolved oxygen, chemical oxygen demand, organic nitrogen, iron and manganese, toxic substances and phenolic compounds in the water sample. Microbial water testing should be for protozoa, helminths, and bacteria.

2.4

The Philippines National Standards for Drinking Water 2007 (PNSDW-2007) provide the minimum standards for quality of potable water. Per PNSDW, drinking water must be clear, colorless and free from objectionable taste and odor. All other standard values are contained in the PNSDW Administrative Order No. 2007-0012 or any other standards more recently issued by the Department of Health.

References

Flood Control & Sabo Engineering Center, June 2010, Technical Standards and Guidelines for Planning of Flood Control Structures, Japan International Cooperation Agency, Philippines.

Republic of the Philippines Department of Health (2007) [PNSDW]. Philippines National Standards for Drinking Water 2007, Administration Order 2007-0012. WPP (Water Partnership Program), 2012, Rural Water Supply Volume 1, Design Manual, The World Bank Office, Manila, Philippines.

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3

Hydrology

3.1

Introduction This section of Volume 3 provides a broad outline of hydrological techniques. It outlines the following steps in the hydrological analysis process:   

3.2

Catchment delineation

Design rainfall analysis

Choice and use of hydrological analysis techniques

Catchment Delineation

One of the basic data required in undertaking hydrological analysis is the catchment area.

The catchment area (Figure 3-1) is derived by delineating the basin boundary in a topographic map. Topographic maps may include:    

1:50,000 or better mapping from National Mapping and Resource Information Administration (NAMRIA) Topographical survey, which may assist particularly for smaller portions of the catchment and for drainage projects Aerial survey, such as LiDAR or photogrammetry

Urban drainage layout, which provides an indication of the runoff characteristics

A discussion on these different data sources is provided in Section 2.

The catchment area is then computed using the following:

   

Planimeter – subjected to regular calibration/maintenance to attain accurate result/reading Triangulation

Cross-section millimeter paper, and CAD / GIS software

CAD and GIS software are likely to be the most common method for delineating catchments in the coming years.

In addition to overall catchment delineation, further sub-catchment delineation is typically undertaken to:

  

Provide flow estimates at different points in a study area Align with key inflow points to a hydraulic model

In drainage studies, to estimate the flows arriving at drainage inlets or culverts. Note that this might change with different drainage layout alternatives that might be considered 3-1

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects



Provide sufficient resolution for models other than the Rational Method (Section 3.4.1).

The level of detail that the catchment is delineated into sub-catchments is highly dependent on the particular project and study area. For large river basins, subcatchments may be in the order of 100 km2 to 200 km2, while for drainage studies catchments could be less than 1ha. More details on the procedure for delineation of catchment areas is provided in FCSEC (2010). Figure 3-1

Typical Catchment Configuration

Source: FCSEC, 2010

3.3

3.3.1

Rainfall Analysis Rainfall analysis includes the formation of design hyetographs for hydrological analysis, as well as the analysis of recorded rainfall data.

Methods to Establish Design Hyetograph

The characteristic of rainfall is expressed by three factors.   

3-2

Amount of rainfall (or rainfall intensity) Temporal distribution of rainfall Aerial distribution of rainfall

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

3.3.1.1

Design Rainfall Intensity

The Rainfall Intensity-duration-Frequency (RIDF) data prepared by the Hydrometeorological Investigation and Special Studies Section of the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) is the technical data used in determining the intensity of rainfall in a particular place. The data is plotted to show values at various return periods.

RIDF is separated into short duration (10 min to 1 hour) and long duration (1 hour to 1 day). Various durations may need to be analyzed depending on the project and the application. For example, longer durations may be more important for storage based analysis. PAGASA operates/maintains 52 Synoptic stations equipped with automatic rainfall gauges. Updates of runoff analysis may be secured from PAGASA.

When a station cannot be located or there is no station, the RIDF can be estimated from the specific coefficient shown in Iso-Specific Coefficient and the probable daily rainfall value shown in lsohyet of Probable 1-Day Rainfall.

3.3.1.2

In the unusual situation of more than one rainfall station in a catchment, the catchment average rainfall can be determined in accordance with the methods described in Section 3.3.4. Establishing a Temporal Pattern

With the exception of the Rational Formula, the majority of hydrological analysis requires the establishment of a temporal pattern. In the absence of other information, the Alternating Block Method is an appropriate approach to determining the temporal pattern. This methodology is described in detail in FCSEC (2010) as well as many hydrological textbooks.

3.3.1.3

Note that an alternative approach is the analysis of recorded temporal patterns from a synoptic gauge within the catchment. This would involve the selection of a representative rainfall pattern based on analysis of historical rainfall events for large floods. This approach is suitable for larger river basins, and is described in Section 5.3.3 of FCSEC (2010).

Area Reduction Factor

Intense rainfall is unlikely to be distributed uniformly over a large river basin. The basin mean rainfall for specified frequency and duration is less than point rainfall. To account for this, Technical Standards and Guidelines for Planning of Flood Control Structures (FCSEC,2010) recommends the use of Horton's formula to convert point rainfall to basin mean rainfall. This is presented in Equation 3-1. Equation 3-1

where: I

Io

0.31 ]

𝐼𝐼 = 𝐼𝐼𝑜𝑜 𝑒𝑒 [−0.1(0.386𝐴𝐴) =

=

basin mean rainfall (mm) point rainfall (mm)

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

A

3.3.2

3.3.3

Fa

=

=

Effective Rainfall

catchment area (km2)

I/Io area reduction factor

The next step after determination of design hyetograph is to estimate the effective rainfall. The effective rainfall or excess rainfall is neither retained on the land surface nor infiltrated into the soil but becomes direct runoff to the outlet of the river basin. A lot of methods have been proposed to estimate effective rainfall; however, when data are available effective rainfall can be established by the relationship between rainfall and runoff.

Analysis of Recorded Rainfall

Recorded rainfall data can be an important tool in hydrological analysis. It can be used: 



 

3.3.4

For validation of RIDF values, where the recording period is sufficiently long. Intensities from the recorded data series and the associated return period can be determined and compared with the RIDF values. This is particularly useful where there is no synoptic gauge within the catchment.

For use in calibration and verification of hydrological and hydraulic analysis. Recorded rainfall can be applied to the analysis, and compared with recorded flow data or water level data. This allows for calibration and verification of parameters adopted for the analysis. For general information on the likely return period of a recorded flood event.

For comparison of the assumed temporal pattern from the design hyetograph with actual recorded temporal patterns.

An analysis technique for the determination of return period for recorded rainfall is presented in Section 5.3.2 and 5.3.3 of Technical Standards and Guidelines for Planning of Flood Control Structures (FCSEC, 2010). Average Rainfall in Catchment Area

There are three methods of determining the catchment area average rainfall, as described below. These are generally applied to the analysis of recorded rainfall data. However, where multiple RIDF gauges exist within or near a catchment, the Arithmetic Mean and Thiessen Method can also be adopted.

3.3.4.1

Detailed examples for undertaking these methods are presented in the Technical Standards and Guidelines for Planning of Flood Control Structures (FCSEC, 2010).

Arithmetic-Mean Method

This is the simplest method by averaging the rainfall depths recorded at a number of gages. This method is satisfactory if the precipitation is almost uniformly distributed within the catchment area.

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3.3.4.2

Thiessen Method

This method assumes that at any point in the catchment area, the rainfall is the same to the nearest rainfall gauge. The value recorded at a given rainfall gauge can be applied halfway of the next station in any direction.

3.3.4.3

The relative weights for each gauge are determined from the corresponding areas of application in a Thiessen polygon network, the boundaries of the polygons formed by the perpendicular bisectors of the lines connected to the adjacent gauges.

lsohyetal Method

This method takes into account the orographic influences (mountains, terrain, etc.) on rainfall by constructing isohyets, using observed depths at rain gauges and interpolation between adjacent rain gauges.

Once the isohyetal map is constructed, the area A, between isohyets, within the catchment, is measured and multiplied by the average rainfall depths P1 of the two adjacent isohyets to compute the average rainfall.

3.4

Information of the storm patterns can result in more accurate isohyets; however, a fairly dense network of rain gauges is needed to accurately construct the isohyetal map from a complex storm.

Runoff Analysis

There are many methods for runoff analysis. following:     

3.4.1

3.4.1.1

Rational Formula

This Volume introduces the

Unit Hydrograph Method

Storage Function Method

Flood Frequency Analysis

Specific Discharge Method

This is not an exhaustive list, and does not mean that other methods cannot be adopted where appropriate. Rational Formula

The Rational Formula Method is one of the most commonly used for estimating flood peak discharge for small watersheds. It is widely applied in rivers where storage phenomena are not required, where the catchment is treated as rectangular, symmetrical about the river course and where the rainwater flows down the river course at a constant speed.

Basic Equation

The principle behind the Rational Formula Method is that a rainfall intensity (I) begins and continues indefinitely and then the rate of runoff increases until it reaches the time of concentration (tc), where all of the watersheds are contributing 3-5

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

to the flow at the outlet point or point under consideration. The Rational Formula is provided in Equation 3-2. The Rational Formula is applicable to a rural or forested catchment area smaller than 20 km2.

For urban catchments, caution should be applied in the application of the Rational Formula for catchments greater than 5 km2. In urban catchments, the impacts of local obstructions, hydraulic controls and localized storages can result in significant impacts on the peak flow estimate. Equation 3-2

where:

𝑄𝑄𝑃𝑃 =

𝑐𝑐𝑐𝑐𝑐𝑐 3.6

QP

=

maximum flood discharge (m3/s)

I

=

rainfall intensity within the time of flood concentration (mm/hr)

c

A

= =

dimensionless runoff coefficient catchment)area (km2)

The key assumptions associated with the Rational Formula Method are: 

 

The computed peak rate of runoff at the outlet point is a function of the average rainfall rate during the time of concentration, i.e., the peak discharge does not result from a more intense storm of shorter duration, during which only a portion of the watershed is contributing to the runoff at the outlet. The time of concentration is the time for the runoff to become established and flow from the most remote part of the drainage area to the outlet point. Rainfall intensity is constant throughout the rainfall duration.

A general overview of the decision of what runoff analysis to adopt is outlined Figure 3-2.

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Figure 3-2

Overview of Rational Formula Applicability* Delineate Catchment Area

Delineate SubCatchments

urban

N

Is the catchment urban or rural

Is Catchment Area < 5km2?

Is Catchment Area < 20km2?

Y Y

Rural

N

Y Y

Are storage issues important? N

Other Hydrological Analysis Method required

3.4.1.2

Rational Formula Appropriate

Other Hydrological Analysis Method required

*Other hydrological analysis method may include the Unit Hydrograph or other computer based methods

Runoff Coefficient (c)

The runoff coefficient (c) is the least precise variable of the Rational Formula implying a fixed ratio of peak runoff rate to rainfall rate for the catchment area, which in reality is not the case. Proper selection of the runoff coefficient requires judgment and experience on the part of the hydrologist/engineer. The proportion of the total rainfall that will reach the river and/or storm drains depends on the percent imperviousness, the slope and the ponding characteristics of the surface. Impervious surface, such as asphalt pavements and roofs of buildings, will produce nearly 100% runoff after the surface has become thoroughly wet, regardless of the slope. Some general guidance on potential ‘c’ values to adopt is provided in Table 3-1.

Field inspection, aerial photographs, and present land use maps are useful in estimating the nature of the surface within the target basin. The runoff coefficient will increase with urbanization due to increased impervious surface and installation of drainage system. In a large-scale development, the projected runoff

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coefficient due to development should be used to determine the design discharge and the expected safety level.

After the existing and the future land uses are obtained, the area is categorized and measured to obtain the percentage of each category to the total catchment area. From the percentage of each area, the weighted average runoff coefficient is calculated. Table 3-1

Values of 'c' Recommended for Rational Formula Land Use

3.4.1.3

Minimum

Maximum

Residential Area - Densely built

0.50

0.75

Residential Area - Not densely built

0.30

0.55

City Business District

0.70

0.95

Light Industrial Areas

0.50

0.80

Heavy Industrial Areas

0.60

0.90

Parks, Playgrounds, Cemeteries, unpaved open spaces and vacant lots

0.20

0.30

Concrete or Asphalt Pavement

0.90

1.00

Gravel Surfaced Road and Shoulder

0.30

0.60

Rocky Surface

0.70

0.90

Bare Clay Surface (faces of slips, etc.)

0.70

0.90

Forested Land (sandy to clay)

0.30

0.50

Flattish Cultivated Areas (not flooded) / Farmland

0.30

0.50

Steep or Rolling Grassed Areas / Steep gullies not heavily timbered

0.50

0.70

Flooded or Wet Paddies

0.70

0.80

Time of Concentration – Rural Catchments

There are a number of methods for calculating the time of concentration. The following provides an overview of Kirpich’s Formula and Kraven’s Formula. FCSEC (2010) recommends the use of Kirpich’s Formula over Kraven’s Formula.

Further discussion on time of concentration methods is described in Chapter 15 of National Engineering Handbook – Part 630 Hydrology (USDA, 2012). Kirpich's Formula

Kirpich’s formula is applicable for agricultural catchments. It is described below in Equation 3-3. An alternative form is presented in Equation 3-4. Equation 3-3

𝑡𝑡𝑐𝑐 =

0.0195𝐿𝐿0.77 𝑆𝑆 0.385

Equation 3-4

𝑡𝑡𝑐𝑐 =

3-8

𝐿𝐿1.15 51𝐻𝐻 0.385

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

where: tc

=

time of concentration (minutes)

S

=

average basin slope (S=H/L)

L

=

H

=

Kraven’s Formula

length of watercourse (m)

difference in elevation (m)

This methodology divides the time of concentration into an inlet time and a flow time. It is applicable for rural catchments.

The time of concentration (tc) for the catchment area is obtained from Equation 3-5. Equation 3-5

where:

𝑡𝑡𝑐𝑐 = 𝑡𝑡𝑖𝑖 + 𝑡𝑡𝑓𝑓

ti

=

L

=

tf

V

=

=

Inlet Time

inlet time = time it takes for flow from the remotest point to the inlet point or farthest point of river channel

flow time = time it takes from the inlet point or farthest point of the river channel to the outlet point or point under consideration = L/ V length of river channel from its outlet point to its farthest point (m)

flow velocity (m/s)

Inlet time is computed as follows.  

Find the inlet point. If the estimated inlet catchment area is over 2 km2, the inlet time is t = 30 min

When the catchment area (A) of the farthest point of the channel is clearly judged to be less than 2 km2, compute the inlet time (min.) from A (km2) in Equation 3-6 Equation 3-6

Flow Time

𝑡𝑡𝑖𝑖 =

30√𝐴𝐴 √2

Flow time is computed from Kraven's Formula (Table 3-2), which gives relations between slope of water course and flow velocity as shown below. The gradient represents the average gradient for the water channel. 3-9

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 3-2

Kravens Formula

Riverbed gradient (Sb)

Sb >1/100 (steep slope)

1/100 >Sb >1/200

Sb 10%), L ≤ 50 m For moderate slopes (< 5%), L ≤ 100 m For mild slopes (< 1%), L ≤ 200 m n* = Horton’s roughness value for the surface (refer Source: DID, 2012 Table 3-4) S = slope of overland flow surface (%) tg = curb and gutter flow time (minutes) L = length of curb gutter flow (m) S = longitudinal slope of gutter (%) td = travel time in minutes n = Manning’s roughness coefficient (Table 3-4) R = Hydraulic Radius (m) S = Friction Slope (m/m) L = Length of Reach (m)

Source: DID, 2012

Table 3-4

Values of Horton's Roughness n* Horton’s Roughness (n*)

Land Surface Paved

0.015

Bare Soil

0.0275

Poorly Grassed

0.035

Average Grassed

0.045

Densely Grassed

0.060

Source: DID, 2012

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

3.4.1.5

3.4.2

Rainfall Intensity

The rainfall intensity is estimated as per Section 3.3.

Unit Hydrograph Method

A unit hydrograph (originally named unit graph) is a surface runoff hydrograph resulting from a unit volume (1 inch or 1 cm depth) of rainfall excess generated uniformly over the drainage area at constant rate for an effective duration. A unit hydrograph is derived streamflow data using its basic definition. It is a simple linear model. The following assumptions are inherent in the model:    



The rainfall excess has a constant intensity within the effective duration.

The rainfall excess is uniformly distributed throughout the whole drainage area.

The base time of the direct runoff hydrograph resulting from rainfall excess of given duration is constant.

The ordinates of all direct runoff hydrograph’s of a common base time are direct proportional to the total amount of direct runoff represented by each hydrograph. These are the principles of superposition and proportionality.

For a given watershed, the hydrograph resulting from a given rainfall excess reflects the unchanging characteristics of the watershed. This is the principle of time invariance, which, together with the principles of superposition and proportionality, is fundamental to the unit hydrograph model.

Once the unit hydrograph has been determined, it may be applied to find the direct runoff and streamflow hydrographs. A rainfall hyetograph is selected, the abstraction or losses are estimated, and rainfall excess is calculated. The discrete convolution equation may be used to yield the direct runoff hydrograph. By adding an estimated baseflow to the direct runoff, the streamflow hydrograph is obtained. The discrete convolution equation derives the direct runoff by applying the following procedure. The individual hydrographs resulting from each rainfall excess, are computed by multiplying ordinates of the unit hydrograph by the rainfall excess. Each individual hydrograph starts at the same time as its respective rainfall excess. The direct runoff hydrograph is obtained by summing the ordinates of the individual hydrographs.

Figure 3-3 illustrates the direct runoff hydrograph from three successive rainfall excess increments. A more detailed breakdown of the procedure is provided in FCSEC (2010).

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 3-3 Unit Hydrograph Method

Source: FC SEC, 2010

It is noted that the above procedure of computing the direct runoff hydrograph from individual incremental hydrographs is time consuming. To facilitate the computation, the matrix form is applied and is expressed as: where: R=

.

Q=RxU

R1 R2 R3

0 R1 R2

Ri

Ri-1

0 …0 0 …0 0 …0 R1

U=

U1 U2 U3 Uj

Q=

Q1 Q2 Q3 Qk

The direct hydrograph (Q) has k values, k = i + j – 1 i 3.4.2.1

j

= =

number of rainfall excess (R) values

number of unit hydrograph (U) values.

SCS Unit Hydrograph

The unit hydrograph method has been applied to many river basins in many countries where several synthetic unit hydrographs have been developed. Synthetic unit hydrograph can be estimated for ungauged river basins by means of relationships between parameters of a unit hydrograph model and the physical characteristic of the river basin. The SCS Unit Hydrograph is detailed in FCSEC (2010).

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3.4.3

Storage Function Method

Storage function model treats the behavior of flow of channel and flood plain as a single unit, and is most suitable when applied to the fixed type of flood flow. The storage function model, when compared with the non-uniform flow model, has an advantage that it can simulate a decrease of discharge to the flow direction by inundation.

The storage function model was derived based on the assumption that the relation between water level (H) and discharge (Q) is a single-valued function. Hence, this model cannot be applied to estuaries and confluence of rivers.

3.4.4

The Storage Function Method is described in detail in FCSEC (2010). This method is typically implemented through computer models. Flood Frequency Analysis

The method of computing flood frequency is generally statistical in nature. It means that historical records of maximum flood should be continuous and long. There should be distinction between the daily maximum flood and the momentary flood peaks. In addition, there should be important distinction between stages and discharges as there are changes in the stage-discharge relationship. It is preferable to work with discharges and if necessary the results are referred to the most recent stage-discharge relationship.

Given the statistical series of flood peaks, a continuous distribution is used to fit the historical sequence. A frequency function can be generalized to represent the series as function of: where: X

Xave

σx

X = Xave + K σx

=

= =

flood of specified probability

mean of the flood series

standard deviation of series

K = frequency factor defined by a specific distribution & is function of the probability level

3.4.5

Flood distribution function is generally skewed and may follow either the Gumbel (Fisher-Tippet Type I) or the Pearson Type III (Gamma Distribution).

Specific Discharge Method

The specific discharge method is an approximate method for the estimation of flows, and is typically used to validate flows from the hydrological analysis technique, rather than for the estimation of flows in its own right. Given the approximate nature, it should be used with caution for infrastructure design. It is typically applicable for river basins rather than urban drainage applications. This method is based on the approach presented in FCSEC (2010). The specific discharge is the flood peak discharge per unit catchment area (refer to Equation 3-7). Generally, the specific discharge for small rivers is comparatively

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larger than that of the bigger rivers. The specific discharge curve explains this (refer to Figure 3.4 Specific Discharge Curve, where the specific discharge is the ordinate and the size of the catchment area as the abscissa). From this curve, design discharge is roughly calculated even without any runoff analysis. The reliability of the design discharge estimated by runoff methods can be easily assessed by comparing it with specific discharge method. The method is best suited to rural catchments, or catchments with a lower proportion of urbanization. Equation 3-7

where:

𝑄𝑄 = 𝐴𝐴𝐴𝐴

q=

specific discharge (m3/s/km2)

A=

catchment area (km2)

Q=

design discharge (m3/s)

Table 3-5 indicates constants of the Creager type specific curve for the following equation. Equation 3-8

where: c=

A=

−0.048 −1)

𝑞𝑞 = 𝑐𝑐𝐴𝐴(𝐴𝐴

constant (Table 3-5)

catchment area (km2)

Table 3-5 Region Luzon

Constant (c) for Regional Specific Discharge Curve Return Period 2-year

5-year

10-year

25-year

50-year

100-year

15.66

17.48

18.91

21.51

23.83

25.37

Visayas

6.12

7.77

9.36

11.81

14.52

1747

Mindanao

8.02

9.15

1006

11.60

12.80

14.00

Using the specific discharge curve or equation, design discharge or probable discharge is obtained as follows.    

3-14

Determine the catchment area (A).

Determine the return period or safety level.

From the specific discharge curve, find the region where the project is located, the return period and the catchment area in Figure 3-4.

Another way is to compute specific discharge (q) from the equation, using catchment (A) and constant (c) from Table 3-5 with corresponding regions and return periods.

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 3-4

Specific Discharge Curve

3.4.6

Other Considerations in Hydrological Analysis

3.4.6.1

Channel Routing Model

Distributed flow routing models are used to describe the transformation of storm rainfall into runoff over a watershed to produce a flow hydrograph for the watershed outlet. This hydrograph becomes input at the upstream end of a river system and routed through the system to the downstream end.

3.4.6.2

Several channel routing models have been proposed: These are (a) Storage function model, (b) Muskingum, (c) Kinematic wave and (d) Muskingum-Cunge standard and so on. These are typically incorporated through computer models. Further details on these methods are provided in FCSEC (2010).

Baseflow

Base flow is sustained runoff of prior rainfall that was stored temporarily in the river basin. The base flow can be assumed to be constant during the flood. When a stream flow gauging station is located in or near the target river basin, the mean daily discharge of one day before the floods is used as the base flow. When there are no data available, 0.05 m3/s/km2 can be used for the base flow (FCSEC, 2010).

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3.5

Computer Models There are numerous computer models available for undertaking hydrological analysis. For complex catchments, where a number of sub-catchments are involved, routing needs to be incorporated and there is the potential for storage effects, it is generally simpler and easier to incorporate these into a computer model rather than undertake the computations manually. There are numerous computer models available, and each one can be applicable in a range of situations. Conceptually, most hydrological models have two functions: 



Conversion of rainfall into a runoff from a sub-catchment. There are numerous approaches, including Unit Hydrograph, Storage Function Method, Lawrence method etc.

Routing of the flow from the sub-catchment (from 1) along the main drainage path or river. Types of routing models are discussed in Section 3.4.5. These routing methods may also allow for the incorporation of storages such as dams and detention basins.

Many of the computer models incorporate similar sub-models for undertaking the above calculations. Computer models are constantly evolving, and it is important for the hydrologist to remain aware of the current software, and advantages and disadvantages of each one. Some current available hydrological software include:

 

 

HEC-HMS – available for US Army Corp of Engineers. This modelling system is probably the most commonly used modelling system in the Philippines to date.

XP-RAFTS & XP-SWMM – these two modelling systems are available from xpsolutions. XP-RAFTS represents a stand-alone hydrological modelling software while XP-SWMM includes hydraulic analysis as well. MIKE Software – Available from DHI, and incorporates hydrological analysis within the hydraulic modelling suite.

SOBEK – Available from Deltares, and incorporates the hydrological model either as stand-alone, or integrated with their hydraulic modelling software.

This is not an exhaustive list, and there are many software available. This Guide does not recommend any particular software over another. However, whatever software is utilized, it is important that key parameters for the model setup be specific in the reporting, to ensure that this can be reviewed appropriately (refer Section 3.6).

3.5.1

Given it wide use within the Philippines, a broad overview of the HEC-HMS software is provided below. As noted above, this does not constitute this Guideline recommending this software over an alternative available software.

The Use of Computer Models

Designers who use computer models have a duty of care to ensure that they are familiar with the software, including the underlying assumptions of the software and algorithms, key input data and interpretation of output data. 3-16

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It is noted that many of the problems that occur with computer models are not in the development of the program itself, but rather in the application of the software. Typical issues include:     

3.5.2

Incorrectly specified input data. Errors in the input data.

Application of the model beyond the scope for which it was intended.

Incorrect schematization of the model or representation of the study area. Incorrect interpretation of the model results.

Refer to Section 4.13 for more details.

HEC – HMS

The Hydrologic Engineering Center’s Hydrologic Modelling System (HEC-HMS) which was developed by the U.S. Army Corps of Engineers, simulates the precipitation-runoff processes of dendritic watershed systems. It is applicable to a wide range of geographic areas for solving the widest possible range of problems. This includes large river basin water supply and flood hydrology, and small urban or natural watershed runoff. Hydrographs produced by the program are used directly or in conjunction with other software for studies of water availability, urban drainage, flow forecasting, future urbanization impact, reservoir spillway design, flood damage reduction, floodplain regulation, and systems operation. HEC-HMS presents the watershed runoff process as shown in Figure 3.5. It uses separate model to represent each component of the runoff process and consists of the following:     

Modelling of Catchment Runoff, through: Models that compute runoff volume

Models of direct runoff (overland flow and interflow) Models of baseflow

Models of channel flow or routing

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 3-5

HEC-HMS Watershed Runoff Processes

Source: US Army, 2000

The HEC-HMS models that compute runoff volumes are listed in Table 3-6. These models address questions about the volume of precipitation that falls on the watershed: How much infiltrates on pervious surfaces? How much runs off previous surfaces? How much runoff of the impervious surfaces? When does it runoff? Table 3-6

Runoff-Volume Models Model

Categorization

Initial & Constant Rate

Event, lumped, empirical, fitted parameter

SCS Curve Number (CN)

Event, lumped, empirical, fitted parameter

Gridded SCS CN

Event, distributed, empirical, fitted parameter

Green & Ampt

Event, distributed, empirical, fitted parameter

Deficit & Constant Rate

Continuous, lumped, empirical, fitted parameter

Soil Moisture Accounting (SMA)

Continuous, lumped, empirical, fitted parameter

Gridded SMA

Continuous, distributed, empirical, fitted parameter

The HEC-HMS models of direct runoff are listed in Table 3-7. These models describe what happens as water that has not infiltrated or been stored on the watershed moves over or just beneath the watershed surface.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 3-7

Direct-Runoff Models Model

Categorization

User-specified Unit Hydrograph (UH)

Event, lumped, empirical, fitted parameter

Clark's UH

Event, lumped, empirical, fitted parameter

Snyder's UH

Event, lumped, empirical, fitted parameter

SCS UH

Event, lumped, empirical, fitted parameter

ModClark

Event, distributed, empirical, fitted parameter

Kinematic

Event, lumped, conceptual, measured parameter

Table 3-8 lists the HEC-HMS models of baseflow. These simulate the slow subsurface drainage of water from the system into the channels. Table 3-8

Baseflow Models Model

Categorization

Constant Monthly

Event, lumped, empirical, fitted parameter

Exponential recession

Event, lumped, empirical, fitted parameter

Linear Reservoir

Event, lumped, empirical, fitted parameter

For modelling channel flow with HEC-HMS are listed in Table 3-9. These are the so called routing models, simulate one-dimensional open channel flow. Table 3-9

Routing Models Model

Categorization

Kinematic wave

Event, lumped, conceptual, measured parameter

Lag

Event, lumped, empirical, fitted parameter

Modified Pulse

Event, lumped, empirical, fitted parameter

Muskingum

Event, lumped, empirical, fitted parameter

Muskingum-Cunge Standard Section

Event, lumped, quasi-conceptual, measured parameter

Muskingum-Cunge 8-point Section

Event, lumped, quasi-conceptual, measured parameter

Confluence

Continuous, conceptual, measured parameter

Bifurcation

Continuous, conceptual, measured parameter

In addition to the model of runoff and channel processes, HEC-HMS includes models for simulating a water control structure such as diversion or reservoir/detention pond.

In the HEC-HMS watershed hydrology, the response of a watershed is driven by precipitation that falls on the watershed and evapotranspiration from the watershed. The precipitation may be observed rainfall from a historical event, it may be a frequency based hypothetical rainfall event, or it may be an event that represents the upper limit of precipitation possible at a given location. Historical 3-19

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

precipitation data are useful for calibration and verification of model parameters, for real-time forecasting and for evaluating the performance of proposed designs or regulations. Data from the second and third categories – commonly referred to as hypothetical or design storms are useful if performance must be tested with events that are outside the range of observations or if the risk of flooding must be described. Similarly, the evapotranspiration data used may be observed values from a historical record or they may be hypothetical values.

3.6

Details of specifying and analyzing historical or hypothetical–storm precipitation and evapotranspiration with HEC-HMS are referred to HEC-HMS Technical Reference Manual.

Reporting Requirements

A hydrological report should be prepared for a project. In some cases, both the hydrology and hydraulic report can be incorporated into a single report. The report should contain, as a minimum, the information on parameters with hyrdrological models provided in Table 3-10 and general information provided Table 3-11. Table 3-10

Information to be Provided on Parameters with Hydrological Models

Hydrological Model Rational Formula

Parameters to Include

 

3-20

Rainfall intensities adopted for the assessment, including either the background on the calculation or the information provided by PAGASA, showing clearly the coordinates or location of the rain gauge Details of the time of concentration calculation, including: – Why the method of calculating the time of concentration was adopted – Key parameters assumed for the calculation (such as flow length ‘L’) and details of their calculation – Calculated time of concentration value

Unit Hydrograph

 

Details on key transformation parameters Details on any routing or lagging that is applied, and why this was adopted

SCS Unit Hydrograph

 

As above Curve Number (CN) and why this was adopted. Provide suitable references and information on land-uses.

General Hydrological Models

For all computer based hydrological models:  The software that was used and the version number of that software  Key runoff generating parameters. These may include: – Catchment slopes and areas; – Horton roughness parameters or similar – Rainfall loss models, such as curve numbers or initial/ continuing losses  The type of routing model (e.g. Muskingum-Cunge) that was adopted and why  Details of any storages (such as dams) included in the model  Comparison of the rainfall volume (total rainfall that fell within the design rainfall event) with the runoff volume generated in the model

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 3-11

Minimum Hydrological Reporting Requirements

Component

3.7

Description

Project Description

A brief description should be providing outlining:  Purpose of the Project  What the project involves  Why the hydrological analysis is required

Study Area

A description of the study area should be provided, sufficient so that the reader is aware of the location of the project. This will include:  Description of the study area  Map showing locality of the study area, including key features such as road names, river names etc.  Coordinates of the project site location. If it is a linear structure (e.g. levee), then the approximate centroid of the structure will suffice.  Why the hydrological analysis is required

Catchment Details

A map showing the catchment, either identified on topographical, aerial mapping or similar. The catchment map should show the overall catchment and any subcatchments. A description should be provided on the catchment, particularly focusing on the land-uses within the catchment, soils, vegetation, slopes and other key features.

Rainfall Data

As a minimum, a summary table should be provided with the rainfall intensities for different size rainfall events and different durations. This may be the tabular information provided by PAGASA, for example. This should clearly show the location of where the rain data is representing (i.e. the rain gauge location)

Hydrological Analysis

The following should be provided as a minimum:  Statement as to what hydrological analysis technique was adopted, and why.  Summary of all key input parameters for the analysis. A justification for key parameters, such as the Rational Formula ‘c’ value should be provided (refer Table 3-10)).

Results

The results should be summarized in a clear and concise format. The results may include:  Peak flows for different size floods;  Discharge hydrographs presented in graphical formats;  Runoff volumes. The results presented should be suitable for the project application.

Validation

For catchments where the Rational Formula is not adopted, either the specific discharge may be used (for rural catchments) or the Rational Formula can be used for a sub-catchment, where this sub-catchment is sufficiently small to meet the criteria in Section 3.4.1. As noted above, it is not intended that the two methods will match but rather that this provides a method for cross checking the magnitude of the results.

References DID (Department of Irrigation and Drainage, Malaysian Government), 2012. Urban Stormwater Management Manual for Malaysia, 2nd Edition, Government of Malaysia, Kuala Lumpur.

Flood Control & Sabo Engineering Center, June 2010, Technical Standards and Guidelines for Planning of Flood Control Structures, Japan International Cooperation Agency, Philippines. Ministry of Public Works and Highways, 1984, Design Guidelines Criteria and Standards for Public Works and Highways, Philippine Government, Manila.

United States Department of Agriculture, 2012. National Engineering Handbook – Part 630 Hydrology – Chapter 15, Natural Resource Conservation Service, May.

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4

Hydraulic Analysis

4.1

Introduction

4.2

The hydraulic analysis chapter provides an overview of the core hydraulic principles that will underlie the subsequent chapters in this volume. It provides background on some general hydraulic principles. It is recommended to refer to appropriate hydraulic manuals and textbooks where appropriate. Some references to these have been provided in this chapter.

Types of Channels

Open channels are a natural or constructed conveyance for water whereby the water surface is exposed to the atmosphere, and the gravity force component in the direction of motion is the driving force. Stream channels are:   

Natural channels with their size and shape determined by natural forces.

Compound in cross section with a main channel for conveying low flows and a floodplain to transport flood flows. Shaped in cross section and plan form by the long-term history of sediment load and water discharge over time.

Artificial channels include roadside channels, irrigation channels, storm drains and drainage ditches, which are:

 

4.3

Constructed channels with regular geometric cross sections.

Unlined or lined with artificial or natural material to protect against erosion.

Natural Channel Flow (River Flow)

Rivers originate from mountains and hills, pass through valleys, plains and wetlands and then exit to the sea. A drainage basin can simplistically be divided into three zones: headwaters (an upper erosional zone of sediment production), transfer zone (a middle zone of sediment transport with simultaneous erosion and deposition), and depositional zone (and a lower zone of sediment deposition) (Figure 4-1). The actual situation is often more complex, because local geological controls or other factors can produce local depositional zones in the upper basin or local erosional zones in the lower basin.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 4-1

Drainage Basin Zones

Source: Living in the Environment, 1990

The longitudinal profile of the river system tends to flatten through time by degradation in the upper reaches and aggradation in the lower reaches (Figure 42). In most natural systems this process is slow enough to be of little engineering concern. However, where the river system or catchment has been interfered with historically, profile flattening may proceed at noticeable rates. In some channelization projects, response of this type has been dramatic. Figure 4-2

Typical Longitudinal River Profile

Source: Ohio DNR, undated

4.3.1

Headwaters (Mountain Torrents)

These are high-velocity river on steep slopes, often exhibiting a sequence of drops and chutes controlled by large boulders, fallen timber, etc. They are also commonly referred to as Production Zones, through the source of sediment that they can provide for the downstream part of the river. Erosion and deposition are sometimes confined to severe flood events. Some mountain torrents on very steep slopes are subject to the phenomenon of “debris flows” or “debris torrents” whereby under severe flood conditions the bed becomes fluid and a virtual avalanche of boulders and gravel runs down the mountainside. 4-2

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4.3.2

4.3.3

4.4

Non-alluvial channels have highly developed meanders in solid rock valleys and may have degrading beds. Many mountain rivers are classified as non-alluvial. Transfer Zone

This zone is effectively a transitional zone between mountain torrents and alluvial fans. Stream velocities decrease as the river channel slope also decreases. The sediment sizes also decrease and meanders start to form.

Depositional Zone (Alluvial Fans)

Alluvial fans generally occur where a stream emerges from a mountain valley onto relatively flat land. They are depositional features typically characterized by alluvial materials and unstable multiple channels subject to frequent shifts or “avulsions.”

River Geomorphology

Scientists, engineers and water resource managers are faced on how to work with rather than against nature. Engineers working on flood defense, land drainage, channel stability and navigation interest should balance the design with environmental and other considerations. The need to balance the needs of different interests, sometimes conflicting, makes it essential to take a multifunctional approach. Engineers seek to solve river–related problems while retaining those natural forms and features that allow rivers to transmit the inputs of water and sediment, support diverse habitats and provide a pleasing landscape for river centered recreation. Hence, a comprehensive and reliable morphological analysis and classification system form the essential basis to sound engineering geomorphology. The following section gives brief geomorphological principles. Most alluvial channels exhibit a natural instability that results in continuous shifting of the river through erosion and deposition, formation and destruction of islands, development of oxbow lakes, and formation of braided channel sections.

The degree of channel instability varies with hydrologic events, bank and bed instability, type and extent of vegetation on the banks, sediment mobility and floodplain use.

Rivers have inherent dynamic qualities by which changes continually occur in the channel position and shape. Changes may be slow or rapid, but all streams are subjected to fluvial forces that cause changes to occur. In these streams, banks erode, sediments are deposited, and islands and side channels form and disappear in time. River mechanics involves identifying the physical characteristics and understanding the relationship of the actions and reactions of fluvial forces tending to effect change in channel and floodplain morphology. This knowledge enables us to estimate the likely morphological change for river channels and floodplains as a result of fluvial forces, which assists when planning and maintaining the built environment. The potential effect of these dynamic systems

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

on public infrastructure such as highways and bridges should be identified and understood.

4.4.1

A brief introduction to river geomorphology is provided in the following sections. More detailed descriptions and information is available in references (such as Thorne C R, Hey R D , Newson M D, 1999 and Melville & Coleman, 2000). Further discussion on sediment processes are also provided in these references, together with the FCSEC guideline for Sabo Engineering.

Stream Types

A general overview of different stream types are provided in Table 4-1. Table 4-1

Stream Types

Stream Types

Sinuosity Index

Straight

1.5

Example Figure

Source: Geocaching, 2009

Sinuosity provides an indicative measure of the stream type. It is measured by the stream length divided by the valley length, as indicated in Figure 4-3. Figure 4-3

Sinuosity

Source: AMWS, 2007

4.4.2

Straight Streams

Straight channels are sinuous to the extent that the thalweg usually oscillates transversely within the low flow channel, and the current is deflected from one 4-4

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

side to the other. The current oscillation usually results in the formation of pools on the outside of bends while alternate bars, resulting from deposition, form on the inside of the bends.

4.4.3

In alluvial channels, straight stream may only be a temporary condition particularly in sandy channel rivers that are prone to erosion/deposition of mobile sediments. Aerial photography and topographic maps may reveal former locations of the channel and potential directions of further movement.

Braided Rivers

Braiding is caused by mass bank failure (slumping) as well as large quantities of sediment load that is either deposited or remain where the stream is unable to transport. Deposition occurs when the supply of sediment exceeds the stream’s transport capacity. As the streambed aggrades from deposition, the downstream channel reach develops a steeper bed slope. Multiple channels develop on the flatter upstream slope as additional sediment is deposited within the main channel.

4.4.4

The aggraded material may be deposited within the channel to form bars that may build over time to become islands supporting vegetation. At the flood stage, the flow may inundate most of the bars and islands, resulting in the complete destruction of some and reworking of others. A braided stream is generally unpredictable and difficult to stabilize because the channel changes alignment rapidly, is subject to continual degradation and aggradation, and is very wide and shallow even during flooding.

Meandering Streams

A meandering stream consists of winding channel planform with alternating Sshaped bends (Figure 4-4). In alluvial streams, the channel is subject to lateral movement through the formation and destruction of bends (Figure 4-4). Bends are formed by the process of erosion and scouring of the banks on the outside of bends and by the corresponding deposition of bed load on the inside of bends to form point bars. The point bar constricts the bend and causes erosion in the bend to continue, contributing to the lateral migration of the meandering stream (Figure 4-5). Figure 4-4

Meandering Stream Processes (Source: Ohio DNR, undated)

Source: Ohio DNR, undated

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Meandering streams can experience processes such as avulsion and meander cutoff where the stream experiences a wholesale shift in alignment. This commonly occurs when the channel breaks its banks in alluvial rivers with expansive floodplains. The out of bank flows rework the floodplain and short circuit meanders, creating oxbow lakes, or occupy secondary flowpaths. After a cutoff is formed, the stream gradient is steeper; the stream tends to adjust in response to the increase in stream power. Prediction of the rate and direction of the meander movement can be difficult. A review and comparison of historical mapping and aerial photographs can assist, together with local knowledge and observations. Complex morphological modeling, requiring detailed physical and hydrological data, can also be undertaken to predict the movement.

4.4.5

4.5

Meandering streams and rivers with bridge crossing present challenges as the rivers are highly rich in mobile sediment and unpredictable channel planform. Likewise, highway embankments which may form part of flood mitigation scheme do present the same and similar challenges. Careful consideration is required when works are proposed in the vicinity of type of rivers as they may be flood prone.

Sedimentation Transport

The concept of sediment transport is provided in Annex B.

Open Channel Flow

Design analysis of both natural and artificial channels proceeds according to the basic principles of fluid mechanics. They are namely: continuity, momentum and energy and are applied in open channel flow. Several important open channel flow concepts and relationship are described in the succeeding sections.

4.5.1

Definition & Basic Principles

4.5.1.1

Energy

As shown in Figure 4-5, the total energy at a given location in an open channel is expressed as the sum of the potential energy head (elevation), pressure head, and kinetic energy head (velocity head). The total energy at a given channel cross section can be represented as: Equation 4-1

𝐸𝐸𝑡𝑡 = 𝑍𝑍 + 𝑦𝑦 +

𝑉𝑉 2 2𝑔𝑔

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

where: Et

=

total energy, m

y

=

flow depth, m

Z

V g

=

=

=

elevation above a given datum, m mean velocity, m/s

gravitational acceleration, 9.81 m/s2

Written between an upstream cross section designated 1 and a downstream cross section designated 2, the energy equation becomes: Equation 4-2

𝑉𝑉12 𝑉𝑉22 𝑍𝑍1 + 𝑦𝑦1 + = 𝑍𝑍2 + 𝑦𝑦2 + + ℎ𝐿𝐿 2𝑔𝑔 2𝑔𝑔 where: hL

=

head or energy loss between section 1 and 2, m

The energy equation states that the total energy head at an upstream cross section is equal to the total energy head at a downstream section plus the energy head loss between the two sections. Figure 4-5

Energy Grade Line

Source: Virginia DOT, 2002

4.5.1.2

Steady and Unsteady Flow

A steady flow is one in which the discharge passing a given cross section is constant with respect to time. The maintenance of steady flow in any reach requires that the rates of inflow and outflow be constant and equal. When the discharge varies with time, the flow is unsteady.

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4.5.1.3

4.5.1.4

4.5.1.5

Uniform Flow and Non-uniform Flow

A non-uniform flow is one in which the velocity and depth vary in the direction of motion, while they remain constant in uniform flow. Uniform flow can only occur in a prismatic channel, which is a channel of constant cross section, roughness and slope in the flow direction. Non-uniform flow can occur either in a prismatic channel or in a natural channel with variable properties.

Gradually Varied and Rapidly Varied Flow

Gradually varied flow is a non-uniform flow in which the depth and velocity change gradually enough in the flow direction that vertical accelerations can be neglected. Otherwise, it is considered to be rapidly varied

Specific Energy

Specific energy, E, is defined as the energy head relative to the channel bottom (refer to Figure 4-6). If the channel is not too steep (slope less than 10%) and the streamlines are nearly straight and parallel (so that the hydrostatic assumption holds), the specific energy E becomes the sum of the depth and velocity head: Equation 4-3

𝐸𝐸 = 𝑦𝑦 + 𝛼𝛼 ( where:

y

=

depth, m

V

=

mean velocity, m/s

α g

4.5.1.6

𝑉𝑉 2 ) 2𝑔𝑔

=

=

velocity distribution coefficient gravitational acceleration, 9.81 m/s2

The velocity distribution coefficient is taken to have a value of one for turbulent flow in prismatic channels but may be significantly different for natural channels.

Critical Flow

Critical flow occurs when the specific energy is a minimum for a given discharge in regular channel cross sections. The depth at which the specific energy is a minimum is called critical depth. At critical depth, the Froude number has a value of one. Critical depth is also the depth of maximum discharge when the specific energy is held constant. These relationships are illustrated in Figure 4-6. During critical flow, the velocity head is equal to half the hydraulic depth. The general expression for flow at critical depth is: Equation 4-4

𝛼𝛼

𝑄𝑄 2 𝐴𝐴3 = 𝑔𝑔 𝑇𝑇 4-8

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

where:

α

=

velocity distribution coefficient

g

=

gravitational acceleration, 9.81 m/s

Q A

T

=

=

=

Figure 4-6

3

total discharge, m /s

cross-sectional area of flow, m

2

2

channel top width at the water surface, m Specific Energy Diagram

Source: Virginia DOT, 2002

4.5.1.7

4.5.1.8

4.5.1.9

Subcritical Flow

Depths greater than critical depth occur in subcritical flow, and the Froude number is less than one. In this state of flow, small water surface disturbances can travel both upstream and downstream, and the control is always located downstream.

Supercritical Flow

Depths less than critical depth occur in supercritical flow, and the Froude number is greater than one. Small water surface disturbances are always swept downstream in supercritical flow, and the location of the flow control is always upstream.

Froude Number

The Froude number, Fr, represents the ratio of inertial forces to gravitational forces and is defined by: Equation 4-5

𝐹𝐹𝑟𝑟 =

4-9

𝑉𝑉

√(𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 𝜃𝜃⁄𝛼𝛼)

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

where:

α

=

velocity distribution coefficient

g

=

acceleration of gravity, 9.81 m/s

V d θ

4.5.1.10

=

= =

mean velocity = Q/A, m/s hydraulic depth = A/T ,m

2

channel slope angle, m/m

This expression for Froude number applies to any open channel or channel subsection with uniform or gradually varied flow. For rectangular channels, the hydraulic depth is equal to the flow depth.

Hydraulic Jump

A hydraulic jump occurs as an abrupt transition from supercritical to subcritical flow in the flow direction. There are significant changes in depth and velocity in the jump, and energy is dissipated. For this reason, the hydraulic jump is often employed to dissipate energy and control erosion downstream of structures such as highway culverts and spillways. A hydraulic jump will not occur until the ratio of the flow depth (y1) in the approach channel to the flow depth (y2) in the downstream channel reaches a specific value that depends on the channel geometry. The depth before the jump is called the initial depth (y1), and the depth after the jump is the sequent depth (y2). Refer to Figure 4-8. Figure 4-7

Hydraulic Jump Diagram

When a hydraulic jump is used as an energy dissipater, controls to create sufficient tailwater depth are often necessary to control the location of the jump and to ensure that a jump will occur during the desired range of discharges. Sills can be used to control a hydraulic jump if the tailwater depth is less than the sequent depth. If the tailwater depth is higher than the sequent depth, a drop in the channel must be used to ensure a jump.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

4.5.2

Flow Classification

The classification of open-channel flow can be summarized as follows: Steady Flow

 

Uniform Flow

Non-uniform Flow

- Gradually Varied Flow - Rapidly Varied Flow

Unsteady Flow  

Unsteady Uniform Flow (rare) Unsteady Non-uniform Flow

- Gradually Varied Unsteady Flow - Rapidly Varied Unsteady Flow

4.5.2.1

The steady, uniform flow case and the steady, non-uniform flow case are the most fundamental types of flow treated in most hydraulic conditions. Steady Uniform Flow

For a steady, uniform flow, the mean velocity, V, can be computed with Manning’s equation: Equation 4-6

where:

𝑉𝑉 =

1 2⁄3 1/2 𝑅𝑅 𝑆𝑆 𝑛𝑛

V

=

velocity, m/s

R

=

hydraulic radius = A/P, m

n

P S

=

= =

Manning’s roughness coefficient wetted perimeter, m

slope of the energy grade line, m/m (For steady uniform flow, S = channel slope, m/m)

The selection of Manning’s n is generally based on observation; however, considerable experience is essential in selecting appropriate n values. Typical ranges of n values for various types of channels and floodplains is given in Table 4-2, Table 4-3, Table 4-4 and Table 4-5.

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Table 4-2

Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) - Natural Channels Description

Minimum

Maximum

1. Some grass & weeds, little or no brush

0.028

0.033

2. Dense growth of weeds, flow depth greater weed height

0.033

0.040

3. Some weeds, light brush on banks

0.035

0.050

4. Some weeds, heavy brush on banks

0.050

0.070

5. Some weeds, dense trees

0.060

0.080

For trees within channel, with branches submerged at high flood increase all above values by

0.010

0.020

6. Winding, some pools & shoals, clean (1.)

0.035

0.045

7. Winding, some pools & shoals, clean, lower stages, more ineffective sections

0.045

0.055

8. Winding, some pools & shoals, clean, some weeds & stones (3.)

0.040

0.050

9. Winding, some pools & shoals, clean, lower stages, more ineffective sections, stony sections

0.050

0.060

10. Sluggish river reaches, rather weedy or with deep pools (4.)

0.060

0.080

11. Very weedy reaches (5.)

0.100

0.150

Irregular sections, with pools, slight meander; increase above values by about

0.010

0.020

Fairly Regular Section

Mountain streams, no vegetation in channel, bank steep, tree & brushes along banks submerged at high flood 1. Bottom of gravel, cobbles & few boulders

0.040

0.050

2. Bottom of cobbles, with large boulders

0.050

0.070

Large Stream Channels (top width greater than 30m) Reduce smaller stream coefficients by 0.10

Table 4-3

Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) - Floodplains Description

Minimum

Maximum

1. Pasture, short grass, no brush

0.030

0.035

2. Pasture, tall grass, no brush

0.035

0.050

3. Cultivated land-no crop

0.030

0.040

4. Cultivated land, nature field crops

0.045

0.055

5. Scrub& scattered brush

0.050

0.070

6. Wooded

0.120

0.160

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 4-4

Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) – Man-made Channels & Ditches Description

Minimum

Maximum

1. Earth, straight & uniform

0.020

0.025

2. Earth bottom, rubble sides / riprap

0.030

0.035

3. Grass covered

0.035

0.050

4. Dredged

0.028

0.033

5. Stone lined & rock cuts, smooth &uniform

0.030

0.035

6. Stone lined & rock cuts, rough & irregular

0.040

0.045

7. Lined - smooth concrete

0.014

0.018

8. Lined - grouted riprap

0.020

0.030

9. Winding sluggish canals

0.025

0.030

10. Canals with rough stony beds, weeds on earth banks

0.030

0.040

Table 4-5

Values of Manning’s Roughness Coefficient 'n' (Uniform Flow) - Pipes Description

Minimum

Maximum

1. Cast Iron, Uncoated

0.013

0.015

2. Cast Iron, Coated

0.012

0.013

3. Wrought Iron, Black

0.013

0.015

4. Wrought Iron, Galvanized

0.014

0.017

5. PVC, HDPE

0.009

0.013

If the normal depth computed from Manning’s equation is greater than critical depth, the slope is classified as a mild slope while a steep slope is classified as one where the normal depth is less than critical depth. Thus, uniform flow is subcritical on a mild slope and supercritical on a steep slope.

Strictly speaking, uniform flow conditions seldom, if ever, occur in nature because channel sections change from point to point. For practical purposes in most hydraulic engineering problems, however, the Manning equation can be applied to most streamflow problems by making judicious assumptions. When the requirements for uniform flow are met, the depth (yn) and the velocity (Vn) are said to be normal and the slopes of the water surface and channel are parallel. For practical purposes, in open channel design, minor undulations in streambed or minor deviations from the mean (average) cross-section can be ignored as long as the mean slope of the channel can be represented as a straight line.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

4.5.2.2

Non-Uniform Flow General

For the gradually varied flow condition, the depth of flow must be established through a water surface profile analysis. The basic principles in water surface profile analysis are where:    

Water surface approaches the uniform depth line asymptotically Water surface approaches the critical depth line at a finite angle Subcritical flow is controlled from a downstream location

Supercritical flow is controlled from an upstream location

There are twelve (12) possible water surface profiles (see Figure 4-9) depending on the particular flow conditions. A complete discussion of water surface profile analysis is contained in most open channel hydraulics textbooks, such as Chow (1959) and Henderson (1966).

Methods of Analysis

Two methods of performing a water surface profile analysis are:  

The Direct Step method

The Standard Step method

Both methods make use of the energy equation to compute the water surface profile. The direct step method can be used to analyse straight prismatic channel sections only. The standard step method is applicable to non-prismatic and nonstraight channel alignments. For a complete discussion of both refer to Open-Channel Hydraulics (Chow, 1959) or numerous other textbooks on open channel hydraulics.

4.6

The analysis of water surface profile problems is best performed by computer. Available computer models are discussed in Section 4.10.

Closed Conduit Flow Calculations (Drainage Systems)

Flow conditions in a closed conduit can occur as: 

 

Open-channel flow - analysis of open-channel flow in a closed conduit is no different than any other type of open-channel flow, and all the concepts and principles discussed in Section 4.5 are applicable. Gravity full flow - occurs at that condition where the conduit is flowing full, but not yet under any pressure. Pressure flow - occurs when the conduit is flowing full and under pressure.

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Figure 4-8

Non-Uniform Flow Profiles

Due to the additional wetted perimeter and increased friction that occurs in a gravity full pipe, a partially full pipe will actually carry greater flow. For a circular conduit the peak flow occurs at 93% of the height of the pipe, and the average velocity flowing one-half full is the same as gravity full flow.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 4-9

Part-Full Flow Relationship for Circular Pipes

Source: FHWA, HDS4, 2001

Gravity full flow condition is usually assumed for purposes of storm drain design, as it provides a margin of safety over designing for pressure flow. However, it may not always be possible or suitable to avoid pressure flow. The Manning's equation combined with the continuity equation for circular section flowing full can be rewritten as: Equation 4-7

where:

𝑄𝑄 =

𝐾𝐾𝑢𝑢 5⁄3 1⁄2 𝐷𝐷 𝑆𝑆 𝑛𝑛

Q

=

discharge, m3/s

D

=

pipe diameter, m

Ku

=

0.312

n S

=

=

Manning's coefficient

slope, m/m

This equation allows for a direct computation of the required pipe diameter. Note that the computed diameter must be increased in size to a larger nominal dimension in order to carry the design discharge without creating pressure flow.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

4.6.1

Energy Equation

In simple terms the energy equation states that the energy head at any cross section must equal that in any other downstream section plus the intervening losses (as per Section 4.5.1). The energy head is divided into three components: the velocity head, the pressure head and the elevation head. The energy grade line (EGL) represents the total energy at any given cross section. The energy losses are classified as friction losses and form losses. The hydraulic grade line (HGL) is below the EGL by the amount of the velocity head. In open-channel flow the HGL is equal to the water surface elevation in the channel, while in pressure flow the HGL represents the elevation water would rise to in a stand pipe connected to the conduit (refer to Figure 4-10). Figure 4-10

Hydraulic Gradeline and Energy Grade Line for Piped Drainage Systems

Source: Inst. of Eng. Aust, 1977

4.6.2

Energy Losses

When using the energy equation all energy losses should be accounted for. Energy losses can be classified as friction losses or form losses. Friction losses are due to forces between the fluid and boundary material, while form losses are the result of various hydraulic structures along the closed conduit. These structures, such as access holes, bends, contractions, enlargements and transitions, will each cause velocity head losses and potentially major changes in the energy grade line and hydraulic grade line across the structure. The form losses are often called "minor losses," which is misleading since these losses can be large relative to friction losses.

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4.6.2.1

Calculating Friction Losses

Friction losses are calculated as: where: L

Sf

ℎ𝑓𝑓 = 𝐿𝐿𝑆𝑆𝑓𝑓

=

=

length of the conduit

friction slope (energy grade line slope)

Uniform flow conditions are typically assumed so that the friction slope can be calculated from either Manning's equation, or the Darcy-Weisbach equation. Rewriting Manning's equation for Sf: Equation 4-8

𝑄𝑄𝑄𝑄 2 𝑆𝑆𝑓𝑓 = ( 2⁄3 ) 𝐴𝐴𝑅𝑅

The Darcy-Weisbach equation for open-channel flow: Equation 4-9

𝑆𝑆𝑓𝑓 =

𝑓𝑓 𝑉𝑉 2 4𝑅𝑅 2𝑔𝑔

and for pressure flow in circular conduit: Equation 4-10

ℎ𝑓𝑓 = where: Q

=

𝑓𝑓𝑓𝑓 𝑉𝑉 2 𝐷𝐷 2𝑔𝑔

flow

n

=

Manning’s roughness coefficient

R

=

hydraulic radius = Area/Perimeter

A V

D

=

= =

area

velocity (m/s) diameter (m)

L

=

length (m)

hf

=

head loss due to friction

f

=

pipe friction factor

Manning's equation is more commonly used by practicing engineers, even though the Darcy-Weisbach equation is a theoretically better equation since it is dimensionally correct and applicable for any fluid over a wide range of conditions. 4-18

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

However, the possibilities for greater accuracy with the Darcy-Weisbach equation are limited by determination of the Darcy f and a generally more complicated application than the Manning's equation. Typical Manning's n values for closedconduit flow are given in Table 4-5.

4.6.2.2

No matter which formula is used, judgment is required in selecting roughness coefficients. Roughness coefficients are primarily defined by the type of pipe material. However, many other factors can modify the value based on pipe material. Other important factors include the type of joint used, poor alignment and grade due to settlement or lateral soil movement, sediment deposits and flow from laterals disturbing flow in the mainline.

Calculating Form Losses

Form losses occur when flow passes through structures such as access holes, junctions, ends, contractions, enlargements and transitions. These structures can cause major losses in both the energy grade line and the hydraulic grade line across the structure, and if not accounted for in design, the capacity of the conduit may be restricted. Form losses may be evaluated by several methods. The simplest method is based on a coefficient times the velocity head, with different coefficients tabulated for access holes, bends, inlets, etc. The general form of the equation is: Equation 4-11

𝑉𝑉 2 ℎ𝐿𝐿 = 𝐾𝐾 2𝑔𝑔

Some representative values of K are provided in Table 4-6.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 4-6

4.6.3

Local Losses Coefficient (K)

Pipe Network Analysis

Pipe network analysis involves the detailed and careful scrutiny of the fluid flow through a hydraulic network containing several interconnected branches and loops. In the design of a distribution system, a pipe network analysis must be done to determine the flow rates and pressure drops in the individual sections of the network, giving the basis for selecting pipe diameters.

The basic principles governing network hydraulics are: 

Conservation of mass – the fluid mass entering any pipe system will be equal to the mass leaving the system. In network analysis, outflows are lumped in nodes. A related principle is that at each junction (node), the algebraic sum of the quantities of water entering and leaving the node is zero. 4-20

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects



Conservation of energy – In any closed path or circuit in a hydraulic network, the algebraic sum of the energy (head losses) in the individual pipes is zero.

Another way of stating it is that the difference in energy (head loss) between two nodes in a system must be the same regardless of the path that is taken (Bernoulli principle).

4.6.3.1

One important tool that a network designer may use is the equivalent pipe method. It is the substitution of a complex system of pipes by a single pipe that will give an equivalent head loss at a given flow. Network Analysis by Conventional Method (Hardy Cross)

The most common conventional method (not using computers) that is used in designing hydraulic networks is the Hardy Cross algorithm method. It involves iterative trial and error.

One approach of Hardy Cross is the method of balancing the heads on the nodes by adjusting assumed flows in the pipe elements. Clockwise flows and corresponding head losses are assigned negative signs, and vice versa for positive signs. In the initial trial, initial values of flows in all pipe elements are assumed subject to the second principle above. The corresponding head losses in one closed circuit are calculated using the Hazen Williams formula. The head losses are then added considering their signs. This same head loss calculation and addition are done to each of the other closed loops. The assumed flow values are adjusted and the above procedure is done repeatedly until the summation of the head losses in the closed circuit becomes zero.

4.6.3.2

Nowadays, manual computation for hydraulic network analysis is only acceptable when applied to systems with only a single pipeline or branched network with no loop. For networks with loops, it is highly recommended to use the more accurate, fast and convenient network modeling computer software, which is discussed in the following section.

Network Analysis by Computer Software

There are a number of pipe network analysis software (also called network simulation software, or hydraulic network modeling software) which mathematically solve hydraulic equations for all interconnections, branches and loops of the pipe network.

With the advent of such powerful software, the conventional methods of water distribution design have been mostly discarded. The computer software requires the designer to create a water supply system model by inputting in the computer program information that includes pipe lengths, junction or node elevations, connectivity of the pipes and nodes, demand in each node, information on pumps, elevations of reservoirs, elevations and yield of sources. An example of such a computer model is EPANET, which is freely available from the US Environmental Protection Agency, although there are many different software available.

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4.7

Highway & Bridge Deck Drainage Structures

4.7.1

Cross Drainage (Culverts)

A culvert is a conduit that conveys flow through a roadway embankment or past some other type of flow obstruction. Culverts are typically constructed of concrete (reinforced and non-reinforced), corrugated metal (aluminum or steel) and plastic in a variety of cross sectional shapes. The most common cross sectional shapes for culverts are illustrated in Figure 4-11 and typical inlet structures are shown in Figure 4-12. The selection of culvert material depends on structural strength, hydraulic roughness, durability, and corrosion and abrasion resistance. Figure 4-11

Commonly Used Culvert Shapes

Source: FHWA, HDS4, 2001

Figure 4-12

Standard Inlet Types (Schematic)

Source: FHWA, HDS4, 2001

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Flow conditions in a culvert may occur as open-channel flow, gravity full flow or pressure flow, or in some cases a combination of these conditions. A complete theoretical analysis of the hydraulics of culvert flow is time-consuming and difficult. Flow conditions depend on a complex interaction of a variety of factors created by upstream and downstream conditions, barrel characteristics and inlet geometry.

4.7.2

For purposes of design, standard procedures and nomographs have been developed to simplify the analysis of culvert flow. These procedures are detailed in the Hydraulic Design Series Number 5 – Hydraulic Design of Highway Culverts (FHWA, 2005). Roadside & Bridge Deck Drainage

Roadway drainage involves the collection, conveyance, removal, and disposal of surface water runoff from the carriage way, shoulders, and adjoining roadside areas. Major roadside drainage facilities are ditches, gutters and swales; median drainage and slope drains.

4.7.2.1

The following provides a brief overview. Further discussion on specific aspects of roadside and bridge drainage is provided in Volume 4: Highway Design and Volume 5: Bridge Design.

Roadside Channels (Ditches and Gutters)

Roadside channels are commonly used with uncurbed roadway sections to convey runoff from the highway pavement and from areas which drain toward the highway. Curbs are normally used at the outside edge of pavements to contain the surface runoff within the roadway and away from adjacent properties, to prevent erosion on fill slopes, to provide pavement delineation and to enable the orderly development of property adjacent to the roadway Gutter cross sections usually have a triangular shape with the curb forming the near vertical leg of the triangle. The gutter may have a straight cross slope or a cross slope composed of two straight lines.

Modification of the Manning equation is necessary for use in computing flow in triangular channels because the hydraulic radius in the equation does not adequately describe the gutter cross section, particularly where the top width of the water surface may be more than 40 times the depth at the curb. To compute gutter flow, the Manning equation is integrated for an increment of width across the section as shown Figure 4-13.

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Figure 4-13

Definition Sketch - Triangular Section

The resulting equation in terms of cross slope and spread on the pavement is: Equation 4-12

where:

𝐾𝐾 5⁄3 1⁄2 8⁄3 𝑆𝑆 𝑆𝑆 𝑇𝑇 𝑛𝑛 𝑋𝑋

K

=

0.016

T

=

width of flow (spread), (m)

Q Sx S

4.7.2.2

𝑄𝑄 = = = =

flow rate (m3/s)

cross slope, (m/m)

longitudinal slope, (m/m)

Table 4-2 shows typical values of the Manning’s roughness for various gutter or pavement materials.

Median Barriers & Median Channels (Swales)

Medians are commonly used to separate opposing lanes of traffic on divided highways. Median areas should preferably not drain across traveled lanes, and often times the inside lanes and shoulder of multi-lane highways will drain to the median area where a center swale collects the runoff. Based on capacity or erosion considerations, it is sometimes necessary to place inlets in medians to remove some or all the runoff that has been collected. Medians may be drained by drop (grate) inlets similar to those used for pavement drainage.

4.7.2.3

Where median barriers are used and, particularly on horizontal curves with associated super-elevations, it is necessary to provide inlets and connecting storm drains to collect the water that accumulates against the barrier. Slotted drains adjacent to the median barrier and in some cases weep holes in the barrier can also be used for this purpose.

Storm Drain

The total drainage system which conveys runoff from roadway areas to a positive outlet including gutters, ditches, inlet structures, and pipe is generally referred to as a storm drain system. In urban areas a highway storm drain often augments an

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4.7.2.4

existing or proposed local drainage plan and should be compatible with the local storm drain system.

Bridge Decks

Effective bridge deck drainage is important for several reasons, including hydroplaning and associated traffic safety. While bridge deck drainage is accomplished in the same manner as any other curbed roadway section, bridge decks are often less effectively drained because of lower cross slopes, uniform cross slopes for traffic lanes and shoulders, parapets that collect debris, and drainage inlets that are relatively small and susceptible to clogging.

4.8

Because of the limitations of bridge deck drainage, roadway drainage should be intercepted where practical before it reaches a bridge.

Flood Control Structures

Rivers generally commence in the mountains or hills, then flow along the plains and finally join the oceans. They form more or less defined channels; drain away the surface runoff produced by rainfall; and discharge the unutilized water back into the sea. The rivers not only carry water but also sediments washed down from the catchment area and eroded from the beds and the banks of rivers.

Channels are formed by the interactions of water and sediments. During large floods, floodwaters not only overflow and bring about inundation to riverine areas, but also cause serious sediment related damages. These include: 1) bank erosion/collapse including dike and revetment and 2) river bed degradation.

In order to protect the populace and properties, and to reduce the extent of overflow in the flood plain, flood control structures are planned and designed. River channel improvement is the most common flood control scheme which aims to either increase the carrying capacity of river, or to reduce the flood stages.

River channel improvement includes:     

Dikes - embankment built parallel to the river banks. Levees are earth embankment, whereas floodwalls are generally concrete walls. Revetments - embankment protection against erosion and scouring. Floodway - diverts river flow from one river to another water body

Dams/ Detention Structures – these attenuate the flood flow and reduce the peak flow arriving to critical areas downstream in the floodplain. Cut-off Channel - connects the beginning and end of a meandering portion of a stream so as to straighten the river course.

The planning and design of this river channel improvement are discussed in Section 5.

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4.9

Regulating Structures Flood protection can also be achieved by providing a reservoir to retard or delay excessive runoff for purpose of reducing heights of floods. The function of reservoirs is to store water when streamflow is excessive and release it gradually after the threat of flooding has passed. This can be accomplished by building dams across main rivers and/or tributaries. In cases where the maximum flood level is reached, the flood flow is regulated either by sluiceway, weirs, or spillways, and/or combinations. The flow for this type of hydraulic structures behaves in rapidly varied flow condition and they are treated with special hydraulic principles.

Flood waters which cannot be contained in allotted storage space of reservoirs, detention or retention reservoirs may to be released through regulating structures such as weirs and sluiceways.

4.9.1

Ordinarily, the excess is drawn from the top of reservoir or dikes and is conveyed through constructed water way back to river or some natural drainage channel. In this case, they are called weirs. On the other hand, water withdrawn below dam wall is known as sluiceways.

Weirs

Weirs may be classified as broad, roof shaped, round or sharp crested. The discharge is estimated using weir formula of Poleny as expressed as: Equation 4-13

where:

𝑄𝑄 = 0.66𝑐𝑐𝑐𝑐𝑐𝑐(2𝑔𝑔)0.5 ℎ𝑢𝑢0.66

Q

=

discharge (m3/s)

μ

=

weir coefficient

c

B g

hu

= = =

=

correction factor for submerged overfall weir crest width, m

acceleration due to gravity 9.81 m/s2

weir head, m.

Weir coefficient μ depends upon the crest form of the weir. Table 4-7 exhibits the correction factor.

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Weir Coefficient, μ for Different Weir Shape

Table 4-7

Crest Form

μ

Broad sharp edges

0.49-0.51

Broad round edges

0.50-0.55

Round overfall

0.70

Sharp edged

0.64

Rounded

0.75

Roof shaped

0.79

Source: Lauterjung, H., Schmidt, G., 1989

Figure 4-14 shows the weir coefficient c for submerged overfall. Whether an overfall is free or submerged depends upon the height of the tail water level in relation to the position of the weir crest as shown in Figure 4-15. Figure 4-14

Weir Coefficient with Tailwater Submergence

Source: Lauterjung, H., Schmidt, G., 1989

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Figure 4-15

Tailwater Conditions for Submerged Overfall

Source: Lauterjung, H., Schmidt, G., 1989

4.9.2

Sluiceways

In case a barrier is placed in a stream in which the flow takes place through a geometrically fixed opening located under the upstream water level the flow is analyzed by orifice formula. Once the orifice is considered as square or rectangular section then it is known as sluiceway. The sluiceway as shown in Figure 4-16 serves to regulate the outflow from the upstream section. Figure 4-16

Profile of Rectangular Sluiceway

Source: Lauterjung, H., Schmidt, G., 1989

The discharge is calculated using the following equation: Equation 4-14

𝑄𝑄 = 𝑘𝑘𝑘𝑘𝑘𝑘𝑘𝑘(2𝑔𝑔ℎ)0.5

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where: K

=

correction factor for submerged discharge; for free discharge k = 1 (See Figure 4-17)

a

=

height of outflow opening, m

μ

B h g

Figure 4-17

= =

= =

discharge coefficient. The coefficient takes the jet contraction into account. For vertical sluiceways, μ = 0.55 to 0.60. width of outflow opening, m

impounding height in front of the sluice or dam acceleration due to gravity 9.81 m/s2

Sluiceway Discharge Coefficient as a Function of h/a & hu/a

Source : Lauterjung, H., Schmidt, G., 1989

The correction for the degree of submergence is presented in Figure 4-18 and Figure 4-19.

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Figure 4-18

Free Discharge (Top) and Submerged Discharge (Bottom)

Source: Lauterjung, H., Schmidt, G., 1989 Figure 4-19

Limit between Free & Submerged Discharge

Source: Lauterjung, H., Schmidt, G., 1989

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4.10

4.11

Bridge Hydraulics A discussion on bridge hydraulics is presented in Volume 5 of the DGCS.

Downstream Influences

Downstream influences are an important consideration in hydraulic analysis. The influence of factors outside of the hydraulic analysis or modelling area can change the capacity of a channel or drainage network within the study area.

Methods such as Manning’s equation assume that there are no backwater effects influencing the capacity of the channel. However, where these are likely to occur, then a more complex analysis of the flow conditions are required and most likely this will need to be undertaken in a computer model. For example, a Manning’s calculation can be undertaken on a channel to determine the capacity of that channel. However, if the channel is in an estuarine environment then tidal influences may result in a reduced capacity and this will not be taken into account using a simple Manning’s equation.

Key influences from downstream of a study area include:    

4.11.1

Tidal effects.

Tributary and river flow.

Downstream structures or controls. Downstream tributary inflow.

These are discussed in the following sections.

Tidal & Storm Surge Effects

Hydraulic analysis should take into consideration the impact of tidal and coastal storm events on the capacity of river, channel and drainage infrastructure. This should be undertaken where drainage or rivers are influenced by coastal levels. For the purposes of this Guide, it is assumed that a channel or drainage infrastructure may be influenced by coastal inundation where the invert is below 3 m above Mean Sea Level and it is within 5km of the coastline. The key challenge is adopting an appropriate tidal level for use in hydraulic assessments.

Coastal events and flood events do not always occur at the same time, nor may be driven by the same storm systems. Furthermore, the timing can be significantly different. For example, a storm may pass across a catchment, which results in peak discharges prior to it passing over the coastal area.

One option, for example, is to adopt a 100 year flood with a 100 year ocean level. However, the challenge is that this is likely to be particularly conservative, with the resulting estimated flood levels likely to be much higher than a 100 year frequency.

A thorough assessment would involve a joint probability analysis, analyzing historical rainfall, streamflow and ocean records to determine the likelihood and 4-31

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probability of different events occurring concurrently. However, this is generally too onerous for the majority of studies.

In the absence of joint probability analysis, the following is suggested as a potential approach:  

4.11.2

Analyze the X year flood with the Mean Higher High Water tide level, where X is the flood from the catchment to be analysed.

Analyze the 5 year flood with the X year storm surge level, as defined in Engineering Standards for Port and Harbor Structures, Volume II, Chapter 6 of PPA (2009).

The maximum of the above two results will be the X year flood level.

Tributary & River Flow

The analysis of tributaries in isolation of the influence of downstream river flows can result in an overestimation of channel or drainage capacity. Engineering judgment may be required based on a review of the study area as to whether this is a key issue.

As with tidal influences, the key challenge is estimating the joint probability of a river flood with a flood in the tributary. For very large river systems, for example, a flood may occur in the river with no significant flood flows from the tributary catchment.

In the absence of more detailed information and probability analysis, the following is suggested as a potential approach: Where the catchment area of the downstream river is more than 5 times the catchment area of the tributary  

Analyze the X year flood in the tributary with the 5 year flood in the river, where X is the flood event to be analyzed. Analyze the 5 year flood in the tributary with the X year flood in the river.

The maximum of the above two results will be the X year flood level.

Where the catchment area of the downstream river less than 5 times the catchment area of the tributary When this is the case, the timing of the flood in the river is more likely to coincide with the flooding in the tributary. Therefore, an X year flood in the tributary should be analyzed together with the X year flood in the river.

4.11.3

It is noted that for large river systems, it may be difficult to estimate the discharge in the river. When this is the case, the specific discharge method is one alternative for providing a representative flow for the river (refer to Section 3.4.5). Downstream Structures or Controls

Downstream structures may include weirs, culverts, bridges etc. These types of structures have the potential to create an upstream afflux, which may create a backwater into the area of analysis. 4-32

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4.11.4

4.12

Where a structure is located in close proximity to the study area, it is recommended that a hydraulic analysis be undertaken on the structure to determine the likely upstream afflux. This afflux can then subsequently be used for the hydraulic calculations.

Downstream Tributary Inflow

When analyzing a river or channel, it is important to consider the influence of a large inflow downstream of the hydraulic analysis area. Where there is a large tributary entering the main river or channel downstream, and this is likely to affect the upstream levels, this should be included in the analysis.

GeoHazard Impacts on Hydraulics

GeoHazard risks and implications are discussed in Volume 2A: GeoHazard Assessment. Some specific GeoHazards (Table 4-8) have the potential to influence the hydraulic analysis, and should be taken into consideration where appropriate.

4.13

It is noted that there are other factors, such as landslides and volcanic lahar, which result in increased sediment runoff and movement of debris, which should also be considered. Reference should be made to guidance of FCSEC for Sabo Engineering.

Computer Models

Many of the calculations and methods presented in this chapter are typically undertaken through computer models. There are numerous computer models available, and these models are constantly evolving to include greater levels of complexity and application.

4.13.1

It is not appropriate to recommend a specific software type due to the constant changing nature of the different software, but rather to provide sufficient background so that the selection of a suitable model might be made to a specific project or application.

The Use of Computer Models

Designers who use computer models have a duty of care to ensure that they are familiar with the software, including the underlying assumptions of the software and algorithms, key input data and interpretation of output data.

It is noted that many of the problems that occur with computer models are not in the development of the program itself, but rather in the application of the software. Typical issues include:     

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Incorrectly specified input data. Errors in the input data.

Application of the model beyond the scope for which it was intended.

Incorrect schematization of the model or representation of the study area. Incorrect interpretation of the model results.

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

4.13.2

Types of Flood and Drainage Models

Flood and drainage computer models can generally be characterized as onedimensional (1D), two-dimensional (2D) or some combination of the two. Table 49 provides an overview of these types of models and their application. A summary of the key advantages between 1D and 2D models are provided in Table 4-10. Table 4-8

GeoHazard Subsidence

GeoHazard Impacts on Hydraulics

Description Regional subsidence tends to be more of an issue in coastal areas, where the global landform elevations changes relative to ocean levels. Where subsidence occurs in a coastal area, it may be necessary to account for this in the design. This is because the landform may settle over time relative to ocean levels, and reduce the flood immunity or coastal protection offered by a structure. A flood control or coastal protection measure should incorporate the expected subsidence or settlement over the design life. This identification of the potential for subsidence will be identified with the best available information at the time under Volume 2A: GeoHazard Assessment. The designer will need to undertake further investigations and assessments to identify the scale of the subsidence and mitigating actions to overcome this.

Seismic

Similar issues may result from changes in landform from seismic activity. However, the key difference is that seismic may result in less uniform changes in landform, and therefore can have an impact on flood levels and flood behavior. The PGA (Volume 2A: GeoHazard Assessment) will identify the potential level of risk. The designer will then need to incorporate this within the design. This should also be included in hydraulic analysis and subsequent design of infrastructure.

Storm Surge

Particularly for rivers and drainage structures close to the ocean, storm surge has the potential to influence the capacity and flood levels in these systems. The likely occurrence of storm surge with a catchment flood can be difficult to estimate. A suggested approach is provided in Section 4.11.1. Storm surge will also influence the design of coastal structures. This is further discussed in Section 7.

Sediment and Debris Flow

Factors such as landslides and volcanic lahar, which result in increased sediment runoff and movement of debris, should also be considered. These have the potential to reduce the capacity of dams, block hydraulic structures such as bridges and change the path of rivers. For mitigation measures, reference should be made to guidance of FCSEC for Sabo Engineering.

Floating Debris

Floating debris includes both anthropogenic (e.g. rubbish) and natural sources (e.g. trees). These have the potential to obstruct culverts, bridges and other hydraulic structures. In catchments where a significant level of debris is likely, then additional blockage factors should be applied in the design of structures, or suitable debris control structures should be planned upstream. Some general guidance on blockage of culverts is provided in Section 6.5.6, while for the design of Bridges reference should be made to Volume 5 ; Bridge Design.

Increased Development and Settlements

Increase development in a catchment can impact on the runoff, by increasing the impervious area. Both the existing and likely future catchment conditions should be considered when undertaking hydrological analysis. Similarly, the level of demand for water supply systems should consider the changes in development. Development may also impact on the capacity of floodplains and drainage channels, where development impinges upon the flow area. This may happen through both formal and informal settlement. The impact that this has on the capacity of the floodplain or drainage area should be considered.

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Table 4-9 Model 1D models

Overview of Different Model Types Structure

With these models the main channel and floodplain of a waterway is schematized as a single 1D channel, comprising a series of spaced crosssections.

Typical Application The use of 1D models is generally restricted to modelling single waterway branches, or simply connected (dendritic) channel systems, where flow in the floodplain is well connected to the main channel. Due to their inherent limitations, 1D models have generally been replaced by more flexible 1D branched, full 2D or combined 1D/2D models.

Branched 1D Models

These models allow arbitrary connections of multiple channel systems, and are an evolutionary development of simpler 1D models. Floodplains can be represented as separate flowpaths and there can be multiple flowpaths within a single floodplain. This provides a more realistic description of flows through a street network for example.

These models are sometimes referred to as quasi-2D models, but should not be confused with genuine 2D models. These models can be applied where flowpaths are well defined and clear controls exist between flowpaths.

It is noted that within each branch or flowpath the flow is represented by the one-dimensional crosssectionally averaged equations of motion. 2D Models

With these models survey information for the study area is projected onto a 2D model grid or mesh. Grids may be a square or rectangular, as typically the case in finite difference models. A mesh may be variable-sized quadrilaterals, triangles or of curvilinear nature, as is typically the case for finite element or finite volume models. The flow solution is based on the numerical solution of the full 2D depth-averaged equations of motion computed at each active computational element.

Integrated 1D/2D Models

With these models the main channel(s) and/or structures (such as culverts, bridges or pipe networks) are described by the 1D domain that is connected dynamically to the 2D domain of the overbank area. There can be a number of independent 1D domains within the overall integrated model.

These integrated models aim to provide a more comprehensive, efficient and accurate representation of a hydraulic system by making the most of both branched 1D and full 2D model capabilities.

3D Models

These models are similar to 2Dmodels; however there is the opportunity for non-uniform vertical velocity profiles. This allows for the computation of 3D phenomena such as wind circulation in shallow areas or density-stratification within the water column.

These models are typically not used in urban or rural flood situations, as flow depth is too shallow and/or velocities too great to develop stratified conditions.

Source: Engineers Australia, 2012

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Full 2D models are capable of providing a detailed description of the flow in urban or rural floodplains and overbank areas. Full 2D models are more computationally demanding than 1D models. This may be a factor when considering long simulations or real-time forecasting applications. In addition, fixed grid models may have problems in providing adequate resolution of in-bank flows.

Other 3D cases such as weir flow occur at a scale too small to resolve in a typical flood model

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 4-10

Advantages and Disadvantages of Model Types

Features

Advantages

Disadvantages

1D Models Series of linked channels with discrete cross-sections at regular intervals Output at each cross-section can include water level, depth and velocity (averages)

Relatively fast to run (run time typically < 1 hour) Can be time consuming to build, but relatively quick to modify Result files are relatively small

Requires cross-sections to be input to model, extracted either from field survey or DEM Can be time consuming to build, but relatively quick to modify Requires more interpolation and interpretation of results

2D models Detailed grid or mesh-based topography with element resolutions for an urban environment typically ranging from 1m to 10m. For more extensive floodplain environments, element resolution can typically range from 10m to 100m. Output at each grid/mesh element can include water level, depth and velocity.

Less interpolation of results required and more readily linked to GIS Modeller is not required to identify flowpaths in advance Can model complex flowpaths Floodplain storage is implicitly defined Inputs and outputs defined spatially in GIS type environments, results in better data continuity and more readily accessible/understandable results for community/ stakeholders

Requires detailed grid/mesh to be interpolated from aerial and/or field survey based DEM (plus roughness mapping over study area) Can be time consuming to build modifications often not as easy as for 1D Relatively slow to run (run times typically range from hours to days) Result files are relatively large (up to GB per simulation) Can in some cases instil overconfidence in the result that may not be justified if the underlying data are inadequate

Source: Engineers Australia, 2012

4.13.3

4.13.4

Available Software

There are numerous available software packages to undertake modelling for flood and urban drainage analysis. An overview of different software that is currently available is provided in Table 4-11. This is current as at the time of issuing of this report, but the software and the capabilities are likely to change as developments occur. Therefore, it is recommended that the reader familiarize themselves with the current software at the time and select appropriately. Utilities

In addition to the above, model types, there are also numerous utilities that are available. These types of utilities include:    

Culvert calculation utilities. Bridge calculation utilities.

Simple pipe capacity analysis.

Rip rap, scour and other related utilities.

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Each of these utilities typically assesses particular features (such as a hydraulic structure) in isolation. They are useful for smaller projects or for validation of a hydraulic structure in a larger floodplain or urban drainage model. They generally do not allow for the estimation of the larger floodplain behavior. Some examples of software available for these types of calculations include:  

4.13.5

HY-8, distributed through FHWA for culvert calculations.

Hydraulic Toolbox, provided by FHWA that includes calculations in a number of different simplified applications.

Checking and Review of Model Results

Understanding and checking of model results is critical. It is important that the modelling system not be treated as a “black box”. The following (primarily based on Engineers Australia, 2012) provides some broad guidance on how to check the results of the model. However, it is noted that this is not exhaustive and should be based on experience and knowledge of hydraulics. 







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Mass balance – Some models can generate or lose water as a result of the computational scheme. The mass balance is a quick way of checking this, and is measured as Input Volumes = Outflow Volumes less Storage at the end of the model run. Errors greater than 1% to 2% should generally be investigated, and the cause of the errors identified and rectified where possible. Continuity – discharge hydrographs should be obtained at several locations along each flowpath, and at locations upstream and downstream of major flowpath intersections, to check that the continuity and attenuation of flows is reasonable.

Stability – the results should be checked for signs of instability, such as unrealistic jumps or discontinuities in flow behavior, oscillations (particularly around structures or boundaries), excessive reductions in time step or iterations required to achieve convergence.

Froude numbers – Froude numbers should be checked to identify areas of trans-critical and super-critical flow, and the implications of this flow behavior on the model results considered. In general, model results in areas of transcritical flow should be used with extreme caution.

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 4-11 Software

4.14

Overview of Different Software for Flood and Drainage Analysis Hydraulics

Hydrology

Drainage

Flood Estimation

1D or 1D/2D

Applicability

XP-RAFTS

Y

N

N

N/A

Primarily a hydrological model. Modelling of reservoirs possible and simple hydraulic structures possible.

XP-SWMM

Y

Y (some pipe and drainage design)

Y

1D/2D

This model actually has a similar hydrological modelling engine as XP-RAFTS, and also includes hydraulics. It has integrated hydrology and hydraulics. Note that there are also design alternatives that are available for this software (available from xp solutions)

XPDRAINAGE

Y

Y

N

1D

Sustainable Drainage Design

DHI (MIKE)

Y

Y (more analysis than design)

Y

1D/2D

DHI produces a number of components under the MIKE banner, which include drainage (pit and pipe) modelling software, channel and 2D modelling software.

HEC-RAS

N

N

Y

1D only

HEC-RAS (and associated software) is freely available, so it tends to get used extensively. Very good for bridge and culvert assessments, but it is not as good at defining complex floodplains

HEC-HMS

Y

N

N

N

Hydrological analysis only.

SOBEK

Y

Y (more analysis than design)

Y

1D/2D

Deltares (formerly WL|Delft Hydraulics) distributes this software. Similar in complexity to DHI.

Tuflow

N

Y (more analysis)

Y

1D/2D

Distributed by BMT WBM. Similar to MIKE and SOBEK in complexity.

DRAINS

Y

Y (more design oriented)

N

1D

This is primarily a drainage design software for analysis of HGL etc. Relatively simple, but good for modelling of pit and pipe infrastructure. Very good functionality in terms of optimizing designs etc. Primarily setup for Australian applications however.

WinDES

Y

Y

N

1D

Similar to DRAINS.

RORB

Y

N

N

N/A

Hydrological analysis only.

WBNM

Y

N

N

N/A

Hydrological analysis only.

12D and other CAD based design software

Y (although simple)

Y (design orientated)

N

1D

Many of the CAD packages, 12d etc. have drainage design incorporated into them. Good for simple, straight forward design of drainage systems.

Physical Models Complex flow patterns may defy accurate or practicable mathematical modelling. Physical models should be considered when:   

hydraulic performance data are needed that cannot be reliably obtained from mathematical modeling. risk of failure or excessive over-design is unacceptable.

flow behavior is outside that represented by known hydraulic behavior and research is required.

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4.15

Understanding Accuracy The accuracy of hydrological and hydraulic analysis is dependent on a number of factors such as the quality of the data used in the development of the analysis (such as topographical data, boundary conditions), assumptions in undertaking the analysis and any calibration data that might be used. Hydrology, in particular, can be subject to wide ranges in estimates. Even flow gauges can high wide error ranges, particularly in large flood flows. It is important that this be considered in undertaking any design. Particularly in the use of computer models, there is a tendency to believe the accuracy of the results is the same as the number of decimal places that are produced.

4.16

Freeboard on flood level estimates is often used as a way to make allowance for the various uncertainties in the estimates. Gillespie (2005) argues that uncertainty in factors typically included in the freeboard, such as model error, waves, afflux and climate change, may vary between studies or locations. For this reason the freeboard allowance should be based on the best estimate of uncertainty in the factors relevant to the specific study, rather than be a blanket adoption of a standard or default value. Therefore, this Guide generally provides minimum freeboard allowances. However, higher freeboards may be adopted in some situations, particularly where uncertainty is expected to be higher.

Reporting Requirements

A hydrological report should be prepared for a project. In some cases, both the hydrology and hydraulic report can be incorporated into a singular report, based on the information identified in Table 4-12.

4.16.1

The report should contain, as a minimum, the information identified in Table 3-10.

Computer Model Specification

When a computer model is used for hydraulic analysis, the following should also be provided:    

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Name and version of the software package; Full details on the modelling assumptions;

Specification of key parameters adopted for the analysis and why these parameters were adopted. Mass Balance Error report from the model (refer to Section 4.13).

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 4-12 Component

Minimum Hydrological and Hydraulic Reporting Requirements Description

Project Description

A brief description should be provided outlining:  Purpose of the Project  What the project involves  Why the hydraulic analysis is required

Study Area

A description of the study area should be provided, sufficient so that the reader is aware of the location of the project. This will include:  Description of the study area  Map showing locality of the study area, including key features such as road names, river names etc.  Coordinates of the project site location. If it is a linear structure (e.g. levee), then the approximate centroid of the structure will suffice.  Why the hydrological analysis is required

Hydrology

Where a separate hydrological report is prepared, then a reference is required for the hydrology report. This should include the date of the report, document reference number etc. Where the hydrological report is not combined with the hydraulic report, then a brief description of the catchment should be provided together with a catchment map. If the hydrological report is combined, then the requirements of Section 3 of the DGCS for hydrology reports should be included in this report.

Design Criteria

Where the hydraulic analysis is being undertaken as part of a flood control or drainage project, a discussion should be provided on the selection of the design flood for the design. This should take into consideration the discussion in Section 5.2 and Section 6.2, as appropriate. Overview of general approach to the design, such as planning considerations etc.

Hydraulic Analysis

The following should be provided as a minimum:  Statement as to what hydraulic analysis technique was adopted, and why.  Summary of all key input parameters for the analysis. A justification for key parameters, such as the Mannings ‘n’ value should be provided.  Details of any storages, such as dams, should be included.

Results

The results should be summarized in a clear and concise format. The results may include:  Peak flood levels, depths, velocities  Water level time series  Plans showing flood extents  Discussion and interpretation of the results. The results presented should be suitable for the project application.

Design

Where the hydraulic analysis is being undertaken as a part of a flood control or drainage project, then details will be required on the design of the structure. This will include information on the following (where relevant):  Specification of the freeboard above the design flood achieved, and how this compares with the requirements of this Guide.  Estimated scour depth for the structure, calculated in accordance with Annex A, where appropriate  Sizing of any protection methods, and how this was calculated.  For urban drainage pipe design, traditional HGL calculation spread sheets are not required where computer modelling is undertaken. However, the following should be provided: – Identification numbers or equivalent for manholes and pipes, to provide reference, together with suitable reference plans or maps – Tabulation of the following:  Catchment area assumed for each inlet  Sizing of inlets  Size of each pipe  Invert on the upstream and downstream of each pipe  Flow capacity of each pipe  Details of other key calculations as identified in Section 5 and Section 6.

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4.17

References Arizona Master Watershed Steward (AMWS), 2007. Accessed 16 September 2013, http://ag.arizona.edu/watershedsteward/resources/module/Stream/stream_pr oc_page5.htm Chow, V.T., 1959, Open Channel Hydraulics, McGraw-Hill Book Co. Inc., New York.

Engineers Australia, 2012. Project 15 – Two Dimensional Modelling in Urban and Rural Floodplains, Australian Rainfall and Runoff Revision Projects, Draft, November, Australia.

Federal Highway Administration, 2008, Introduction to Highway Hydraulics, Hydraulic Design Series No. 4, U.S. Dept. of Transportation, Washington.

Federal Highway Administration [FHWA], 2005. Hydraulic Design of Highway Culverts, Hydraulic Design Series No. 5, Revised Version (May, 2005), U.S. Department of Transportation, Washington. Federal Highway Administration, 2001, River Engineering for Highway Encroachment, Hydraulic Design Series No. 6, U.S. Dept. of Transportation, Washington. Geocaching, 2009. Menomonee River: Straight, Sinuous or Meandering?, accessed 16 September 2013, http://www.geocaching.com/geocache/GC1TGAY_menomonee-river-straightsinuous-or-meandering?guid=b89306b0-083e-427e-94be-f4b0367b47fd Henderson, F.M., 1966, Open Channel Flow, Macmillan Co, New York.

Institution of Engineers, Australia, 1977. Australian Rainfall and Runoff – Flood Estimation and Design, Canberra, ACT. JICA/DPWH, 2010, Technical Standards and Guidelines for Design of Flood Control Structures.

JICA/DPWH, 2010, Technical Standards and Guidelines for Planning of Flood Control Structures.

Kinori, B.Z., 1970, Manual of Surface Drainage Engineering Vol. 1, Elsevier Publishing Co., Amsterdam. Kinori, B.Z., Mevorach, J, 1984, Manual of Surface Drainage Engineering, Vol. 2: Stream Flow Engineering and Flood Protection (Developments in Civil Engineering).

Lauterjung, H., Schmidt, G., 1989, Planning of Intake Structures, Deutsches Zenturm fur Entwicklungstechnologien (GATE).

Melville, B. W., Coleman, S. E. (2000) Bridge Scour. Water Resources Publications, Highlands Ranch, Colorado. Ministry of Public Works and Highways, 1984, Design Guidelines Criteria and Standards for Public Works and Highways, Volume II.

Ohio Department of Natural Resources [DNR], undated. Ohio Stream Management Guide, Guide No. 3 – Natural Stream Processes. 4-41

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Philippine Port Authority [PPA] (2009). Engineering Standards for Port and Harbor Structures, March.

Texas Department of Transportation, 2009, Hydraulic Design Manual.

Thorne C R, Hey R D , Newson M D 1999, Applied Fluvial Geomorphology for River Engineering and Management.

US Army Corps of Engineer, 1994, Channel Stability Assessment for Flood Control Projects, Em 1110-2-1418, US Dept. of Army.

US Army Corps of Engineers [HEC-RAS], 2008, HEC-RAS River Analysis System, Hydraulic Reference Manual Version 4.0, The Hydrologic Engineering Center, Davis, CA. Utah Department of Transportation, 2004, Manual of Instruction Roadway Drainage. Virginia Department of Transportation, 2002, Drainage Manual,

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5

Flood Control

5.1

Introduction This chapter of the guideline focuses on the design of flood control structures. It is largely derived from the Technical Standards and Guidelines for Design of Flood Control Structures (FCSEC [1], 2010) with updates and revisions. These revisions are based on currently international practice as well as a review of the usability of the guideline.

Many of the structures identified in this chapter require multi-disciplinary approaches to engineering. Depending on the size of the flood control structure, the failure of one of these structures may result in significant impacts to property and risk to life. Therefore, it is important that the design of this infrastructure be undertaken by suitably qualified engineers with experience in undertaking this work.

This guideline on flood control structures provides an overview of some of the key issues, considerations and items to be incorporated into design. As with the entire Guide, this is not meant to be an exclusive list of design criteria or a manual for the design of these infrastructures. The infrastructure covered in this chapter includes:       

5.1.1

Dikes (levees) Spur Dikes

Revetments Ground Sill

Small Dams

Sluiceway and Conduits for Embankments/Dikes Overtopping Embankments

Planning is required for the implementation of this infrastructure. It is generally recommended that a catchment or floodplain wide approach be adopted for planning, as outlined in Technical Standards and Guidelines for Planning of Flood Control Structures (FCSEC [2], 2010).

Bank Stabilization

Bank stabilization and river training is a specialized field requiring familiarity with the stream and its propensity to change, knowledge of the bed load and debris carrying characteristics of the stream, and experience and experimentation at similar sites on the same or similar streams.

Attempts at localized control should be avoided where the river is in the midst of changes by studying long reaches. Regardless of the size of the stream and the control measures used, consider stream response to the installation of the measure. For instance, bank stabilization at a crossing can cause scour in the bed 5-1

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

of the channel or redirect the current toward an otherwise stable bank downstream.

To a large extent, design is an art, and many questions concerning the relative merits of various measures have not been definitively answered. General principles for the design and construction of bank protection and training works are:  



  

The cost of the protective measures should not exceed the cost of the consequences of the anticipated stream action.

Base designs on studies of channel morphology and processes and on experience with compatible situations. Consider the ultimate effects of the work on the natural channel (both upstream and downstream).

Site reconnaissance is imperative. This may include a combination of on-site inspection, aerial reconnaissance, or aerial photographs taken over a period of years. Consider the possibility of using physical model studies at an early stage.

Inspect the work periodically after construction with the aid of surveys to check results and to modify the design, if necessary. The protective measures themselves are expendable.

5.2

General Criteria

5.2.1

Design Flood

The design flood for bank protection works and dikes is typically determined at the master plan stage. However, where a master plan has not been undertaken, then a design flood will need to be determined in consideration of a number of factors, including economic, environmental and social.

In determination of a design flood for a project, a risk based approach is often adopted, where risk can be represented as: Risk = Likelihood x Consequence

Likelihood refers to the frequency of the flood event, such as a 100 year flood or a 5 year flood. Consequence refers to the implications of the inundation occurring, and may include the economic damages of inundation and potential risk to life. These factors should be weighed up in the determination of a suitable design flood.

Some considerations for the design flood are as follows: 

Where protection of new assets or new developments is to be constructed, then the design flood may be based on the design flood for development of that asset, so as to provide suitable protection. For example, it may be appropriate to adopt a larger flood for urban areas where inundation will cause potential risk to life and property, and a smaller flood for rural land where inundation may not be a key issue.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects





For protection of existing assets, then the design flood may be a level that is achievable considering available space and constraints in installing protection measures. Where a revetment or spur dike is adopted, then the aim may be to alleviate more frequent erosion problems rather than from larger events.

In the absence of a risk assessment or master plan, Table 5-1 provides design floods that can be adopted for different river sizes. Table 5-1

Design Flood - Suggested Protection Levels River Type

5.2.2

Design Flood

Principal and Major Rivers (40km 2 drainage area and above)

100 year

For Small Rivers (below 40km2 drainage area)

50 year

Consideration of Afflux and Impact on Surrounding Areas

Consideration should be made on the potential impact of implementation of various flood control structures on surrounding lands. It is important that, by implementing flood control works and protection one area of land, that another area of land is not significantly impacted as a result. Examples of this include: 



Construction of a levee to protect a village or urban area. The levee removes the floodplain storage, and results in increases in flood levels for a village on the other side of the river.

Construction of spur dikes to protect against erosion issues on the outer bend of a river. As a result, the inner bed starts to erode, placing properties on the inner bend at risk.

Therefore, any assessment of potential flood control works will need to consider changes to the flood behavior in the surrounding areas. Ideally, this should be undertaken as a part of a wider flood control master plan, as defined in FCSEC [2] (2010). However, where this is not available, the following is recommended:   

5.2.3

5.2.4

No more than a 50 mm change in flood levels on urbanized areas, and planned future urban areas No more than 100 mm change in flood levels on rural properties

No change in the design flood immunity of critical infrastructure such as highways and bridges.

Downstream Influences

Downstream influences, as identified in Section 4.11, should be accounted for.

Climate Change

Climate change should be considered as a part of the design and scoping for the project. This is outlined in Section 7.

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5.3

Dike/Embankment/Levee

5.3.1

Basic Concept

A dike is an embankment or levee constructed along the banks of a stream, river, lake or other body of water for the purpose of protecting the landside from overflowing floodwater by confining the stream flow in the regular channel. Internationally, a dike may be referred to as a “dike”, “levee” or “embankment”, and the terms can generally be used interchangeably. It is generally preferred to consider dikes/ levees as a last resort alternative for river improvements. Key considerations for the design of a dike include: 

   

5.3.1.1

5.3.1.2

Difficulty of implementation due to land acquisition (right of way) or existence of important facilities such as ports or harbors. Note that in these cases a concrete retaining wall type dike might be adopted. Maintenance and access considerations.

Potential for breach of the dike, and the subsequent damages as a result. Overtopping of the dike in floods larger than those planned.

Local tributary and drainage problems, and the need to allow for these in the design of the dike.

Types of Levees

Levees can be applied for the protection of urban areas or agricultural areas. In general, given the lower value of agricultural land, and the lower impact from resulting inundation, urban levees tend to be the most common type. Agricultural levees may be used in some situations to reduce more frequent, nuisance flooding that can impact on the use of the land.

Location and Alignment

The alignment and location of a levee should consider:  

    

The alignment should be as straight as possible, as sharp curves are subject to direct attack from flow and should be avoided. Where there is sufficient space, the embankment should not be close to the river banks otherwise it may be damaged due to undermining. If this is unavoidable, then it may need to be constructed together with a revetment.

The embankment should be well away from the estimated meander belt of the meandering river. Valuable tillable land, wells and historical or religious structures should be avoided. Significant environmental assets should be avoided.

When a dike is designed in rural or undeveloped areas, consideration should be made for future development potential of that land. Reduction of the existing stream area shall be avoided as much as possible.

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

5.3.1.3

The new dike shall be designed to protect the affected flood prone areas. In consideration of the stability of the structure, the dike alignment shall avoid unstable peat and muck, weak subsoil, and loose sand foundation to prevent settlement.

Materials

Dikes generally consist of soil and sand. The advantages of using earth materials are:        5.3.1.4

Economical because of the availability of materials. It will last for a long period of time.

It could be easily mixed with the ground materials.

It follows the ground deformation/settlement of foundation.

If the scale of flood control plan is increased in the future, it is easier to improve. If the dike is damaged by flood, earthquake or other inevitable disasters, it is easy to restore. For environmental consideration.

Causes of Dike Damages and Proposed Countermeasures

There are many potential causes of damage and breaching of dikes. A general overview of some potential causes of damage and potential countermeasures are provided in Table 5-2. However, it is important to note that each individual case may be different, and consideration of the specific issues will need to be made. Table 5-2

Causes of Dike Damage and Potential Countermeasures

Causes of Damage

5-5

Countermeasures

Erosion (Scouring)

The surface of the dike on both sides shall be covered with vegetation for protection against erosion. The riverside should be protected with revetment, if necessary. Further details on suitable protection is provided in Section 5.5.

Overflow

Sand bagging is an option for emergencies. If overtopping is expected, then suitable protection will need to be incorporated on the dike to allow for this overtopping. (refer to Section 5.9).

Seepage

Seepage can cause the dike to potentially collapse. Further details for dealing with seepage are provided in Section 5.3.3.

Earthquake

Immediately repair/restoration after the earthquake.

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-1

5.3.2

Design Criteria

5.3.2.1

Parts of Dike

Example Countermeasure against Seepage

An overview of the key components of a dike are provided in Figure 5-2. Figure 5-2

5.3.2.2

Key Components of a Dike

Height and Freeboard

The height of the dike is determined from the design flood level elevation plus an additional freeboard allowance depending on the design discharge as shown in Table 5-3. Freeboard is the margin of height maintained between the top of the embankment and the design flood level to guard against over-topping and wave wash. Table 53 shows the freeboard allowance corresponding to the design flood discharge. Table 5-3

Freeboard Allowance for Dikes

Design flood discharge Q (m3/s)

Freeboard (m)

Less than 200

0.6

200 and less than 500

0.8

500 and less than 2,000

1.0

2,000 and less than 5,000

1.2

5,000 and less than 10,000

1.5

10,000 and over

2.0

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

For levees around lakes and swamps or at the high tide areas where the design discharge is not fixed, the height of the dike shall not be less than the value to be obtained by allowing for wind driven waves. A minimum freeboard in these situations should be 0.6 m. Figure 5-3

Dike Height

For the backwater effect in a tributary, the height of the dike in the transition stretch shall not be lower than that of the main river or even higher at the confluence in order to prevent inundation in the subject areas. In general, the dike’s height of the main river at the confluence point is projected following its design flood level. Note that the alternative is to undertake more complex hydraulic analysis of the combined flows between the main river and the tributary, as discussed in Section 4.11. Figure 5-4

5.3.2.3

Freeboard due to Backwater Effects

Top Width / Crest Width

The crest width of the dike shall be based on the design flood discharge, and shall not be less than the values given in Table 5-4.

When the landside ground level is higher than the design flood level, the crest width shall be a minimum of 3 m regardless of the design flood discharge. Crest width shall be designed for multi-purpose use, such as for patrolling during floods and in the execution of emergency flood prevention works

Adequate widths of the top of the embankment are generally required to serve as a road for facilitating the transport of materials during the construction stage and maintenance operations.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

For backwater effect on the affected tributary, the crest width of the dike shall be designed such that it is not narrower than the dike crest width of the main river. Table 5-4

Recommended Crest Widths for Dikes

Design Flood Discharge, Q (m3/sec)

Crest Width (m)

Less than 500

3

500 and less than 2,000

4

2,000 and less than 5,000

5

5,000 and less than 10,000

6

10,000 and over

7

The levee shall be provided with a maintenance/access road for river maintenance and emergency flood prevention activity during the occurrence of a flood. The width of the maintenance/access road shall be 3 m or more. It shall be constructed near the existing peripheral and/or river side road with entrance facing the downstream side. A built-in stairway is also necessary and should be built strong enough to withstand the expected external forces acting on it.

The dike shall be provided with a maintenance road for patrolling the river during emergency flood prevention activities as well as routine maintenance. The maintenance road shall be 3.0 m or more. The crest of the dike may be used for the maintenance road.

A maintenance road is no longer necessary when a permanent road is to be built and the difference in height between the dike crest and the landside is below 0.6 m. Figure 5-5

Plan and Perspective of Dike Showing the Location of Access Road

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Further considerations for the levee crest: 





5.3.2.4

Levee crests should be constructed with a small cross fall, to shed heavy rainfall. This will help prevent the pooling of water and possible piping damage. If the crest of the embankment is proposed to be used for the dual purpose of a public access road, then extra care will be required for maintenance and repair to ensure that the crest level is maintained.

The crest of the levee should be protected against drying out or cracking by the use of gravel sheeting, or in the case of trafficked urban areas or roadways used as levees, the crest can be sealed. This assists in maintaining the levee at close to optimum moisture content.

Slopes

The slope of the embankment can be dependent on a number of factors such as the soil type, access arrangements, construction methods and maintenance arrangement. Typically, the soil type is one of the driving factors in determining slope. The slopes should consider: 



  

5-9

The side slopes should be gentler on both landside and riverside of the embankment than 1V:2H for low embankments (6.0 m).

A minimum side slope of 1V:4H is typically adopted for embankments consisting of sand and shall be protected by providing a total cover of 300mm thick of a good soil and sodding. On the landward side, steeper slopes can be achieved with crib walls or concrete walls where space may be restricted. A slope gentler than 1V:4H to 1V:5H should be adopted if maintenance and mowing of the surface is required.

A steeper slope may be adopted on the riverside where this is protected by a revetment (refer to design of revetments in Section 5-5).

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-6

5.3.2.5

Example of Crib-Wall used with Restricted Space

Berms

Berms are provided for stability, repair and maintenance purposes. 





On the riverbank side, when the crest height from the river bed is more than 6 m, berms shall be provided at every 3 to 5 m. These should have a width of 1 m or more. On the landward side, when the crest height from the existing ground is more than 4 m, berms shall be provided at every 2 to 3 m in height with a width of 1 m or more. A masonry dike may have a minimum berm width of 1 m when necessary, for stability purposes.

Berms should include swale drains that run parallel to the slope, and aim to reduce the velocity of water running down the slope. These are discussed in more detail in Volume 4: Highway Design. Figure 5-7

5.3.2.6

Arrangement of Berm

Slope Stability

The slope stability should be confirmed and checked against the techniques identified in Section 7.4 of Volume 4: Highway Design.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.3.2.7

Allowance for Settlement

Additional height, or “extra-embankment”, should be incorporated to allow for settlement of the dike over time. The additional height to be allowed for is based on the soil type and should be determined in accordance with appropriately qualified geotechnical engineers. Figure 5-8

5.3.2.8

5.3.2.9

Incorporating Settlement into Design of Levee

Vegetation

The preferred vegetation for levees is grasses. Trees and shrubs should not be planted on or near batters as they increase the potential of risk of failure of the levee due to cracking, piping failure or falling over.

Toe Protection Work (Landside)

When the levee is constructed along the road or the drain, toe protection is required. It should have a height of 0.5-1.0 m and be made of dry stone masonry to secure the drainage in the levee body. Figure 5-4

5.3.2.10

Toe Protection work

Drainage (Cross Drainage)

Levees generally result in the obstruction of overland flow paths and drainage from the landward side to the river. It is important that adequate drainage is incorporated to ensure that there are no adverse local drainage issues caused by the levee. In some situations, these adverse drainage impacts may be worse that the flooding that the levee is intending to protect against.

Pipes or drainage culverts will be required to cross through the levee to relieve local drainage. A discussion on the design of this cross drainage is provided in Section 5.8.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

In order to maintain adequate drainage during a flood event, it may be necessary to incorporate pumps. Further detail on the design of pumps is provided in Section 6.10.

5.3.3

Geotechnical Considerations

5.3.3.1

Scope of Geotechnical Investigation

The scope of a properly prepared geotechnical investigation should be based on the type of dike being proposed or the particular dike rehabilitation or modification being proposed, in conjunction with the geological complexity at the site.

For proposed small-sized embankment dams, determining the feasibility may only necessitate a review of commonly available data, including:     

   5.3.3.2

5.3.3.3

Maps (soil survey, topographic, geological, river survey, aerial photography, etc.). Well and spring data.

Geological surveys or investigation.

Construction records of nearby structures (highway or railroad cuts, building excavations, soil pits, rock quarries, etc.).

The presence of adequate soils-including the identification of dispersive soils, collapsible soils (slaking shales, gravelly materials, etc.) sands susceptible to liquefaction, etc.

The limits and orientation of geological features such as joints, bedding and sheared zones. The potential for surface subsistence.

The potential for landslides at or around the proposed site.

Selection of Materials for Dike

Suitable materials for the Dike should be selected in accordance with the earthworks and fill discussion provided in Volume 4: Highway Design and taking into consideration the DPWH Standard Specifications for Highways, Bridges and Airports (2013).

Classification of Foundation

The ground condition of the proposed dike alignment shall be investigated in consideration of the foundation. Weak and permeable foundations are major issues which should be verified during the preparation and the survey and investigation stages. This should be undertaken by a suitably qualified geotechnical engineer, and taking into consideration the discussions in Volume 4: Highway Design.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.3.3.4

Common Foundation Problems and their Treatments

The preferred solution for most foundation problems, usually incurred by the presence of deficient or unsatisfactory materials, is to remove the unwanted material and, if necessary, replace it with suitable material (fill, concrete, grout, etc.). Sometimes, however, this solution is not cost-effective—sometimes removal of one layer of unsatisfactory material will expose additional or more deficient material. At other times, the solution is simply not feasible. In these cases, an engineered solution is necessary to treat the foundation so that the potential adverse effects are eliminated or minimized to an acceptable standard. Here are some common foundation problems and their respective treatments: 







5.3.3.5

Permeable foundation. This can be alluvial (sandy, silty, or gravelly) foundations or rock foundations with joints, fissures, crevices, permeable strata, fault planes, etc. This condition can result in seepage that causes erosion and/or excessive uplift pressure (both of which are structurally detrimental) or that can be considered an excessive loss from the reservoir (which is an economic or functionality concern, rather than a structural one). Filtered drainage systems (chimney drains, downstream blanket, toe drains, pressure relief wells) can be installed to provide a free flow of seepage and dissipation of water pressure without soil loss or disturbance to the foundation structure. Saturated foundation. A foundation of this kind can be susceptible to sliding due to low shear strengths associated with saturated fine-grained soils as well as sands containing enough fines to be considered impermeable. Proper drainage system during the construction stage shall be installed to adequately drain the foundation. The width of the dam can also be increased to achieve gentler side slopes. This will decrease the normal shear stress along the length of the foundation’s critical slide plane. Highly plastic clay (expansive soil) foundation. This can result in excessive shrinking and swelling, which may cause damage to the dike’s structure. The plastic foundation soil may possibly be treated with hydrated lime. The resultant soil-lime mixture tends to be more granular in texture, which increases its strength and decreases its shrink-swell potential. Weak foundation. This condition can be caused by the presence of cohesionless or dispersive soils, slaking shales, gravelly materials, or clean, saturated sand of very low density. It can result in significant deformation or even shear failure. If highly dispersive soils cannot be effectively removed, provisions for adequate drainage (due to the high susceptibility of erosive piping condition), such as downstream engineered filter (an upstream filter may also be necessary for rapid drawdown condition).

Core & Cut-off Arrangement

The use of a core and/or cut-off arrangement in an earthen bank depends on geotechnical advice and on the nature of the foundation material, as well as the material available for use in the levee. Where geotechnical investigations indicate the presence of sand lenses, it may be necessary to use a core trench to provide a suitable cut-off arrangement for the levee. Where there is a limited availability of 5-13

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5.3.3.6

good quality clay, it may be necessary to use a core, or zoned cross-section, in which the material in the central zone is either selected clay, or treated with gypsum or lime, and placed as specified (Victoria NRE, 2002).

Embankment Zoning

A core or zoned type of earth embankment is constructed from different types of materials to reduce the cost of construction. The main body is made of pervious earth fill which is less costly. In the design of such types of embankments, a provision for a core at its center is made. The primary water barrier (also called core) will have low permeability but, as is typical for such materials, will have relatively low strength. Material for these purposes must be impermeable, free from shrinking and swelling characteristics and resistant to erosion from water intrusion. The best material for these purposes is a well graded gravel with clay (GW-GC), which offers both impermeability and excellent erosion resistance. A clayey gravel (GC) material or a silty gravel (GM) material is necessary.

5.3.3.7

The core covers the entire height of the dam at its center, that is, it extends beyond the height of the water surface in the reservoir. The foundation is based on an impervious stratum, so that seepage below the foundation is prevented, and to check the seepage from the reservoir, a blanket is sometimes also laid out at the upstream face of the embankment.

Stripping

The correct stripping of topsoil from the levee site is critical to ensuring the necessary bonding of the bank with the underlying material. Stripping should be carried out down to clay. Where clay is not within the sub-soil, then alternative measures will be required. This is discussed in USBR Design of Small Dams (1987).

5.3.3.8

Where topsoil is shallow a minimum stripping depth of 300 mm should be adopted to ensure all surface roots and vegetation are removed. If a core/key trench is not being incorporated in the levee, the foundation clay must be ripped and recompacted to remove any further roots from the bank and ensure a good bond. (Victoria NRE, 2002)

Safety against Seepage

During flood, the pore pressures of the dike will increase due to the seepage of the floodwater, which eventually decrease the shearing strength of dike. As a result, the safety of the dike will be decreased. In the evaluation of seepage reduction or seepage control measures, the following shall be reviewed and evaluated: 

 

Protective control measures such as relief wells, weighted graded filters, horizontal drains, or chimney drains which prevent seepage forces from endangering the stability of the downstream slope.

Filters and transition zones designed to prevent movement of soil particles that could clog drains or result in piping. Drainage blankets, chimney drains, and toe drains designed to ensure that they control and safely discharge seepage for all conditions. The designs of these 5-14

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects



  

features must also provide sufficient floe capacity to safely control seepage through potential cracks in the embankment impervious zone. Contacts of seepage control features with the foundation, abutments, embedded structures, etc., designed to prevent the occurrence of piping and/or hydrofracturing of embankment and/or foundation materials. If conduits or pipes exist through the embankment, they should be inspected to ensure that they are functional or have been properly sealed. Grouting, cut-off trenches, and impervious blankets.

Construction records for foundation shaping, treatment and grouting at the contact between the impervious core and foundation.

For existing embankments, all seepage records complied during the existence of the structure should be reviewed for significant trends or abnormal changes.

5.3.4

Specific Applications and Considerations

5.3.4.1

Floodwall

If land acquisition or available space is a major consideration for dike in an urban area or in areas close to important facilities, a floodwall may be an alternative. Specific considerations for the floodwall are:   

The floodwall should not impair scenic views from properties. A desirable height for a floodwall should be less than 0.8 m.

Structural stability of the floodwall will need to be considered.

Figure 5-9

5-15

Self-Standing Retaining Wall (Example)

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-10

5.3.4.2

Parapet Wall (Example)

Dike Affected by Tidal Fluctuation

The dike height affected by high tide (section at which design high-tide level is higher than the design flood level) shall be designed in consideration of the hightide level plus the surge height due to wave action. It is necessary to provide drainage at the dike’s heel in order to collect local runoff and the floodwaters resulting from the wave overtopping action.

5.3.4.3

In such applications, it is recommended that the dike be designed in accordance with the sea wall and revetment design methods outlined in Engineering Standards for Ports & Harbors (Philippine Port Authority, 2009).

Overflow Dike/ Lateral Weir

The dike for special purpose, such as overflow levee, guide levee, separation levee, etc. shall be planned to allow sufficient demonstration of the functions.

The height, length, width, etc. (of overflow levee, guide levee, separation levee, etc.) depend on the place of construction, purpose, etc.; and therefore, must be thoroughly analysed on a case by case basis. In some cases, hydraulic model tests, etc. must be conducted to confirm the appropriateness of the design of each structure. Suitable protection works will need to be incorporated allowing for overtopping of the dike. Refer to Section 5.9.

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Figure 5-11

5.3.4.4

Illustrative Example of Overflow Dike

Retrofitting Improvements

Whenever there is a necessity to heighten/widen the levee on the landside or riverside, the position depends on the alignment. Generally, it is preferred to widen the levee on the landside to prevent reducing capacity of the river. When there is a right of way problem or when there is adequate water way, widening may be applied on the riverside. However, when the toe of the dike is close to the low water channel in case of a compound cross section, it is suggested to avoid widening on the riverside even if there is sufficient river width. Figure 5-12

Widening and Increasing the Height of Dike

Existing

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5.4

Spur Dikes

5.4.1

Basic Concept

Spur dikes are river training structures constructed along the banks of rivers and flood dikes to deflect or repel the flow for the purpose of training the course of the river channel and to protect the banks from scouring by inducing siltation in the area. A spur dike is a river structure with the following functions:  

Increases the flow roughness and reduces the flow velocity around the riverbank. Redirects river flow away from the riverbank.

Corollary to the above functions, installation of spur dikes has the following purposes:

 

Prevents bank erosion and damage to revetment. Deepens water depth for navigation.

Figure 5-13

Example of Spur dikes used to protect outer River Bank

Source: CIRIA, 2007

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Figure 5-14

Example of Spur dikes used with Bridge Design

Source: PNG DoW, 1987

5.4.2

Types of Spur Dike

Spur dikes can be broadly classified into permeable and impermeable/semipermeable. The permeability of spurs is defined simply as the percentage of the spur surface area facing the streamflow that is open. 



5-19

Permeable type – the spur dike of this type is made of piles and frames, preferably in series. Its purpose is to reduce the river flow velocity at the immediate downstream of the spur dike and induce sedimentation. In cases where piles cannot be driven due to the presence of boulders on the riverbed, crib frame, skeleton works or concrete block type shall be used.

Impermeable/semi-permeable type - This type of spur dike is made of masonry (impermeable) or concrete blocks and loose boulder (semi-permeable), preferably in series. Its purpose is to divert the river flow direction away from the riverbank. These types of spur dikes can be further classified:

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

 

Overflow type – the main purpose is to reduce the river flow velocity. This type of spur dike can be considered as a series of spur dikes. Non-overflow type – its main purpose is to change the river flow direction away from the riverbank.

Impermeable spurs provide more positive flow control but cause more scour at the toe of the spur and, when submerged, cause erosion of the streambank. High permeability spurs are suitable for use where only small reductions in flow velocities are necessary as on mild bends but can be used for more positive flow control where it can be assumed that clogging with small debris will occur and bed load transport is large. Permeable spurs may be susceptible to damage from large debris. Figure 5-15

Example Permeable Spur Dike

Source: PNG DoW, 1987

5.4.3

Spur Dikes vs Revetments

The choice between the adoption of spur dikes and revetments is not always straight forward in riverbank protection. Also, in some situations it may be appropriate to adopt a combination of the two, where the spur dike can provide protection for the revetment. Some considerations are provided below. Revetments:  

Typically used where it is intended to protect the riverbank in its existing position.

Where it is necessary to re-instate the riverbank before protecting it, then a revetment may be appropriate.

Spur dike: 



If it is necessary to re-instate the riverbank, but re-instatement is expensive, then a spur dike might be appropriate. This will result in gradual re-alignment of the riverbank.

If the cost of a continual revetment is expensive, then a spur dike, or small spur dikes, may in some situations be more economical. 5-20

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

  

Typically used on wide, shallow rivers, rather than narrow deep channels.

May not be appropriate where the variation in water level from low flow to flood level is large. Can be useful for navigational rivers, where they can assist in defining the navigational channel.

5.4.4

Design Criteria

5.4.4.1

Design Water Level

The design flood level and the ordinary water level during the rainy season shall be considered in the design of a spur dike. These should be indicated on design plans.

5.4.4.2

5.4.4.3

The design water level needs to be calculated based on the hydraulic methods presented in Section 4, and allowing for the reduced cross sectional area presented by the spur dikes. It should generally be assumed that the cross sectional area is the portion of the channel clear of any spur dikes, and that there is no effective flow within the spur dike field.

Design Velocity

The design velocity, used in the sizing of any protection measures for the spur dike, needs to be increased for the local velocity acting on the spur dike. It is recommended to adopt a design velocity of 2 times the cross sectional average velocity for the design of the spur dike (following Maynard, 1978).

Orientation

Permeable retarders are designed to provide flood retardance near the streambank, and this is typically achieved regardless of orientation. Therefore, for construction cost purposes, the cheapest alternative is to typically construct the permeable retarder spur dike perpendicular to the bank.

As identified in HEC23 (2009), there is no clear consensus of orientation of impermeable/ semi-permeable spurs. Spur orientation at approximately 0 degrees (perpendicular)has the effect of forcing the main flow current (thalweg) farther from the concave bank than spurs oriented in an upstream or downstream direction. Therefore, more positive flow control is achieved with spurs oriented approximately normal to the channel bank (HEC23, 2009).

As per HEC23 (2009), it is recommended that the spur furthest upstream be angled downstream to provide a smoother transition of the flow lines near the bank and to minimize scour at the nose of the leading spur. Ideally, this first spur dike should be located upstream of the most severe scouring area, to ensure that it remains during larger flows. Subsequent spurs downstream should generally all be set normal to the bank line to minimize construction costs.

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5.4.4.4

Height

The height of impermeable spur dikes should not exceed the top of the banks. Otherwise, erosion can occur at the overbank end of the spur dike. Where it does not exceed this, the following shall also apply:  

The height of a non-overflow type spur dike should be at the level of the design flood. The height of overflow type spur dike shall be the maximum of:

- 10% to 40% of the distance reckoned from the average riverbed to the

design flood level.

- 0.5 to 1.0 m above the ordinary water level during rainy season.

5.4.4.5

5.4.4.6

Permeable spurs, and in particular those constructed of light wire fence, should be designed to a height that will allow heavy debris to pass over the top. Top Width/ Crest Width

Usually, the top width or crest width of impermeable spur dikes ranges from 1 to 3 m.

Slopes 



5.4.4.7

A spur dike should slope from the bank to the river, to prevent overtopping occurring at a low point on the spur dike. The longitudinal slope of the spurdike should be 1V:20H to 1V:100H toward the center of the river. The side slopes shall depend on the quality of the subsoil, groundwater flow and the type of structure. Slopes are typically between 1V: 1H and 1V:2H on the upstream side and 1V: 1H and 1V:2H on the downstream side.

Length

Spur dikes should have lengths up to 10% to 15%of the width of the river or channel but not to exceed 100 m. The river flow capacity should be examined when the length of the spur dike is more than 10% of the river width (distance of left to right bank); or when the spur dike is to be constructed in a narrow river, since this could affect the opposite bank and considerably reduce the river flow capacity. Figure 5-16

Dimensions of Spur Dike – Impermeable Overflow Type

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5.4.4.8

Spacing

The spacing of spur dikes is related to the length of the spur dike, the angle of the spur dike, permeability and the degree of curvature of the bend.

As a general rule of thumb, the spacing for semi-impermeable (up to 35% permeable) or impermeable spur dikes should be less than 2 times its effective length at flow attack zones and 2 to 4 times at straight sections of channel.

The effective length is the length from the desired bankline to the tip of a spur. Where it is proposed to protect the bankline it its existing position, then the effective length will be the same as the length of the spur. Where the spur dikes are expected to result in an increase in the bankline, then the effective length is the length from the planned bankline to the end of the spur. This is demonstrated by “L” in Figure 5-7. Permeable spurs should be spaced closer together. Based on the procedures identified in HEC23, for a 75% permeable spur, the spacing should be approximate 70% of that for an impermeable spur.

A more detailed procedure for determining the spacing of spur dikes is provided in HEC23, in Design Guidance 2.12, Section 2.2.7. Figure 5-17

Effective Length of a Spur Dike

Source: HEC23, 2009

5.4.4.9

Embedment Depth

For concrete and stone masonry type spur dike, a minimum embedment depth of 0.5 m is recommended.

For gabion-type, boulder type and concrete block type spur dikes, only a provision of about 0.2 m layer of gravel before placement of the main body is sufficient.

Piles supporting permeable structures can also be protected against undermining by driving piling to depths below the estimated scour. Round piling are recommended because they minimize scour at their base. Extending the facing material of permeable spurs below the streambed also significantly reduces scour. If the retarder spur or retarder/deflector spur 5-23

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.4.4.10

5.4.4.11

5.4.4.12

performs as designed, retardance and diversion of the flow within the length of the structure may make it unnecessary to extend the facing material the full depth of anticipated scour except at the nose. Slope Protection

Impermeable spur dikes will require protection of the slope. Furthermore, if the spur dike is expected to overtop during design flows, then the crest will also require protection. Typical protection for spur dikes includes gabions, gabion mattresses, concrete blocks and rip rap. The design of these can be adopted as per revetments, which is detailed in Section 5.5.4.2. Impact Loading

The structural design of any permeable spur dikes will need to be able to resist dynamic and hydraulic loads based on the bankfull condition. An appropriate design debris loading conditions will need to be selected by the designer. The proposed log debris loading condition for bridges in Volume 5 could be adopted as an initial estimate. However, this should be based on review and judgment by the designer. Toe Protection Works

Toe protection should be provided to prevent collapse of the spur dike due to riverbed degradation or scouring. Riprap or gabion can be used for toe protection work. The design of these can be based on the approach for revetments, which is detailed in Section 5.5.6. However, the methods identified in Section 5.5.6 should be adjusted to account for the scour estimated for a transverse structure. This is identified in Annex A.

When the spur dike is not orientated at a right angle to the bank, then Figure 5-18 should be used to adjust the estimated scour depth calculated in Annex A. Figure 5-18

Scour Adjustment for Spur Orientation

Source: HEC23, 2009

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Figure 5-19

5.4.4.13

Toe Protection Works for Spur Dike

Shape of Spur dikes

In general, straight spurs should be used for most bank protection. Straight spurs are more easily installed and maintained and require less material. The shape of permeable spur dike will depend on the material adopted.

For impermeable and semi-permeable spur dikes, they should be straight with a rounded nose, as identified in Figure 5-20. Figure 5-20

Shape of Spur Dike

Source: HEC23, 2009

5.4.4.14

Base Protection

The base of spur dike is the joint to the bank or to the revetment usually prone to damage and outflanking. Therefore, the gap between the base and bank shall be filled up by adequate materials, such as riprap and gabion. 5-25

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5.5

Revetments

5.5.1

Basic Concept

Revetments are flood control structures constructed along river banks subjected to direct attack of the river flow and along levee slopes for protection against erosion, scouring, riverbed degradation and wave wash. They are used in many situations where the riverbank is to be protected in its existing location.

A revetment should be designed based on the existing site conditions, such as river flow velocity and direction, embankment material, topographical, morphological, and geological conditions of the riverbank, etc. Further, the revetment should be designed to withstand the lateral forces due to high velocity flow, when located in flow attack zone, on a weak geological condition of riverbank, and with poor embankment materials. It is important to note that most flexible revetments (riprap, gabion mattress (spread type), concrete blocks) do not provide resistance against geotechnical instability, such as slumping failure in saturated streambanks and embankments (HEC-23, 2009). Typical applications of revetments include:   

Along meander bends of the river, to prevent scouring.

At downstream and upstream of hydraulic and other related structures where turbulent flow usually occurs. Alongside slopes of irrigation canals to prevent loss of water due to percolations.

Figure 5-21

5.5.1.1

Location of Revetment at River Bend

Types of Revetment  

Rigid (concrete slab)

Flexible (riprap, quarry stones)

Revetment may range from rigid to flexible. Concrete slab-on-grade is an example of rigid while riprap and quarry stones are an example of flexible. Rigid revetments tend to be more massive but are generally unable to accommodate 5-26

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5.5.1.2

settlement or adjustments of the underlying materials. Flexible revetments are constructed with lighter individual units that can tolerate varying amounts of displacement and shifting.

Components of a Revetment

The typical components of a revetment include:   

  

  

Slope covering work: directly covers and protects the bank slope from direct attack from flood water, boulders and floating debris. Foundation work: constructed at the toe of the slope that supports the slope covering works.

Foot protection work: constructed to prevent scouring in front of the foundation work and outflow of material from the back of the slope covering work. Shoulder beam work: headwall installed at the shoulder of the revetment to prevent damage.

Backfilling material: materials which are backfilled to the slope covering work to prevent residual water pressure underneath the slope covering work. Filter material/cloth: installed behind the backfilling material to prevent the coming out of fine materials underneath the revetment due to flow forces or the residual water pressure. Crest work: protect the crest of the slope covering work.

Key: installed at the end portion of the crest work to protect it against erosion at the back of the revetment.

Crest protection work: installed at the end portion of the key to join the crest and the original ground in order to protect against erosion at the back of the revetment.

The components of revetment are illustrated in Figure 5-22 and Figure 5-23. Figure 5-22

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Components of a Revetment

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-23

5.5.1.3

Components of a Revetment Cross-Section

Planning & Considerations

During planning and design stage, the following are some general considerations:  





 





Alignment of revetment shall be as smooth as possible, preferably following the alignment of the existing bank.

Where revetments are used to provide scour or erosion protection, they should be designed to have as little impact on hydraulic performance of the river as possible.

Where the rate of erosion is unpredictable, or future erosion is expected, it may be suitable to set the revetment back from the edge of the river. However, it is important that the toe of the revetment is designed to accommodate future movement of the river. The type of the revetment shall be determined based on the estimated external forces (velocity of flood flow) and the characteristics of river, as well as economic and environmental aspects of alternative options. This should be undertaken early on within the design process. Foot protection works shall be considered based on external forces.

Transition structure (end protection works) of the revetment to the original bank shall be provided. The end of the revetment should run as smoothly as possible into the natural channel to avoid scouring and turbulence.

The revetment should start at a stable, fixed point on the bank and continues downstream to another stable location or to some point below which the river can safely be left uncontrolled. On a meandering river, the revetment will effectively stop the protected bend from migrating. This may have subsequent impacts outside of the protected 5-28

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5.5.2

bend as the rest of the meandering river changes to accommodate this. Therefore, revetments cannot be considered in isolation.

Estimation of Design Velocity

The design velocity is the effective velocity acting on the revetment, and is not equal to the average cross sectional velocity as determined in Section 4.

The cross section average velocity should first be estimated using the procedures as outlined in Section 4. Note that this should be assessed at all cross sections along the revetment. The highest velocity should be adopted in most cases, as for construction purposes it is simpler to adopt a uniform protection measure.

The cross section average velocity then needs to be adjusted to the design velocity, which represents a point approximate 20% up the slope from the toe of the revetment. The following provides a simplified relationship for estimating this, based on HEC23 (2009). Equation 5-1

where: Vdes

Vavg

α

=

= =

𝑉𝑉𝑑𝑑𝑑𝑑𝑑𝑑 =∝ 𝑉𝑉𝑎𝑎𝑎𝑎𝑎𝑎

design velocity

cross section average velocity

velocity adjustment factor, which can be determined based on:

For natural channels:

∝= 1.74 − 0.52log(

For trapezoidal channels:

∝= 1 for RC/W > 26

∝= 1.71 − 0.78log( where: RC

W

∝= 1 for RC/W > 8

= =

radius of bend

width of river/ channel

𝑅𝑅𝐶𝐶⁄ 𝑊𝑊 )

𝑅𝑅𝐶𝐶⁄ 𝑊𝑊 )

The velocity adjustment factor for natural channels is also provided in Figure 524.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-24

Velocity Adjustment Factor

1.8 1.7

Velocity Factor (α)

1.6 1.5 1.4 1.3 1.2 1.1 1

0

5

10

15

20

25

30

Rc/W

5.5.3

Slope Protection Works

There are many types of slope covering work, with some of these shown in Table 5-5. It provides an indication of typical constraints and considerations, but certain slope protection works may be applicable outside of the ranges indicated. The type of slope covering work at the site shall be selected based on the design velocity, slope, availability of construction materials near the site, ease of construction works and economy, etc. When there are constraints due to the required boulder stones during flood and the slope of the bank, a combination of the slope covering works shall be considered.

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Table 5-5

Overview of Different Slope Protection Works & Considerations

Indicative Maximum Design Velocity (m/s)

Slope (V:H)

Remarks

1. Sodded Riverbank with Pile Fence

2.0

Milder than 1:2

Not applicable for places near roads and houses Diameter and length of wooden pile shall be determined considering past construction records. Note that this is not a common technique used for revetments.

2. Dry Boulder Riprap

3.0 to 4.0

Milder than 1:2

Diameter of boulder shall be determined using Table 5-7. Height of generally less than 3 to 5 m.

3. Grouted Riprap (Spread Type)

5.0

Milder than 1:1.5

Use Class “A” boulders for grouted riprap and loose boulder apron.

4. Grouted Riprap (Wall Type)

5.0

1:1.5 to 1:0.5

Use class “A” boulder for grouted riprap.

5. Gabion (Mattress or Spread Type)

5.0

Milder than 1:1.5

Not advisable in rivers affected by saline water intrusion. Not applicable in rivers where diameter of boulders present is greater than 20 cm.

6. Gabion (Pile-up type) – Gabion Wall

6.5

1:1.5 to 1:0.5

Not advisable in rivers affected by saline water intrusion. Not applicable in rivers where diameter of boulders present is greater than 20 cm.

7. Rubble Concrete (spread type)

Milder than 1:1.5

8. Rubble Concrete (Wall Type) 9. Reinforced Concrete

Minimum thickness of 20cm

10. Gravity Wall 11. Sheet Pile

Vertical

In cases where ordinary water level is very high.

12. Vegetation and Reinforced Grass/ TRM

Milder than 1:4

Typically for the upper section of the protection, where the velocities of flow are lower. Should be located above the ordinary water level to ensure only irregular inundation. Refer to Section 5.5.3.7.

Figure 5-25

5-31

Sodding with Grass or Some Other Plans (Natural Type)

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-26

Wooden Pile Fence

Figure 5-27

Dry Boulder Rip Rap

Figure 5-28

Grouted Rip Rap, Spread Type

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5-33

Figure 5-29

Grouted Riprap, Wall Type

Figure 5-30

Gabion Mattress, Spread Type

Figure 5-31

Gabion Mattress (Gabion Wall), Pile-up Type

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-32

Rubble Concrete, Spread Type

Figure 5-33

Rubble Concrete, Wall Type

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5-35

Figure 5-34

Reinforced Concrete

Figure 5-35

Gravity Wall

Figure 5-36

Sheet Pile and Reinforced Concrete (Segment Combination)

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.5.3.1

Dry Boulder (Rip Rap) Sizing

The design of dry boulder revetments requires consideration of the following: Sufficient rock size for the expected velocities (refer Table 5-7)



Adequate grading of the rock to minimize voids within the protective layer



A filter layer is typically provided, as described in Section 5.5.9.3



Appropriate toe protection is provided, as described in Section 5.5.6



There are numerous methods for estimation of rock size for a dry boulder (riprap) revetment. The following equation is recommended for its simplicity and ease of application, while generally providing a relatively conservative estimate of the D50 required (derived based on PNG DoW (1987) and Maynard (1978)). A minimum diameter of 200mm should be adopted for any design. Note that this equation assumes a specific gravity of the rock that is adopted for the riprap of 2.6.

D50 values estimated using this equation should be rounded to the nearest 50 mm. Equation 5-2

where: D50

𝐷𝐷50 = 𝑦𝑦𝑦𝑦𝐹𝐹 3 =

median particle size of the rip rap in meters.

=

Froude Number (refer Section 4.5.1.9). Note that the velocity to be adopted is the design velocity, as discussed in Section 5.5.2.

y

=

g

=

F C

=

Table 5-6

design flow depth in meters. This is typically taken at the base of the toe of the revetment. acceleration due to gravity

coefficient selected from Table 5-6. Note that a safety factor of 1.5 can be generally adopted, unless there are specific uncertainties in flow estimation or site conditions.

Coefficient for Riprap Design

Side Slope of Bank

Factor of Safety

Coefficient (C)

1V:3H or less

1.5

0.25

1V:3H or less

2.0

0.28

1V:2H

1.5

0.30

1V:2H

2.0

0.32

For ease of use, Table 5-7 provides the D50 for different velocities and depths of flow, based on a revetment slope of 1V:2H. Minimum specifications for riprap are provided in Section 9.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 5-7

Dry Boulder Rip Rap Sizing (D50 in mm) Design Velocity (m/s)*

Depth (m)

1

2

3

4

5

1

200

200

300

650

N/A

2

200

200

200

450

900

3

200

200

200

400

700

4

200

200

200

300

650

5

200

200

200

300

550

*refer to Section 5.5.2 for estimation of design velocity

5.5.3.2

Gabion Mattress – Spread Type

Key considerations for a gabion mattress (spread type, also known as reno mattress):  

5.5.3.3

Correct rock size and mattress thickness for the expected velocities Provision of a filter layer

The design of gabion mattress should be undertaken in accordance with manufacturer specifications.

Gabion Mattress (Gabion Wall)

Gabion walls must be checked for stability against earth pressures, as they are designed as mass gravity structures. Stability must be checked for resistance to failure against:   

Overturning of the wall Sliding

Bearing or foundation failure

An overview of the types of forces acting on a gabion wall is provided in Figure 537. The design of a gabion wall is generally well documented in manufacturer guidelines, and reference should be made to these in the design process.

Filters are generally required for gabion mattresses.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-37

5.5.3.4

Typical Forces Acting on a Gabion Wall

Reinforced Concrete Wall/ Slope Protection

A reinforced concrete wall is designed as a retaining wall to resist earth pressures. Key considerations for the design include:  

Depth and foundation requirements required to resist overturning Strength and thickness of wall. As a general guide:

- Typical thicknesses of concrete are a minimum of 150 to 200 mm



 

5.5.3.5

- Typical reinforcement bar or mesh diameter is a minimum of 12.7 mm

Contraction and expansion joints to minimize the risk of cracking and seepage and potential undermining. Vertical expansion joints should run normal to the bank slope. Joints should be constructed careful to ensure that no protrusions into the flow are present, as these may result in the undermining of the structure. Expansion joints should be provided with a waterstop, smooth dowels, sponge rubber filler, and sealant. Allowance for scour, by either extending the wall below the maximum scour depth (refer to Section 5.5.5) or in combination with a sheet pile wall

Special consideration for uplift forces will be needed if the revetment is in a hydraulic jump zone. This will require specialist engineering input

The maximum average flow velocity of 4 – 5 m/s is recommended for concrete revetments.

Sheet Pile Wall

A sheet pile wall must be designed to withstand the earth pressures. In the design, it should be assumed that there is no water on the river side. Key considerations are:  

Generally the sheet pile wall will require anchoring back to the bank.

Commonly sheet pile walls are used in combination with another revetment protection type. In these cases, the revetment provides protection to the bottom portion of the revetment. 5-38

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

   5.5.3.6

The sheet pile wall should extend to below the maximum scouring depth, as identified in Section 5.5.5. Toe protection is not suitable for sheet pile walls. The depth of the sheet pile required should also be based on stability. Weep holes must be provided, as described in Section 5.5.9.1.

Concrete Units

Concrete unit style revetments can either be orderly placed concrete units, or random placed units. They rely on the mass of the concrete units, and the interlocking of the units, to protect the underlying soil from erosion and scour.

The sizing of concrete units can be based on Figure 5-46, and adjusted in accordance with Equation 5-3. Equation 5-3

where:

𝑀𝑀1 (𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 − 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠)3

M

=

mass of concrete unit for slope protection (kg)

θ

=

slope angle of the revetment

M1 5.5.3.7

𝑀𝑀 =

=

mass of the concrete unit based on Figure 5-46 (kg)

Vegetation and Reinforced Grass/ TRM

Vegetation can be a suitable technique for managing the upper bank, and can be used in conjunction with any of the techniques identified above. It is also possible to use reinforced grass or TRM (refer to Section 6.3.2.3– examples include products such as coconet and polypropylene meshes) to provide additional velocity resistance.

The use of vegetation usually occurs in areas outside of the ordinary water level, to ensure that it is only inundated irregularly. Typically, vegetation is used on the top 1/3 of the bank, but this is dependent on the application and the velocities. Below this level other protection measures, such as gabion mattress or rip rap, are used. An example of this type of application is provided in Figure 5-25, with a combination of vegetation and wooden pile fence. An example with rip rap is provided in Figure 5-38. The velocity resistance of vegetation is discussed further in Section 6.3.2.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-38

Example Vegetated Bank Protection

5.5.4

General Design Criteria

5.5.4.1

Height and Freeboard

When a revetment is design to confine flood flows, then a freeboard equivalent to a dike should be adopted. When a revetment is considered in conjunction with another flood control work, such as a levee, then the height of that structure should be considered together with the revetment.

When the revetment is intended to protect against more frequent flow events (such as to control erosion around a bend in up to a 5 year flood), a freeboard of 0.6 m should be adopted above the level of the flood event to be adopted. Figure 5-39

Height of Revetment

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.5.4.2

Slope

After the determination of height of the slope covering work, the slope shall be planned based on the following: 



 

5.5.4.3

The slope of the revetment will generally be the same as those of a dike (refer to Section 5.3) at 1V:2H (vertical and horizontal, respectively) or milder. In the case when the slope of revetment should be steeper than a dike, the slope should aim to be as gentle as possible for stability purposes and should aim to align with the natural slope of the adjacent bank. In case of rapid flow stretches wherein floodwater contains a large quantity of boulders or gravels, the slope shall not be necessarily gentle but shall be milder than 1V:0.5H.

In case of joint portion with a rock-strewn slope, the slope of revetment shall be gradually changed to smoothly connect with the natural slope.

For the retaining wall type revetment (reinforced concrete, gravity wall, rubble concrete etc.), a maximum slope of 1V:0.3H shall be observed considering stability and the resulting residual hydraulic pressure.

Berms 



If the height of revetment is more than 5.0 m, a berm must be provided in order to separate the revetments into segments. This should be identified considering site conditions as well.

A berm is provided for stability, maintenance and construction. The berm should be at least 1 m in width. Where it is provided for a dike, then the width will need to be 1 m.

Figure 5-40

5.5.4.4

Provision of a Berm in a Revetment

Segment Length

For rigid revetments, the length of one segment of revetment along the longitudinal direction should not be more than 50 m in order to prevent damage on the adjoining section of the revetment once it collapses. Edge of the segment shall be provided with end protection and adequately filled with joint filler or sealer to connect with the adjoining section. 5-41

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.5.4.5

Thickness

The thickness of revetment is generally based on the flow velocity, sediment runoff, topography, geological conditions, scouring and degradation, soil and groundwater pressure at the back of revetment and other factors. Minimum thickness should be 300 mm for all types of revetment, except for reinforced concrete type.

5.5.5

Further details on the design of the protection measures are provided in Section 5.5.3.

Depth of Foundation

There are two options for the depth of the top of the foundation for a revetment:

1. The depth of the foundation is below the maximum scouring depth, which is

generally the preferred approach. Refer to Section 5.5.5.1.

2. The depth of the top of the foundation is above the maximum scouring depth,

5.5.5.1

and therefore additional works are required. This should only be done where there is a difficult in achieving point 1 above due to large scouring or riverbed degradation. This is typically only undertaken for flexible revetment types. Refer to Section 5.5.5.2.

Depth of Foundation Below Maximum Scouring Depth

The depth of the foundation shall be deeper than 1 m from the maximum scouring depth. The maximum scouring depth can be estimated based on the procedures identified in Annex A. The top elevation of the foundation work is determined as follows:

1. Plot the 1 m elevations below the maximum scouring level/deepest riverbed

level from the cross-sections and project in the longitudinal profile.

2. Draw the line of the lowest elevation from 1 with the same longitudinal

gradient of the top of slope covering work.

Figure 5-41

Depth of Foundation

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.5.5.2

Depth of Foundation Above Maximum Scouring Depth

The following alternatives can be considered when the depth of the foundation is above the maximum scouring depth:

i.

The top elevation of the foundation work is set at the maximum scouring depth, and the minimum foot protection work shall be installed.

ii. The top elevation of the foundation is set above the maximum scouring depth,

and a flexible foot protection shall be installed to cope with the scouring.

iii. The top elevation of the foundation is set above the maximum scouring depth,

and the foundation work by sheet pile and the foot protection shall be applied in order to cope with scouring.

iv. In cases it is difficult to have adequate depth of embedment for the foundation

work, such as high ordinary water level, tidal river, etc.; cantilever sheet pile shall be installed as foundation work.

These alternatives are shown in Figure 5-42. For cases ii and iii, the top elevation of the foundation work shall be set at 0.5-1.5 m deeper than the average riverbed level. Foot protection work is detailed in Section 5.5.6. Figure 5-42

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Foundation Work

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.5.6

Foot Protection Works

5.5.6.1

Basic Concept

Foot protection works shall be adequately placed in front of the revetment foundation to prevent scouring. Foot protection works are required in cases i, ii and iii as identified in Section 5.5.5.2.

5.5.6.2

The foot protection shall have a minimum width of 2 m towards the centerline of stream.

Types of Protection Works

The type of foot protection work shall be determined based on river conditions, convenience in construction, economy, etc. The basic requirements for the foot protection work are as follows:    

Sufficient weight against the flow forces

Sufficient width to prevent scouring in front of the revetment Durability

Flexibility for the fluctuation of riverbed

Typical types of foot protection works are Figure 5-43. Figure 5-43

Types of Foot Protection Works

Riprap Type

The diameter (D50) of the boulder to be used should be based on Table 5-8. These values are based the simplified equation that is presented in Section 6.5.7.2. It assumes a specific gravity of 2.6. Maintenance is required to ensure that significant movement of rock over time is rectified. Minimum specifications for riprap are provided in Section 5.9.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 5-8

Minimum Diameter of Boulder (Riprap Type) Design Velocity (m/s)

Diameter (mm)

2

200

3

350

4

600

5

950

6

1450

Gabion Type

This type shall not be used for rivers with saline water intrusion and for rivers with riverbed and banks consisting of boulders. The gabions shall be connected to each other. Gabions and gabion mattresses should be designed in accordance with manufacturer specifications. Indicative velocity limits for preliminary sizing are provided in Table 5.9. Note that the critical velocity is the velocity where the mattress reaches the limit of deformation. Mattresses and gabions should be designed in accordance with the critical velocity. Table 5-9 Type

Indicative Velocity Limits for Gabions and Gabion Mattress

Thickness (mm)

Gabion Mattress

Gabions

Rock Fill Size (mm)

D50 (mm)

Critical Velocity (m/s)

Limiting Velocity (m/s)

150

70-100

85

3.5

4.2

180

70-150

110

4.2

4.5

230

70-100

85

3.6

5.5

250

70-150

120

4.5

6.1

300

70-120

100

4.2

5.5

100-150

125

5.0

6.4

100-200

150

5.8

7.6

120-250

190

6.4

8.0

500

Source: DTMR, 2013

Concrete Block Type

Concrete block protection consists of two main types:  

Orderly pile up type

Random pile up type

These are demonstrated in Figure 5-44 and Figure 5-45.

The weight of the concrete block can be estimated based on Figure 5-46 (where orderly pile up types of units represent those that are “embedded” while disorderly or random pile up types represent exposed units). This is based on the equations presented in PPA (2009), which are derived from work undertaken by US Army Coastal Engineering Centre.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-44

Concrete Block Type - Orderly Pile Up - Single Unit

Figure 5-45

Concrete Block Type –Orderly and Random Types

Figure 5-46

Weight of Concrete Block

100000

Weight of Blocks (kg)

10000

1000

100

10

1

1

10 Design Velocity (m/s) Embedded Concrete Units

Exposed Concrete Units

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.5.6.3

Top Elevation of the Foot Protection Work

The top elevation of foot protection work shall be at the same elevation as the top of the foundation work of the revetment.

5.5.6.4

In order to prevent scouring, the top elevation of foot protection work may sometimes be set above the top of foundation work of the revetment. When the thickness of the foot protection work is more than 1 m, the bottom elevation of the foot protection work shall be set at the same elevation with the bottom of the foundation work.

Width of Foot Protection Work

The foot protection work requires sufficient width that will prevent scouring of riverbed in front of the foundation work of the revetment.

The foot protection work shall consider width of at least 2 m in front of the revetment after the scouring. The required width of the foot protection work (B) can be calculated from Equation 5-4. This is represented in Figure 5-47. Equation 5-4

𝐵𝐵 = 𝐿𝐿𝐿𝐿 + where:

∆𝑍𝑍 𝑠𝑠𝑠𝑠𝑠𝑠∅

Ln Φ

=

width in front of revetment (at least 2 m)

∆Z

=

height between the foot protection work and the scoured bed level.

Figure 5-47

=

slope of scour (can generally assume 30 degrees)

Width of Foot Protection Required

An alternative to the above approach is the adoption of the mounded toe approach or falling apron. For a flexible rip rap solution, the toe above the maximum anticipated scour level, but with a flexible toe protection that can respond to scour and thereby protect the revetment from being undermined. This is typically achieve through rip rap, as solutions like gabion mattresses may be less flexible

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

and unable to respond to local scour. This solution is demonstrated in Figure 548. The riprap mound height should be equivalent to ∆Z (maximum scour expected). As scouring occurs at the base of the revetment, the rock will in fill the scour hole and provide the appropriate protection. This falling apron approach is typically only successful in coarse grained rivers (sand and above). Figure 5-48

Riprap Revetment with Mounded Toe Approach

Source: HEC-23, 2009

5.5.7

End Protection Works

Revetments should be provided with end protection works to prevent scouring at the upstream and downstream ends. The scouring causes the escape of backfill materials resulting to the gradual damage of the revetment.

The end protection work is indispensable to the rigid structure type revetments. These types of structure are particularly susceptible to undermining and scouring of backfill materials:  

The end protection shall cover the extent of the covering work and crest work.

The thickness of the end protection work shall be from the surface of revetment up to the backfill material. The thickness of the end protection shall be more than 50 cm.

A flexible transition structure like gabions/boulders should be provided on both ends of the revetment. A key should also be tied back to the bank, as indicated in Figure 5-49. The transition from the existing river bank to the revetment should be as smooth as possible. It may be necessary to undertake works to transition from the existing slope of the riverbank to the slope of the revetment, where it is not possible to match the slope of the revetment to the existing riverbank.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 5-49

5.5.8

End Protection Works

Protection of Revetment Crest

When the design flood event adopted for the height of the revetment is low (such as in a non-diked river), the overflow frequency of the revetment can be high. In these situations it is necessary to protect the crest of the revetment (Figure 5-50). Revetments are particularly susceptible from damage from overtopping flows. For the design of the crest works:  

The width of the crest shall be more than 1 m

The thickness of the crest end work shall be more than 0.5 m.

Figure 5-50

Crest Protection

5.5.9

Other Design Considerations

5.5.9.1

Drainage Pipe/ Weephole

Drainage pipes/weep holes shall be designed and provided for both types of revetment for diked and non-diked rivers. During flood times, the rise of flood water level in the river almost coincides with the rise of groundwater behind the revetment especially when the ground is already saturated. After the floods, the 5-49

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

rate of subsidence of floodwater in the river is usually greater than the recession of groundwater level behind the revetment without drainage pipes/weep holes. If the disparity between the subsiding floodwater and groundwater stages is significantly high, residual hydraulic pressure exists at the back of the revetment (refer to Figure 5-51). Weep holes shall be provided in the revetment using 50~75 mm diameter PVC drainpipes, placed in stagger horizontal direction and spaced 2 m center to center.

One of the main causes of caving in of soil particles behind the revetment is the outflow of backfill fine materials through the joints of revetment and weep holes, which eventually leads to the collapse of the revetment (refer to Figure 5-52). Pervious materials consisting of crushed gravel or geo-textile filters are to be placed between the revetment and original ground to prevent the outflow of the bank materials through the weep holes. The lowest weep holes shall be installed just above the ordinary water level.

5.5.9.2

Figure 5-51

Development of Residual Hydraulic Pressure without Drainage Pipes/ Weep Holes

Figure 5-52

The Need for Filter Cloth/ Gravel

Backfill Material

In situations where backfill is used to reclaim land, it is important that the backfill is as close to the natural material of the riverbank as possible to avoid significant changes of drainage characteristics.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

For backfill immediately adjacent to the revetment: 

  

5.5.9.3

5.5.9.4

5.5.9.5

For rigid type revetment, backfilling materials shall be installed in order to reduce the residual water pressure to the covering work (refer to Section 5.5.9.1) and to fix the covering work to the original bank slope.

For permeable type revetment such as wooden fence type and gabion mattress type, the backfilling materials shall not be installed. The backfilling materials shall be with high permeability, such as crushed gravel etc.

Thickness of the backfilling materials shall be 30-40 cm for wall type and 1520 cm for pitching or lining type.

Filter Layer

For flexible revetments such as (dry boulder or riprap and gabions), a filter layer will generally be required to prevent material being washed from the bank through to the river. Filters should have two key characteristics (PNG DoW, 1987), stability and permeability. It must be fine enough to prevent the base material from escaping through the filter, but it must be more permeable than the bank material (PNG DoW, 1987). Outflow Prevention Materials

Outflow prevention materials (e.g. filter cloth, geotextile) shall be installed behind the permeable revetment types.

Bridge Site and Tributary Confluence

At the upstream and downstream portions of the bridge, sluice gate and culvert, weir, groundsill and confluence of rivers, the river flow is constricted by the presence of these structures, which changes the river conditions. It is, therefore, necessary to provide adequate length of revetment in these areas to prevent bank erosion due to the adverse effects of constricted river flow.

5.5.9.6

5.5.10

It is also important to note that the computation of the scour under these conditions is different to a scenario where no constriction exists.

Construction Joints

Construction joints should be minimized along a revetment. Any construction joints should be adequately strengthened.

Main Causes of Revetment Damage

Understanding of the main types of damages to revetments can assist in designing a stable revetment. The following provides an overview of some typical failure mechanisms for revetments.

5-51

The scouring at riverbed along foundation of revetment is one of the main causes of revetment damages.

Particle(s)/block(s) (e.g., dry boulder riprap) of revetment are detached by strong velocity flow.

Movement/ Extraction of Particle/ Block Caused by High Velocity Flow

Description

Potential Failure Mechanisms for Revetments

Local Scouring and Riverbed Degradation

Failure Mechanism

Table 5-10

Insufficient toe support resulting in gaps in protective blockwork (source : Mott MacDonald http://www.geotechnics.mottmac.com/projects/portsmouthharbourrev/)

Failure of Revetment from undermining (source: Gold Coast Barges, http://www.gcbarges.com.au/waterway-revetment-walls.htm )

Example Figure

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

When the floodwater level is receding, residual water pressure of the remaining groundwater at the back of the revetment may create piping. In case of steep slope revetment, the residual water pressure and earth pressure causes the revetment to collapse.

Residual Water Pressure

5-53

The fine materials behind the revetment are sucked out from the crevice/weep hole of revetment.

Description

Outflow of fine materials from behind the revetment

Damage at the end section due to direct water attack and scouring

Failure Mechanism

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Example Figure

When the floods overtops the revetment and flows back to the river, the back portion of the top of revetment might be damaged.

Logs, rocks and debris carried by strong river flow directly hit the revetment resulting in damages. For gabion style revetments, for example, this may result in puncture of the basket or mattress.

Direct hit by big boulders, logs and/or debris

Description

Erosion on Top of the Revetment

Failure Mechanism

Example Figure

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.6

Groundsill

5.6.1

Definition

Groundsills (also termed check dams) are drop structures located within a channel, commonly used downstream of culverts to prevent head cutting from discharge flows and maintain a consistent streambed profile in the vicinity of the culvert. A typical groundsill layout is shown in Figure 5-53. Figure 5-53

Typical Groundsill Layout

5.6.2

Groundsill Components

5.6.2.1

Main Structure

The main structure of the groundsill is the structure that provides the drop in elevation of the channel base.

The main structure may be constructed of rock riprap, concrete, sheet piles, gabions or treated timber. Riprap and treated timber groundsill have been found to most effective for channels have small drops and relatively narrow widths (up to 30 m).

Gabions, sheet piling and concrete are generally used for larger drops or on wider channel sections. They have been successfully used on channel sections up to 100 m wide.

5.6.2.2

If the design requires a large change in elevation, both a single large structure and multiple small structures should be investigated. A string of small structures may be found to result in reduced erosion potential and cheaper construction costs, if they allow for the use of local materials.

Apron

The apron is provided immediately downstream of the groundsill drop structure to prevent scouring of the downstream channel and undercutting of the groundsill structure. 5-55

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

5.6.2.3

The length and type of apron will be dependent on the flow conditions. A discussion on the design of the apron is presented in FHA (2009).

Channel Protection

Channel bed protection should be provided both upstream and downstream of the groundsill structure, in order to prevent erosion from overtopping flows and the turbulence at the drop structure. In addition to providing protection to the channel bed, it is often necessary to protect the adjacent channel slopes. Groundsills can result in the lateral erosion of channel banks caused by the turbulence produced by energy dissipation at the drop or eddy action at the banks. If this occurs, the erosion may progress upstream, potentially leading to a failure of the groundsill.

5.6.2.4

It is recommended that revetment protection (Section 5.5) be provided along the channel banks immediately upstream and downstream of the groundsill to prevent bank erosion.

Fish Way

A fish way may be required if the river is used for fish migration. As groundsills may interrupt the upstream movement of fishes and other aquatic species, and alternative path for these species should be provided.

The selection and design of fish way options are discussed in Design of Small Dams (USBR, 1987).

5.6.3

Design Criteria

5.6.3.1

Location

Groundsills are typically located downstream of culverts to prevent erosion for discharge flows, or upstream of channel structures (such as bridge piers) to stabilize the channel profile and reduce the risk of erosion of the structure foundations.

5.6.3.2

While proximity to the channel structure they are used to manage is required, they should ideally be placed in straight, constant sections of the channel in order to operate most effectively. Locations on bends, or at changes in channel sections or slopes should be avoided where possible.

Height

It is generally best to keep the height of the groundsill smaller. If a greater height drop is needed, then it is better to separate the groundsill into a number of steps spaced well apart.

Generally, a drop in the order of 2 m or less is recommended, as greater drops will require more dissipation works on the downstream side. However, there will be situations where this is unavoidable.

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5.6.3.3

Alignment

Groundsills should be constructed as straight structures perpendicular to the channel wherever possible, as shown in illustration ‘a’, Figure 5-54. This alignment is typically cheaper to construct, and causes fewer erosion and flooding issues compared against the other alignments shown in Figure 5-54. The alignment shown in illustration ‘b’ results in channel flow being directed against one of the river banks which increases the erosion risk and results in the need for additional protection requirements.

The polygonal and circular groundsill plans shown in illustrations ‘c’ and ‘d’ respectively, direct overtopping flows to the center of the channel. This increases the erosion potential in the channel center and can quickly result in deep scouring occurring if the protection provided is insufficient or experiences failure in a large flood event. Figure 5-54

5.6.3.4

5.7 5.7.1

Groundsill Locations

Scour

The check dam must be designed structurally to withstand the forces of water and soil assuming that a scour hole, is as deep as estimated from Annex A, in order to ensure successful operation over the design life of the groundsill.

Small Dams

Definition

This Guide covers the design of small dams. The definition of a small dam is based on the International Commission on Large Dams (ICOLD) definition. These are any dams that are less than 15 m in height, and which do not fit into the category of large dams as defined by ICOLD (refer to Table 5-11). Table 5-11

ICOLD Definition of a Large Dam

Height (m)*

Length (m)

Volume (m3)

Flood Discharge (m3/s)

Type of Foundation

>15

Any

Any

Any

Any

10 to 15

500

>1 million

>2,000

Unusual

*measured from the foundation to the crest of the spillway. If the spillway has multiple crests (a low flow and high flow crest for example) then the height is measured to the highest crest.

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5.7.2

Purpose of Small Dams

Small dams are hydraulic structures that may be constructed to meet a variety of needs, including:     

Flood control and flood mitigation

Creation of fresh water storage for the provision of clean water Generation of power through hydro-electric schemes Creation of recreation areas, or A combination of the above

5.7.3

Classification of Small Dams

5.7.3.1

Usage Classification

Typical usage classifications are:   

5.7.3.2

Storage – for dams that are designed to permanently retain water, whether for water supply, power, irrigation or recreation. Diversion – for dams that are constructed to divert water into conveyance systems. Commonly used in agriculture to direct water to irrigation canals.

Detention – for dams that retard water during flood events to reduce the flood impacts.

Hydraulic Design Classification

There are two broad hydraulic classifications:  

5.7.3.3

Overflow – these dams are designed to convey water over the dam crest. These dams are typically constructed of concrete.

Non-overflow – these dams are not designed to overtop, but rather to simply store / detain water that will then be removed for usage. As they are not designed to overtop, they are commonly constructed of earth or rock. It is noted that an emergency spillway will still be required.

Material Classification

There are three principal materials used in the construction of small dams; concrete, rock and earth. 



Earthen dams – are the most common type of dam, as they may be constructed from locally available material, that requires little processing. Also, the foundation and topographical requirements for earthfill dams are less stringent than those for other types. Rock dams – are constructed of rock of various sizes to provide stability, and an impervious membrane to prevent water passing through the dam. This impervious membrane may be constructed of soil, concrete, metal sheeting, or other available impervious materials. Rock dams are well suited to remote

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

locations where high rainfall or scarce soils prevents an earthen dam being constructed, or where concrete dams are too costly. Concrete dams – are constructed of concrete that may be reinforced as required. Their durability makes them well suited to overflow dams. Concrete spillways are also commonly constructed on earth or rock dams where high flows may be experienced, in order to protect from dam failure.

5.7.4

Design Considerations

5.7.4.1

Location

The location of the dam should be carefully selected to ensure that is it appropriate. This is highly dependent on local conditions, but the following may serve as a starting point in determining an appropriate location:  





5.7.4.2

Dams should be located at existing local constrictions if possible. This will reduce the amount of construction required to form the dam.

Proximity to existing development should be considered. It is not ideal to build dams above existing development as it puts the development at risk in the case of dam failure. If the dam is to supply fresh water, it may be desirable to locate it remotely in order to help prevent contamination. The impact of the dam on downstream flow conditions should be assessed. A dam location may be unsuitable if it prevents existing downstream water uses, such as extraction for irrigation, from continuing.

The extent of the reservoir should be assessed to ensure that it does not impact existing communities or infrastructure. This may influence the height of the dam wall adopted.

Material

A range of materials are available for the construction of small dams. Typical materials are concrete, rock and earth. The selection of the dam material will be influenced by the proposed use of the dam, as well local conditions.

5.7.4.3

Thought should also be given to what materials are available locally, and what materials local contractors have a familiarity in working with. This will aid in reducing construction costs, and ensuring the dam is correctly constructed. Freeboard

Freeboard is provided on non-overtopping dams in order to prevent unintended overtopping from occur that may result in the erosion and consequent failure of the dam wall. Freeboard requirements may be set as part of the design, such as limiting seepage through core material. These freeboard conditions are set out in Design of Small Dams (USBR, 1987) depending on a number of design parameters. At a minimum, sufficient freeboard should be provided to prevent overtopping from wave action on the reservoir. The height of waves is dependent on the wind 5-59

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speed, the wind duration and the length over which the wind is able the act. Minimum freeboard is typically set assuming a wind speed of 80 km per hour. Based on this wind speed, minimum freeboard requirements may be interpolated from Table 5-12 based on the greatest distance of water across the reservoir. Table 5-12

5.7.4.4

Minimum Freeboard for Small Dams

Greatest Straight Line Distance over Water on Reservoir (km)

Minimum Freeboard (m)

1.2

0.03

0.03

0.03

0.03

0.03

0.03

0.1

* Manning’s values determined from vegetation retardance Chart-D (refer DTMR, 2010). Values are presented to three significant figures for convenience. This should not imply the values are accurate to three significant figures. A Manning’s roughness of 0.03 is adopted for hydraulic radius greater than 1.2 m in accordance with recommendations of original research; however, this may not always be appropriate. Further information is available in DTMR (2010) and FHWA (2005). Source: QUDM, 2013

6.3.2

Permissible Velocities and Channel Types

Channels should be capable of carrying the design discharge at velocities which do not result in excessive scouring or erosion. Indicative permissible velocities for different channels are provided in Table 6-5. Permissible velocities for rigid and vegetated channels are discussed in the subsequent sections Table 6-5

Permissible Velocities for Different Channel Linings

Soil Type

Grain size (mm)

Mean Depth (m) 0.4

1.0

2.0

3.0

>256

4.6

5.1

5.8

6.2

Large Cobbles

256-128

3.6

4.5

4.7

5.0

Small Cobbles

128-64

2.3

2.7

3.1

3.4

Very Coarse Gravel

64-32

1.6

1.9

2.2

2.5

Coarse Gravel

32-16

1.3

1.4

1.6

1.9

Medium Gravel

16-8

1.2

1.1

1.2

1.4

Fine Gravel

8-4

1.0

0.9

1.0

1.2

Very Fine Gravel

4-2

0.8

0.8

0.9

0.9

Very Coarse Sand

2-1

0.5

0.6

0.7

0.8

Coarse Sand

1-0.5

0.5

0.5

0.6

0.7

Medium Sand

0.5-0.25

0.4

0.5

0.5

0.6

0.25-0.125

0.3

0.4

0.5

0.5

Sandy Loam (heavy)

1.0

1.2

1.4

1.5

Sandy Loam (light)

0.9

1.2

1.4

1.5

Loess (settled)

0.8

1.0

1.2

1.3

Boulders

Fine Sand

Source: FHWA HI-90-016 Table 3.5.2

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6.3.2.1

Channels with Rigid Linings

The maximum average flow velocity of 4 – 5 m/s is recommended for hard lined channels. Other considerations in the design of channels with rigid linings, such as concrete, include:











6.3.2.2

Contraction and expansion joints to minimize the risk of cracking and seepage and potential undermining. Note that if a hydraulic jump is intended to move over a joint, then additional joint reinforcing may be required.

Pressure relief weep holes in impermeable linings both within the channel invert and within the channel side slopes. The extent and density of pressure relief weep holes should be sufficient to prevent hydraulic uplift of the channel.

Lateral protection against surface flows undermining the side slopes. A minimum hard faced strip of width 0.5 m on both sides at the top of the channel is recommended.

Vertical cut-off walls should be included at the upstream and downstream extents of a lined channel. These cut-off walls should be provided along the channel invert and up the channel side slopes. The required depth of cut-off walls is dependent on a number of factors including channel flow rate, flow velocity, and type of natural material upstream and downstream of the lined section. A minimum depth of 0.6 m should be adopted. Designers should ensure that scour beyond the downstream end of lined channels is prevented, or at least reduced to an acceptable level. To avoid the scour problems, it is desirable to pass the discharging water over a roughened surface before releasing it into a vegetated channel. This is normally achieved by placing a rock scour pad at the exit of the smooth-bed channel.

Vegetated Channels

For channels with flexible linings, there are generally two approaches:  

Permissible velocity approach

Permissible shear stress approach

Both approaches have been adopted around the world. As identified in FHWA (2005), the permissible shear stress approach better reflects the physical processes that are occurring and is constant over a wide range of channel shapes and slopes. However, the application of the permissible velocity is generally more straightforward to apply and is detailed in this guideline. Details on the permissible shear stress approach are provided in Design of Roadside Channels with Flexible Linings (FHWA, 2005).

A key concern for vegetated channels is what happens when the grass cover cannot be maintained, such as during drought, after fire etc. This aspect should be considered and if there is a reasonable risk of occurrence and channel scour is likely / not desirable, then design should be undertaken assuming bare-earth design values (DTMR, 2010). 6-7

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In designing vegetated / bare-earth channels the following must be considered (DTMR, 2010):   

The material the channel is to be constructed in

A suitable grass species for the channel (where applicable) An appropriate Manning’s n-value

A suitable grass species for a channel should (DTMR, 2010):

  

  

Be quick to establish

Be able to self-repair

Have a relatively short blade length (< 50 mm). Longer blade lengths can increase flow resistance and subsequently result in a reduction in capacity of the channel Be able to survive short durations of inundation

Be able to withstand proposed design velocities, and Be native to the area

Indicative permissible velocities for vegetated channels are provided in Table 6-6. In using Table 6-6, it is important that a good cover of grass be maintained, designers should assess the percentage of stable vegetal cover likely to persist under design flow conditions. Table 6-6 assumes a consolidated surface, rather than a cultivated surface. Table 6-6

Vegetation Bermuda Grass

Buffalo Grass, Kentucky blue grass, Smooth brome, Blue grama

Grass Mixture

Permissible Velocities

Channel bed slope (%)

Stable soils

Erodible soils

0-5

2.40

1.80

5-10

2.10

1.50

>10

1.80

1.20

0-5

2.10

1.50

5-10

1.80

1.20

>10

1.50

0.90

0-5

1.50

1.20

5-10

1.20

0.90

Not suitable for slopes steeper than 10% Weeping lovegrass, Alfalfa, Crabgrass

0-5

1.10

0.80

Not suitable for slope steeper than 5% Sudan Grass & Annual Grasses

0-5

1.10

0.80

Not suitable for slopes steeper than 5% Source: Manual of Surface Drainage Engineering by B.Z. Kinori, 1970

6.3.2.3

Reinforced Grass & Turf Reinforcement Matting

Turf reinforcement matting (TRM) and reinforced grass (using products such as coconet and numerous proprietary polypropylene products) provides additional protection from erosive forces. The concept of turf reinforcement is to provide a 6-8

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structure to the soil/vegetation matrix that will both assist in the establishment of vegetation and provide support to mature vegetation (FHWA, 2005).

There are many products on the market, and the designer should refer to the manufacturer specifications to determine operating flow regimes and velocities that are acceptable, as well as guidance on installation. As an indication, reinforced grasses may have a permissible velocity in the order of 4 m/s, but this should be confirmed by manufacturer specifications. The performance of TRM is subject to vegetation cover, and therefore is subject to some of the key considerations identified in Section 6.3.2.2. Figure 6-1

Turf Reinforcement Matting Profile

Source: FHWA, 2005

6.3.2.4

Riprap or Dry Boulder Channels

Rock lined channels, or rip rap, is a conventional treatment for channels to provide erosion resistance. Typically, the hydraulics of the channel is determined, and then an appropriate rock size is adopted. Some iteration may be required, as the rock size will affect the Manning’s ‘n’ value adopted (refer to Section 6.3.1). When designing and constructing a rock lined channel, the specification for riprap as identified in Section 5.9 should be adopted. For mild channel slopes (less than 5%), angular rock and a specific gravity of 2.6, the following simplified equation can be adopted to determine an appropriate rock size (QUDM, 2013): Equation 6-2

𝑑𝑑50 = 0.04𝑉𝑉 2 where: d50 V

6-9

= =

mean rock size for which 50% of the rocks are smaller (m) average cross sectional velocity (m/s)

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

A more refined version of the equation is provided below, which allows for different types of rock and flow conditions: Equation 6-3

where: d50 V sr K1 K

6.3.2.5

𝑑𝑑50 = =

𝐾𝐾1 𝑉𝑉 2 2. 𝑔𝑔. 𝐾𝐾 2 (𝑠𝑠𝑟𝑟 − 1)

mean rock size for which 50% of the rocks are smaller (m)

=

average cross sectional velocity (m/s)

=

1.0 for angular rock, 1.36 for rounded rock

= =

specific gravity of rock

1.1 for low-turbulent deepwater flow, 1.0 for low-turbulent shallow water flow, and 0.86 for highly turbulent flow

Rock Filled Wire Mattress or Gabion Box or Mattress

Rock filled wire mattresses or gabions may also be used to line the channel bank or bed. Smaller sized rocks can be used because the wire basket surrounding the rock in the mattress or gabion tends to make the mass act as a unit while retaining flexibility. Some specific design considerations include:  

 

6.3.3

The potential for damage to the baskets from debris.

Deterioration of the wire baskets due to pollution or saline environments. This can result in a reduction in the design life and require more frequent maintenance. Plastic coated wire can provide some benefits. The establishment of vegetation over the baskets can limit some of the issues identified above.

Maintenance of the wire baskets needs to be considered, particularly in regards to access. Maintenance needs to be incorporated into future maintenance programs to ensure that they are checked and repaired as necessary.

Design and construction of gabion protection should be in accordance with manufacturer’s specifications and shall be consistent with the latest Standard Specifications. Side Slopes

Recommended side slopes for design of different types of channels are provided in Table 6-7. In specifying a side slope, consideration should also be made for maintenance and safety. For channels adjacent to highways, the following should apply: 

Generally not more than 1V:5H, for traffic safety

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

Where the above cannot be achieved, or the depth is greater than 3 m, then a safety barrier is required

Table 6-7

Recommended Side Slope Material

Stream Bank Materials

Side slope (V:H)

Rigid Lined Channels

nearly vertical

Grass Lined Channels

Not steeper than 1:4, generally aiming for 1:6 to assist in maintenance and for public safety

Rock (Dry Boulder Rip Rap) lined channel

1:3

Gabion Mattress

Refer to manufacturer specifications.

Reinforced Grass/ TRM

Refer to manufacturer specifications. Consideration should be given for maintenance access as per grass lined channels, and therefore 1:6 would generally be preferable.

Hard Clay

1:2 to 1:1

Clay loam and silty loam

1:2

Sandy Loam

1:2

Sand

1:3

Source: DID, 2012, Kinori, 1970, QUDM, 2013 & DPWH, 1984

6.3.4

Freeboard

Freeboard refers to the height from the top of the channel to the water surface at the design capacity (refer to Figure 6-2). A freeboard is allowed to account for effects like waves and water surface fluctuations, sedimentation and water surface estimation errors. A freeboard should be selected that is 15% of the depth of flow in the channel at the design capacity, with a minimum of 100 mm. Figure 6-2

6.3.5

Open Channels and Freeboard (Source: QUDM, 2013)

Minimum Velocities

In hard lined channels, a minimum velocity of 0.8 m/s should be maintained in the channel to prevent deposition and sedimentation. This also has the added advantage of minimizing stagnant water and associated mosquito growth. During dry weather flows, it may become difficult to maintain this velocity. In such situations, it is possible to introduce a smaller channel in the bottom of the drain

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to confine these smaller flows to a smaller cross section (refer example in Figure 6-3). Dry weather flows can be estimated by using the baseflow estimate discussed in Section 3. Figure 6-3

6.3.6

Example Low Flow Channel for Dry Weather Flows

Sub-Critical Flow

The design of channels with flow approaching supercritical conditions should generally be avoided. Where it cannot be avoided, specialist design knowledge may be required as well as additional erosion protection (QUDM, 2013).

6.3.7

Flows between a Froude number of 0.8 to 1.2 are unstable and unpredictable and should be avoided (UDFCD, 2008). As general practice, Froude numbers below 0.8 should be adopted for design.

Transitions

Changes from one channel cross section to another cross section should be undertaken smoothly, with no sudden changes in cross section. An expansion rate (Figure 6-4) of 1 on 4 is recommended as a minimum, while a contraction of 1 on 1 is recommended as a minimum. Typical transition losses are shown in Table 6-8. Figure 6-4

Maximum Rate of Expansion

Source: QUDM, 2002

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Table 6-8

Typical Transition Losses Transition Type

Contraction Coefficient

Expansion Coefficient

Gradual channel transition

0.1

0.3

Typical bridge transition

0.3

0.5

Square edged abrupt transition

0.6

0.8

Source: QUDM, 2013

6.3.8

Bends

The radius of any horizontal curve in a channel should be as large as possible, to reduce super elevation and friction losses, as well as local erosion due to complex flow. A horizontal curve should have a minimum radius of the centerline of the channel of 3 times the width of the channel (PUB, 2011).

The superelevation around a bend may be calculated from Equation 6-4 (FHWA, 2001). The height of the channel on a bend should be designed to accommodate the expected water elevation on the bend at the design capacity as identified in Section 6.2.2, as well as freeboard as identified in Section 6.3.4. Equation 6-4

where: ∆d

∆𝑑𝑑 = =

difference in water surface elevation between the inner and outer banks of the channel in the bend (m)

=

surface width of the channel (m)

V

=

g

=

T

Rc

𝑉𝑉 2 𝑇𝑇 𝑔𝑔𝑅𝑅𝑐𝑐

=

average velocity (m/s)

gravitational acceleration (9.81m/s)

radius to the centerline of the channel (m)

Conclusive values for head losses in open channels are not available. A conservative estimate for bends between 90 and 180 degrees may be calculated using: Equation 6-5

where:

2B 𝑉𝑉 2 ) × ( ) 𝑅𝑅𝑐𝑐 2𝑔𝑔

hb

=

channel bend head loss

V

=

average flow velocity

B

g

Rc 6-13

ℎ𝑏𝑏 = (

=

=

=

channel width gravity

centerline radius of the bend

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

6.3.9

The equation is applicable for bends between 90 and 180 degrees. For bends between 0 and 90 degrees, linear interpolation is recommended.

Safety

The recommended inclusions for safety in channels are provided in Table 6-9. Table 6-9

Recommended Inclusions for Safety

Safety Feature

6.4

Comments

Safety Railings

To be provided for all channels where the design capacity depth is greater than 1 m.

Rungs in Channels

Non-skid aluminum rungs shall be provided 60 m apart for channels with slopes steeper than 2V:1H and where the depth exceeds 1 m.

Closed Conduit Network (Pipe Network) All pipes should be designed using the Hydraulic Grade Line (HGL) method, as described in Section 4.6.

Appropriate energy losses should be accounted for in the design. Losses include:      6.4.1

6.4.2

6.4.3

6.4.4

Losses at junctions Inlets and outlets

Obstruction and penetrations Pipe branch losses, and Transition losses

Minimum Size

The minimum size of pipe to be adopted shall be 910 mm in order to allow the passage of debris and minimize the risk of blockage.

Minimum Velocity

In order to encourage self-cleaning, and minimize sediment build up, pipes should be designed to ensure a minimum flow velocity of 0.8 m/s at pipe full.

Maximum Velocity

The maximum velocity to be adopted for piped drainage systems is 5 m/s.

Cover

Cover refers to the distance from the top of the pipe to the surface. A minimum cover of 600 mm should typically be adopted.

For pipes under highways, or heavily trafficked areas, a cover of 900 mm should be adopted.

A cover depth of 450 mm may be adopted on private property or under open space that experiences only occasional traffic.

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6.4.5

Alignment

Pipes should run straight between pits wherever possible. Where curves in the pipe are absolutely required, standard curved pipes from suppliers should be adopted. 6.4.6

Deflecting joints to achieve curvature is not recommended.

Capacity

The capacity of a pipe flowing full, but not under pressure, should be calculated using Manning’s equation, as discussed in Section 4.5.

6.4.7

6.4.7.1

6.4.8

6.5

6.5.1

It is generally recommended to avoid pipes flowing under pressure in drainage applications, although this may not always be possible.

Outlet Scour Control

Outlet scour control is discussed in Section 6.5.7.1.

Orientation of the Outlet

Refer to Section 6.5.7.3.

Backflow Control Structures

Backflow control structures are discussed in Section 6.5.7.1.

Culverts

Culverts are a relatively short length of pipe or closed conduit used to convey stormwater through an embankment or road, connected at each end to an open channel.

Minimum Sizing

For culverts crossing under local roads, a minimum internal width and clear depth of 910 mm is required.

6.5.2

6.5.3

6.5.4

For culverts crossing under expressways, a minimum internal width and clear depth of 1 m is required.

Minimum Velocity

In order to encourage self-cleaning, and minimize sediment build up, culverts should be designed to ensure a minimum flow velocity of 0.8 m/s at pipe full.

Maximum Velocity

The maximum velocity to be adopted for culverts is 5 m/s.

Flow Conditions

Flow behavior through culverts varies depending on whether the inlet and outlet are submerged.

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Computer design programs will automatically adjust the culvert flow conditions based on the upstream and downstream water levels. Culvert flow calculations are discussed in Section 4.7

6.5.5

6.5.6

Further details on calculating culvert flow are provided in the Urban Drainage Manual (Federal Highways Administration, 2001).

Cover

The cover for a culvert depends on the concrete/ loading class. In general, a minimum cover of 600 mm should typically be adopted. A cover depth of 300 mm may be adopted on private property or under open space that experiences only occasional traffic.

Blockage

Blockage of a culvert is possible through debris as well as siltation of the culvert. The effect of potential blockage should be considered in the design of the capacity of the culvert. While blockage of culverts tends to be associated with forested catchments, where wooded debris may mobilize during floods, urban catchments can also represent sources of debris through mobilization of man-made debris such as cars, garbage and other objects. To date, there have been no studies of blockages of culverts within the Philippines, and in particular the likely blockages for different catchment types and land-uses. In the absence of historical observations or studies, blockage factors as identified in Table 6-10 should be adopted in determining the discharge capacity. When assessing blockage, blockage of the handrails should also be considered for overtopping flow. Table 6-10

Blockage Factors to be Applied to Culverts Culvert Size

Blockage Factor *

Width < 5 m or Height < 3 m

20%

Width > 5 m and Height >3 m

10%

Handrails

50%

* Blockages are applied from the bottom of the culvert, upwards.

Minimization of blockages can be achieved through implementation of features such as debris deflector walls (as shown in Figure 6-5).

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Figure 6-5

Debris Deflector Walls

Source: QUDM, 2013

6.5.7

Inlet and Outlet Structures

Inlet and outlet structures are provided to direct the flow between the open channel and the culvert. Typical structures are shown in Figure 6-6. Figure 6-6

Typical Inlet Structures

Source: DID, 2012

6.5.7.1

Backflow Control Structures

Outlet flow controls include structures such as tidal flaps, flood gates and duck billed valves. These structures control the backflow of water from the receiving water body into either the culvert or pipe. They may be incorporated for a variety of reasons, including:   

6-17

To prevent tidal backflow into a culvert or pipe network

To prevent floodwaters from a river or creek from backwatering through a pipe network or culvert, particularly under a levee or dike To provide water quality controls between two areas

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

These structures introduce additional head losses. Reference should be made to the appropriate manufacturer guidelines. 6.5.7.2

Maintenance of these structures is also critical for their performance.

Outlet Scour Control

Outlet scour control may be required at outlets to reduce flow velocities prior to discharging to watercourses in order to reduce the risk of erosion. Outlet protection may be required where:   

The outlet velocity exceeds the scour velocity of the bed or bank material The outlet channel and banks are actively eroding

There is a bend in the channel a short distance downstream

Protection requirements may range from a riprap apron to stilling basins and concrete structures.

In all cases, a concrete cut-off wall is required at the end of the culvert to prevent undermining.

Rock pad outlets or dry boulder outlets are commonly adopted for culvert outlets (refer to Figure 6-7). These should generally be considered where outlet velocities are less than 5 m/s and the Froude number of the flow is less than 1.7. Figure 6-7

Dry Boulder (Riprap) Outlet

Source: QUDM, 2013

Figure 6-8 and Figure 6-9 provide guidance on the selection of mean rock size (d50) and the length of the dissipater (L). Note that these design graphs assume a specific gravity of 2.6. Refer to standard specification for riprap in Section 5.5.6. The minimum recommended width of the rock pad is defined as:  

Immediately downstream of the outlet: the width of the outlet apron, or the width of the outlet plus 0.6 m (if there is no apron). At the downstream end of the rock pad: the above width plus 0.4 times the length of the rock pad (L) as shown in Figure 6-10.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

If the width of the outlet channel is less than the recommended width of the rock protection, then rock protection should extend up the banks to either the height of the pipe’s obvert or to the design tailwater level. Note that this type of protection is only applicable for slopes of less than 10%.

For information on designing alternative dissipation structures, refer to Hydraulic Design of Energy Dissipaters for Culverts and Channels (FHWA, 2006). Figure 6-8

Sizing of Dry Boulder Outlet Structures for Single Pipe or Box Culverts

Source: QUDM, 2013

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-9

Sizing of Dry Boulder Outlet Structures for Multiple Pipe or Box Culverts

Source: QUDM, 2013 Figure 6-10

Typical Rock Pad Outlet Configuration

Source: QUDM, 2013

6.5.7.3

Orientation of Outlet

Where practical, storm water outlets should be recessed into the banks of any watercourse that is likely to experience bank erosion, channel expansion, or channel migration. Typically the minimum desirable setback (Figure 6-11) is the greater of (based on QUDM, 2013):

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

 

3 times the bank height from the toe of the bank

10 times the equivalent pipe diameter (single cell) or 13 times the equivalent diameter of the largest cell (multiple outlets) measured from where the outlet jet would strike an erodible bank.

Figure 6-11

Typical Orientation and Set-Back of Outlet

Source: QUDM, 2013

Outlets that discharge into a ‘narrow’ receiving channel should be angled 45 to 60 degrees to the main channel flow. A receiving channel is considered ‘narrow’ if:   

6.6 6.6.1

The channel width at the bed is less than 5 times the equivalent pipe diameter, or The distance from the outlet to the opposite bank (along the direction of the outlet jet) is less than 10 times the equivalent pipe diameter, and The inflow is more than 10% of the receiving channel flow

Stormwater outlets that discharge in an upstream direction need to be avoided wherever practical (QUDM, 2013).

Inlet Manholes

Inlet Manhole Location

Inlet pits should be located: 

6-21

Such that the capacity of the reach between inlet pits is not exceeded. This will require an iterative process of pit location. An initial spacing can be determine based on individual pit catchment areas and pit inlet capacities. For a worked example, refer to FHWA (2001), Example 4-15.

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

   

6.6.2

In all low points/depressions in order to prevent the unwanted collection of stormwater. Upstream of bridges/crossings to prevent stormwater flowing onto the bridge/crossing.

In locations were overland flow may present a hazard to pedestrians or vehicles. Where they do not interfere with pedestrian or vehicular access (for example, driveways).

Inflow Capacity

The capacity of an inlet is dependent on the depth of water over the inlet. Under shallow flow conditions the inflow behaves as for a sharp crested weir. As the depth increases, the inlet becomes submerged, and the inflow behaves as for an orifice.

Equations for determining the inflow capacity under weir flow conditions and orifice flow conditions are provided in Section 6.6.2.1 and Section 6.6.2.2, respectively. Alternatively, the inflow capacity can be estimated from the inlet rating curves shown in Figure 6-12 and Figure 6-13.

Note that these curves are applicable only to pits located in low points and depressions. For grated pits on grade (such as roadside drains) refer to Volume 4: Highway Design.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-12

Grated Pit (in depression) Inflow Rating Curves

Source: FHWA, 2001

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-13

Side Opening Pit (in kerb or gutter) Inflow Rating Curves

Source: FHWA, 2001

6.6.2.1

Weir Flow

Weir flow behavior is illustrated in Figure 6-14. Inflow under weir flow conditions can be derived based on the simplified version of the weir formula, as identified below: Equation 6-6

where:

𝑄𝑄𝑔𝑔 = 𝐵𝐵𝐵𝐵 × 1.66. 𝐿𝐿. ℎ3⁄2

Qg

=

inflow

1.66

=

weir coefficient

BF L

h

= =

=

blockage factor

perimeter of the grate, disregarding any sides against vertical edges (such as kerbs or walls)

height of the energy level above the weir crest (Equal to the water level at low velocities)

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-14

Inlet Weir Flow Behavior

Source: QUDM, 2013

6.6.2.2

Orifice Flow

Orifice flow occurs under two conditions. Free flow, where a free surface remains within the inlet and atmospheric pressure is within the chamber, and fully drowned, where the pit is filled with water and the pressure within the pit is governed by the head and flow conditions. These flow conditions are illustrated in Figure 6-14. The flow under both conditions should be assessed and the less capacity adopted in design. Orifice flow is given by the orifice equation: Equation 6-7

where: Qg

=

inflow

Co

=

orifice coefficient

BF Ag h g

6-25

𝑄𝑄𝑔𝑔 = 𝐵𝐵𝐵𝐵 × 𝐶𝐶𝑜𝑜 . 𝐴𝐴𝑔𝑔 . (2𝑔𝑔. ℎ)1⁄2 =

= = =

blockage factor (refer Section 6.6.3) clear opening area

=

0.67

average depth of water over grate

acceleration due to gravity (9.8m/s)

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-15

Inlet Orifice Flow Behavior

Atmospheric

Non-Atmospheric

Source: QUDM, 2013

6.6.3

Blockage

In determining the inflow capacity of inlets, an appropriate blockage rate should be adopted. For inlets located on-grade, a blockage of 20% should be adopted.

6.7

6.7.1

For inlets located in depressions and low points, a blockage of 50% should be adopted.

Manholes & Access Chambers

Manholes and access chambers are used to provide access to the drainage system for inspection and maintenance. Inlet pits (Section 6.5.7.3) can also serve as access points and should be used in lieu of access chambers where possible as they provide the additional benefit of stormwater interception at a minimal additional cost. Inlet pits used as access locations, and dedicated access points are to follow the guidelines below. Location & Spacing

At a minimum, access points should be provided at:     

6.7.2

The convergence of two or more pipes Changes in pipe size

Changes in alignment Changes in grade

Immediately upstream of outlets to tidal waterways

In addition to the above locations, access points should also be provided along straight pipe sections to facilitate cleaning and maintenance. The maximum recommended distance between access locations is recommended at 50 m.

Entry

In order to allow safe entry and exit from access points, all access chambers should have a minimum inside diameter of 1.2 m, or 1.5 m for pipes larger than 2.1m. The top of the access shaft may taper to 0.9 m, so long as the tapered zone does not affect working at the base of the shaft. 6-26

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

6.7.3

Access

Access down the access chamber may be either by steps or rungs embedded in the chamber wall, or by a ladder that workers carry with them.

If steps or rungs are used, they should be made from a non-corrosive material and be maintained appropriately. They should provide a secure grip to allow safe entry and exit.

6.7.4

The use of ladders reduces risks from rust damages steps, and helps to prevent unauthorized access. If ladders are used, the geometry of the access shaft must allow for the safe usage of the ladder. Access Chamber Cover& Frame

Access chamber covers must:     

6.8 6.8.1

Possess adequate strength to resist surface loads

Provide a good fit between the cover and the frame Be finished flush with the surrounding terrain Prevent unauthorized opening

Be easily opened by authorized personnel

If the hydraulic grade line of the system extends above the surface, the cover must be secured so that they remain in place during peak flood events.

Detention Basins

Purpose

Detention basins are used to reduce the peak outflow from a location. Urban development results in increased impervious areas which causes faster catchment responses and higher peak flow rates. Basins are often employed to return peak flow rates and volumes to the pre-developed condition to prevent the development resulting in adverse flood impacts downstream. They can assist in meeting the requirements of Section 6.2.3. Basins can also be used to reduce upgrade works that might be required for stormwater drainage, and may be more economical than increasing pipe sizes or channel dimensions.

Basins perform this through intercepting stormwater flows, and releasing the stormwater volume in a controlled manner over a period of time.

There are many types of basins and configurations. In general, detention basins may be either open air basins, located within parkland areas, road reserves etc., or underground systems. A typical schematic of an open air basin is provided in Figure 6-16 and Figure 6-17, while an underground system is shown in Figure 618. Underground systems will generally require design in accordance with the manufacturer’s specifications.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Basins also provide the opportunity to incorporate water quality management features. For open basins, these might include wetlands or bioretention (raingarden) systems.

Above ground detention basins are a type of small dam structure, and reference should be made to Section 5.6.3.4 for specific design considerations on small dams. Figure 6-16

Typical Schematic of Detention Basin

Source: NJDEC, 2004 Figure 6-17

Example of Above Ground Detention System after Heavy Rain

Source: SUDSnet, http://sudsnet.abertay.ac.uk/DetentionBasinJun24159.htm, 2014

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-18

Example Underground Storage System

Source: Brentwood Industries, 2012 Figure 6-19

Example Underground Detention System using Permeable Pipes

Source: ACME General Engineering Contractors, http://s401908300.initialwebsite.com/services/underground-detention-basins/, 2014

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-20

Example Underground Detention System

Source: http://www.lakesuperiorstreams.org/stormwater/toolkit/underground.htm

6.8.2

Sizing

The size of the basin will be governed by the volume of flow generated from the upstream catchment, and the amount of retardation required of the flow.

It is preferable to utilize a computer model to size the basin, in order to properly simulate the hydraulic conditions at the outlet.

If an appropriate computer program is not available, the sizing can be undertaken using manual flow routing based on the storage equation. This requires the upstream hydrograph. The storage equation is: Equation 6-8

where:

𝑆𝑆𝑛𝑛+1 =

𝑇𝑇 (𝐼𝐼𝑛𝑛 +𝐼𝐼𝑛𝑛+1 ) 2

−

𝑇𝑇 (𝑄𝑄𝑛𝑛 +𝑄𝑄𝑛𝑛+1 ) 2

+ 𝑆𝑆𝑛𝑛 .

I

=

inflow rate

Q

=

outflow rate

1,2

=

start and finish times of the routing step.

S

T

= =

volume in storage routing time step

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

An example of using the generic formula is provided below.

Given the initial inflow and outflow values shown in Table 6-11, the first three steps of calculating the storage volume is shown below. The completed table is shown in Table 6-12. Table 6-11

Time (min)

Inflow (m3/min)

Outflow (m3/min)

Count (n)

0

0

0

1

1

1

0.5

2

2

3

0.8

3

3

5

1.5

4

Step1:

𝑆𝑆2 = 𝑆𝑆2 =

Example Hydrograph Inputs

𝑇𝑇 (𝐼𝐼1 +𝐼𝐼2 ) 2

1 (0+1) 2

𝑆𝑆2 = 0.25

Step2:

𝑆𝑆3 = 𝑆𝑆3 =

1 (1+3) 2

𝑆𝑆3 = 1.6

Step3:

𝑆𝑆4 = 𝑆𝑆4 =

1 (3+5) 2

𝑆𝑆4 = 4.45

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−

𝑇𝑇 (𝑄𝑄2 +𝑄𝑄3 )

−

𝑇𝑇 (𝑄𝑄3 +𝑄𝑄4 )

−

𝑇𝑇 (𝐼𝐼3 +𝐼𝐼4 ) 2

𝑇𝑇 (𝑄𝑄1 +𝑄𝑄2 )

−

𝑇𝑇 (𝐼𝐼2 +𝐼𝐼3 ) 2

−

−

2

1(0+0.5) 2

2

+ 0.

1(0.5+0.8) 2

2

+ 𝑆𝑆2.

+ 0.25.

1(0.58+1.5) 2

+ 𝑆𝑆1.

+ 𝑆𝑆3.

+ 1.6.

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Table 6-12 Time (min)

Worked Example Detention Routing Hydrograph Flow (m3/min)

Discharge (m3/min)

Storage (m3)

0

I1

0

Q1

0

S1

0

1

I2

1

Q2

0.5

S2

0.25

2

I3

3

Q3

0.8

S3

1.6

3

I4

5

Q4

1.5

S4

4.45

4

I5

10

Q5

2.5

S5

9.95

5

I6

4

Q6

3.4

S6

14

6

I7

3

Q7

3

S7

14.3

7

I8

8

Q8

3

S8

16.8

8

I9

5

Q9

2.8

S9

20.4

9

I10

2

Q10

2.7

S10

21.15

10

I11

0

Q11

2

S11

19.8

11

I12

0

Q12

2

S12

17.8

12

I13

0

Q13

1.9

S13

15.85

13

I14

0

Q14

1.9

S14

13.95

14

I15

0

Q15

1.9

S15

12.05

15

I16

0

Q16

1.8

S16

10.2

16

I17

0

Q17

1.5

S17

8.55

17

I18

0

Q18

1.3

S18

7.15

18

I19

0

Q19

1.2

S19

5.9

19

I20

0

Q20

1

S20

4.8

20

I21

0

Q21

0.8

S21

3.9

21

I22

0

Q22

0.8

S22

3.1

22

I23

0

Q23

0.5

S23

2.45

23

I24

0

Q24

0.5

S24

1.95

24

I25

0

Q25

0.2

S25

1.6

25

I26

0

Q26

0.2

S26

1.4

Alternatively, the storage volume may be determined from the inflow and outflow hydrographs, with the storage being equal to the difference in the hydrographs. This option should be used only to determine an initial estimate, as it requires the outflow hydrograph to be estimated. The approach is shown in Figure 6-21.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-21

Basin Volume Estimation

Source: FHWA, 2001

6.8.3

An example of some of the models which can be utilized in the design and analysis of detention basins are HEC HMS, xpswmm, HYDRAIN and xpstorm. The HEC HMS is the widely use within the Philippines and a detailed description is provided in Section 3.5.2. It is noted that this is not necessarily a recommendation of these particular software, as many suitable software exist in the market.

Freeboard

Recommended freeboard requirements for basins are provided in Table 6-13. Table 6-13 Basin Freeboard Requirements Scenario

6.8.4

Freeboard Requirement

Basin formed by road embankment

Bottom of pavement box 0.3m below edge of shoulder

Basin formed by railway embankment

Underside of ballast

Large basins with separate high level spillway

10% of the design flood depth, or 0.3m, whichever is greater

Basin Drainage

The basin floor should be graded at a minimum cross gradient of 1 in 80 for grassed or concrete basins, or 1 in 100 for vegetated basins, in order to allow efficient surface drainage. The outlet structure should be located at the low point of the basin, typically near to the downstream end. The outlet may be constructed as a pipe, culvert, orifice plate, drop pit or similar. The construction method may be selected based on local constraints. The important factor is that whatever solution is adopted, it is capable of restricting basin outflow to the desired rate under a range of storm events. 6-33

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

The intake to a detention basin outlet should be protected against expected debris blockages and designed to minimize the safety risk to a person trapped against the outlet structure, where public access to the basin is possible. The level of protection will vary depending on the consequences of failure caused by blockage of the intake and the potential frequency of blockage. Protection can be achieved by the installation of a trash rack, bar screen and/or a fence. Outlet pipes should have spigot and socket rubber-ring joints and lifting holes should be securely sealed. Pipe and culvert bedding should be carefully specified to minimize its permeability. Cut-off walls or seepage collars must be installed where appropriate, to control seepage and prevent piping failure adjacent to the outlet pipe.

6.8.5

Appropriate measures, such as internal sealing of pipe joints and lifting holes, and bolting down of access chamber lids, should be applied to any existing downstream systems which could be pressurized by the discharge from the outlet. Alternatively, surcharge chambers may need to be incorporated into the outlet pipe to limit the internal pressure.

Emergency Spillway

Basins should incorporate an emergency spillway (Figure 6-22) to safely discharge water once the basin is filled. This may occur in large storm events that result in a large volume of water reaching the basin. The overflow needs to be controlled to prevent the failure of the basin wall. Further details are provided in Section 5.6.3.4.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-22

Typical Spillway Design

Source: FHWA, 2001

6.8.6

6.8.7

Outlet Protection

Protection must be provided as required at the basin outlet to prevent erosion and scour. Refer Section 5.7.4.6.

Release Timing

The design of the release of stored storm water is critical to the success of a retention basin. A typical basin with reduce the peak of the downstream flood hydrograph, and produce some attenuation of the flood peak. Care needs to be taken to ensure that the delayed flood peak does not cause adverse effects downstream through coincident flooding. This is particularly important if multiple basins are employed in a single catchment.

6.8.8

It is generally preferable for basins to be considered as a part of a catchment wide analysis.

Embankment

Refer to Section 5.6.3.4.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

6.8.9

Public Safety

Basins can pose a public safety risk due to the depth of the ponding water, and through currents in the basin, particularly around the outlet structure. Hazards may not be immediately apparent to the public, particularly if the basin serves a dual use as public space. A detailed risk assessment of all basins should be undertaken. At a minimum, the assessment should address: 



   6.8.10

Basin grades – these should be 1 in 6 or shallower if there is public access to allow egress up the wet slope. If steeper grades are used, steps with handrails should be provided.

Ponding depth - basin depths should be restricted to 1.2 m for at least the 20 year flood, and preferably larger events. This is unless public access to the basin is excluded completely with fencing (i.e. it is not a dual purpose basin). Signage – warning signs and depth gauges may be appropriate.

Outlet structure safety – methods of preventing trapped persons being drawn into the basin outlet should be employed. Fencing – fences should only be utilized as a last resort.

Maintenance

In order to ensure the continued successful operation of retention basins, a maintenance plan should be prepared as part of the basin design. The plan should address:    

6.9

Inspections frequency – typically monthly following construction, then annually and following any major storm.

Mowing – typically twice a year, but this should be reviewed based on local needs. Sediment, debris and litter removal – typically twice a year. Particular attention should be paid to the control device and any spillways, and

Repairs and Replacement –drainage devices will deteriorate over time and will require replacement when their operation becomes compromised.

Overland Flowpaths

Overland flowpaths are designed to carry flow in excess of the piped capacity.

Where possible, overland flowpaths should not be contained within private properties. If this is unavoidable, a drainage easement should be obtained to contain the overland flow path, in order to allow maintenance teams to undertake any required control works.

Any potential or likely blockage of the flowpath (due to fences, crossings, vegetation, etc.) should be considered at the time of design, and an appropriate safety factor adopted in determining the size of the overland flowpath.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

The design of the overland flowpath should control flow such that:   

The product of flow depth and velocity (V*D) is less than or equal to 0.4 m2/s

The flowpath has sufficient capacity to meet the design discharge for major drainage as identified in Section 6.2.2

If the flow is contained within an open channel, the freeboard is as per Section 6.2

6.10

Pumping Stations

6.10.1

Purpose

6.10.2

Pumping stations allow for the removal of stormwater from pipe systems that cannot be drained through gravity. Pumping stations are complex and expensive, both to construct and maintain, and their use is only recommended if other options are proven to be unfeasible.

Pumping Station Requirements

Pumping stations are required to:   

6.10.3

Have available back up pumps, pumping mains, and a means of supplying back up power

Pumping start / stop shall be automatic with the option for manual override provided Provide an operation and maintenance schedule

These are minimum criteria. Additional features or requirements may be deemed necessary based on the location of the pumping station. Common Pump Types

There are two broad types of pumps – centrifugal pumps and positive displacements pumps

Centrifugal pumps operate using a rotating impeller to move water into the pump and pressurize it. The difference in pressure between the water in the pump, and the water at the outlet, generates a flow of water through the pump. As centrifugal pumps are reliant on pressure differences to generate flow, they are sensitive to changes in pressure heads at the outlet. A common centrifugal pump design is shown in Figure 6-23.

Positive displacement pumps operate by trapping a fixed amount of water within the pump, and then forcing this water through the pipe by displacing it, commonly by a piston or diaphragm. Positive displacement pumps are more tolerant of changes in head levels. A common positive displacement pump is shown in Figure 6-24.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

In general, there are a large range of pumps available that are applicable to different situations. It is important to select a pump that is appropriate for the proposed use. A detailed guide to pump selection, and the relative merits of different systems, is provided in Hec-24: Highway Stormwater Pump Station Design (FHWA, 2001). Figure 6-23

Centrifugal Pump

Source: Water Partnership Program, 2012

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-24

Positive Displacement Pump

Source: Water Partnership Program, 2012

6.10.4

6.10.4.1

Pump Storage

The storage required should be determined based on the pumping rate of the selected pump, and the inflow hydrograph, as shown in Figure 6-25. The storage volume required is the difference between the hydrograph and the pumping rate. Cycling Sequence

Cycling is the starting and stopping of pumps, the frequency of which must be limited to prevent damage and possible malfunction. The pumping system must be designed to provide sufficient volume for safe cycling. The volume required to satisfy the minimum cycle time is dependent upon the characteristics of the power unit, the number and capacity of pumps, the sequential order in which the pumps operate and whether or not the pumps are alternated during operation (FHWA, 2001).

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-25

Estimated Required Pump Storage from Inflow Hydrograph

Source: FHWA, 2001

6.10.5

Collection System

Storm drains leading to the pumping station are typically designed on mild grades to minimize depth and associated construction cost. To avoid siltation problems in the collection system, a minimum grade that produces a velocity of 1 m/s in the pipe while flowing full is suggested. The inlet pipe should enter the station perpendicular to the line of pumps.

The inflow should distribute itself equally to all pumps. Baffles may be required to ensure that this is achieved. Further details on pump station layout are provided in Hec-24: Highway Stormwater Pump Station Design (FHWA, 2001).

6.10.5.1

6.10.6

Collector lines should preferably terminate at a forebay or storage box. However, they may discharge directly into the station. Under the latter condition, the capacity of the collectors and the storage within them is critical to providing adequate cycling time for the pumps and must be carefully calculated. To minimize siltation problems in storage units, a minimum grade of 2% should be used. Debris Screens

It is recommended that screens be used to prevent large objects from entering the system and possibly damaging the pumps. Larger debris may be screened either at the surface or inside the wet well/storage system. The level of maintenance required should be considered when selecting debris removal procedures (FHWA, 2001).

Pump Station Types

There are two key types of pump stations; wet-pit (Figure 6-26) and dry-pit (Figure 6-27) stations.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

The main advantage of the dry-pit station for storm water is the availability of a dry area for personnel to perform routine and emergency pump and pipe maintenance.

Since dry-pit stations are more expensive than wet-pit stations, wet-pit stations are most often used. Dry-pit stations are more appropriate for handling sewage because of the potential health hazards to maintenance personnel.

The station depth should be kept to a minimum. No more depth than that required for pump submergence and clearance below the inlet invert is necessary, unless foundation conditions dictate otherwise. Figure 6-26

Typical Wet-Pit Pumping Station

Source: FHWA, 2001

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 6-27

Typical Dry-Pit Configuration

Source: FHWA, 2001

6.10.7

6.10.8

Submergence

Submergence is the depth of water above the pump inlet necessary to prevent cavitation and vortexing. It varies significantly with pump type and speed and atmospheric pressure. This dimension is provided by the pump manufacturer and is determined by laboratory testing (FHWA, 2001).

Power Supply

Electrical power should be used where possible, as it is usually the most economical and reliable power source. Liquid fuels and gas may also be used if electrical power is not available, or is unreliable.

Pump stations also require a backup power source that is capable of powering the pump in the event of a failure of the main power system. 6-42

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

6.10.9

Pump Control

Pump stations may be either controlled manually or automatically.

Manual controls are well suited to small, non-critical installations that do not warrant the additional expense of automatic control. Automatic control is well suited to large pump systems, critical systems or remote systems.

6.10.9.1

It is important that any automatic system has a manual backup system provided in case of power failures. Water Level Sensors

Water-level sensors are used to activate the pumps and, therefore, are a vital component of the control system. There are a number of different types of sensors that can be used. Types include the float switch, electronic probes, ultrasonic devices, mercury switch, and air pressure switch.

6.10.10

6.10.11

The location or setting of these sensors control the start and stop operations of pump motors. Their function is critical because pump motors or engines must not start more frequently than an allowable number of times per hour (i.e., the minimum cycle time) to avoid damage. To prolong the life of the motors, sufficient volume must be provided between the pump start and stop elevations to meet the minimum cycle time requirement (FHWA, 2001).

Flap Gates and Valving

Flap gates or valving are generally required on the discharge point from the pumps to restrict water from flowing back into discharge pipe. Flap gates and valving are discussed in Section 6.5.7.1.

Number of Pumps

Two to three pumps should be included as a minimum, with considerations included on redundancy and potential failure of one of the pumps. Ideally, the pumps should have sufficient capacity that if one fails, there is remaining capacity to pump the required discharge.

It is recommended that pumps of the same size be adopted, for simplicity of maintenance and to manage the discharge.

It is recommended that an automatic alternation system be provided for each pump station. This system would automatically redefine the lead and lag pump after each pump cycle. The lead pump will always come on first, but this pump would be redefined after each start so that each pump in turn would become the lead pump. This equalizes wear and reduces needed cycling storage (FHWA, 2001).

Where the above approach is adopted, standby pumps are typically not warranted. However, this should be based on a risk assessment and the critical nature of the area being pumped.

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6.10.12

6.10.13

6.11

Pump Foundations

Where the pump is directly connected to the motor (via gears or drive trains for example) a single common foundation should be constructed for both the pump and the motor to prevent misalignment due to differential settlement. Additional Design Details

Further details on pump station layout are provided in Hec-24: Highway Stormwater Pump Station Design (FHWA, 2001).

Water Quality

Urbanization and development result in changes in the catchment, which can result in increases in runoff and pollutants into receiving water bodies. Various types of measures can be introduced to mitigate the impact on receiving water bodies. These measures are termed Best Management Practices (BMPs) in the USA, Water Sensitive Urban Design (WSUD) in Australia and Sustainable Urban Drainage Systems (SUDS) in the United Kingdom. The design of these measures requires a holistic view of both the stormwater quality and quantity. Typical measures may include:     

6.12

Wetlands

Bioretention systems (raingardens) Swales

Gross Pollutant Traps (GPTs)

Rainwater tanks and water re-use

The design of these types of features is detailed in the Urban Stormwater Management Manual for Malaysia (MSMA, 2012).

Design Drawings

The main components that should be included in design drawing are the following:         

Plan and general layout of the scheme

Structure data table including type, surface level and location Hydraulic Design Data

Pipe data table showing diameter, length and level information

Longitudinal sections of pipes including level and grade information, length of section, size and Class of pipe, hydraulic grade line, services crossings etc. Structure detail plan for special structures. Calculation sheets, where appropriate. Bill of quantities.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

6.13

References Sub-Surface Stormwater Management, http://www.brentwoodprocess.com/stormwater.html , accessed : October 2nd, 2013. Brentwood

Industries

(2012).

Federal Highways Administration [FHWA] (2001). Urban Drainage Design Manual, Hydraulic Engineering Circular No. 22, 2nd Edition, US Department of Transportation. Federal Highways Administration [FHWA] (2005). Design of Roadside Channels with Flexible Linings, Hydraulic Engineering Circular No. 15, 3rd Edition, US Department of Transportation.

Federal Highways Administration [FHWA] (2006), Hydraulic Design of Energy Dissipaters for Culverts and Channels, Hydraulic Engineering Circular No. 14, 3rd Edition, US Department of Transportation.

DID (Department of Irrigation and Drainage, Malaysian Government), 2012. Urban Stormwater Management Manual for Malaysia, 2nd Edition, Government of Malaysia, Kuala Lumpur.

Kinori, B.Z., 1970, Manual of Surface Drainage Engineering Vol. 1, Elsevier Publishing Co., Amsterdam.

New Jersey Department of Environmental Conservation [NJDEC] (2004). New Jersey Stormwater Best Management Practices Manual, April, New Jersey, USA.

Public Utilities Board [PUB] (2011). Code of Practice on Surface Water Drainage, 6th Edition, December, Singapore.

Queensland Department of Energy and Water (2007) {QUDM]. Queensland Urban Drainage Manual, Second Edition. Queensland Department of Energy and Water (2013) {QUDM]. Queensland Urban Drainage Manual, Third Edition, Provisional.

Queensland Department of Transport and Main Roads [DTMR] (2010). Road Drainage Manual, March, 2nd Edition, Australia. Urban Drainage and Flood Control District [UDFCD] (2008). Urban Storm Drainage Criteria Manual. Denver, Colorado, USA.

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7

Coastal Structures The design of coastal structures of different types is presented in detail in the Philippine Port Authority (PPA) Engineering Standards for Port and Harbor Structures – Volume II (2009). PPA (2009) provides a comprehensive design manual on the design of ports and harbors, which includes numerous coastal structures and protection measures. This Guide focuses on coastal structures that are relevant to projects undertaken by DPWH. These are namely revetments and sea walls. A general discussion and overview of the considerations is provided, with key referencing to PPA (2009) for more detailed design information.

Coastal structures are a specialized field of design, and should be undertaken by suitably qualified engineers with relevant experience in this discipline.

7.1

General Criteria

7.1.1

Design Event

In selecting an appropriate design event, there are two key considerations: 



7.1.2

Protection level – the level of protection that is provided by the revetment. This reflects the size of the event where significant overtopping of the revetment will occur and impacts of the overtopping will affect the landward side. Structural Event – The structural design refers to the event at which failure will start to occur. This is typically adopted as the 100 year ocean event, although this may depend on specific applications. Due to issues in overtopping, this may sometimes supersede the protection level.

Protection Level

The level of protection offered by the sea wall or revetment should be dependent on the structure or land-use that is protected, and the implications of overtopping waves should this occur. Ideally, this should be identified as a part of a master planning process, similar to that recommended by FCSEC for flood planning (refer to Section 1.3). Similarly, the protection of roads should be based on the road type, the relative importance of the road etc. More discussion on some of these factors is provided in Volume 4: Highway Design.

In the absence of the above, Table 7-1 provides some recommended protection levels for structures. The protection level refers to the frequency of the event that is being protected against – for example, a 25 year protection level refers to an event that will only be exceeded on average once every 25 years. For retro-fitting applications, where a sea wall or revetment is designed to protect an existing asset that is subject to erosion or overtopping, then a lower protection level might be adopted based on other constraints such as available space, social

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

constraints etc. The key aim for these types of applications is to maximize protection for these existing assets within the constraints.

In some applications, it may also be necessary to increase the protection level. For example, for the protection of a critical facility like a hospital or where an asset is particularly susceptible to damage as a result of inundation.

The level of protection should be determined in consideration of the design waves acting on the structure. The design wave should be estimated in accordance with the procedures outlined in PPA (2009). Table 7-1

Protection Levels for Coastal Structures

Asset/ Land-use Protected

Protection Level

Rural Areas, Sport fields and Parks

25 year

Urban Areas

100 year

Roads

7.1.3

Expressway

100 year

National Road

50 year

Other Road

50 year

Tides

Tides are the periodic rise and fall of sea levels in response to the gravitational attraction of the sun and moon. Table 7-2 provides some of the key terminology used for reference to the tides. Table 7-2

Tidal Terminology

Tidal Parameter

7.1.4

Description

Mean Sea Level (MSL)

The average of the sea water surface for all stages of the tide over a 19 year period

Mean Low Water (MLW)

The average height of the low water heights over a 19 year period.

Mean Lower Low Water (MLLW)

The average height of the lowest water heights recorded for each tidal day over a 19 year period.

Mean High Water (MHW)

The average height of the high water heights over a 19 year period.

Mean Higher High Water (MHHW)

The average height of the highest water heights of each tidal day over a 19 year period.

Storm Surge

Storm surge is a combination of wind driven surge and low pressure surge (associated with a low pressure weather system). Typhoons are generally associated with larger storm surges in the Philippines.

7.1.5

Storm surge is typically estimated using computer models, such as ADCIRC and SWAN.

Design Still Water Level

The values for tide and storm surge are considered part of the "design still water level”. 7-2

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Revetments should be designed for the design still water level plus wave runup.

For the design of coastal protection structures, annual risk of exceedance is required to be estimated. This can then be used to determine the appropriate protection level, as identified in Section 7.1.2. The estimation of annual exceedance levels can be estimated through historical or numerical simulations.

Historical analysis on long-term tide gauge data can provide water level-return period information. Typically, determining the return period associated with these tide station record involves application of log-Pearson Type III (or similar) statistical methods. Either graphical or analytical statistical approaches can be used. However, such analyses are typically restricted to locations near one of the long-term tide stations, and these are rarely close enough to a study area. In some cases, a transfer function may be adopted.

7.1.6

Numerical simulations are undertaken with computer models, and utilize historical data as a key input.

Wave Height Estimation

Wave heights at a structure are the result of a range of conditions. It is recommended that numerical simulations, using tools such as SWAN, are adopted in order to undertake wave transformation to a site. As noted earlier, this is a specialized field and should be undertaken by an appropriately qualified specialist.

7.1.7

7.1.8

7.1.9

7.2 7.2.1

An alternative is to use the maximum breaking wave height (Section 7.2.4.1). This provides a conservative estimate of the wave height, but may be too conservative in deeper water scenarios.

Ship Induced Waves

Ship induced waves should be accounted for where these are likely to be important (for example, near navigational channels).

Riverine Applications

Some revetments are located in zones that are influenced by both coastal wave action and also river flows. In these cases, the structure should be designed taking into consideration the guidance in Section 5.5. The worst case in terms of protection requirements should be adopted.

Climate Change

Climate Change should be incorporated into the design process, as identified in Section 8.

Coastal Revetments

General Overview

The terminology used for different types of wall is typically categorized as: 

Bulkheads – typically associated with fetch lengths in hundreds of meters, and are typically in estuarine locations. 7-3

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

 

Revetment – typically associated with lakes and bays and large wave heights.

Seawall (Figure 7-1) – typically for very large wave heights, with large fetch lengths (thousands of kilometers) and in ocean conditions.

There is not always a clear distinction between revetments (Figure 7-2) and sea walls, and quite often the terms are used interchangeably. For this Guide, the terminology of revetment is typically adopted. Figure 7-1

Example of Sea Wall

Source: FHWA, 2008 Figure 7-2

Example of Rock Sea Wall/ Revetment

Source: FHWA, 2008

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

7.2.2

Components of a Coastal Revetment

The key components of a revetment include:    

Armor layer

Underlayer (filter) Toe protection

Splash apron (where overtopping is expected).

These are shown in the typical section in Figure 7-3. Figure 7-3

Typical Revetment Section

Source: FHWA, 2008

7.2.3

Typical Modes of Failure of Coastal Revetments

There are five typical failure mechanisms for coastal revetments, as identified in FHWA, 2008):      7.2.4

Inadequate armor layer design for wave action Inadequate under layer Flanking

Toe scour

Overtopping splash.

Sizing of Armor Units

The weight of rubble or concrete blocks covering a sloped revetment subject to wave action can be sized based on the Hudson formula (USACE, 1984) provided in Equation 7-1. Note that this refers to the sizing of the armor units for the front face of the revetment. For revetments where large overtopping occurs, PPA (2009) should be consulted to design the armor units on the crest.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Equation 7-1

where:

𝑤𝑤𝑟𝑟 𝐻𝐻 3 𝐾𝐾𝐷𝐷 (𝑠𝑠𝑟𝑟 − 1)3 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐

W50

=

median weight of armor units

H

=

design wave height (refer discussion below)

wr

KD

=

=

Sr

7.2.4.1

𝑊𝑊50 =

=

θ

=

Design Wave Height

unit weight of armor units (kN/m3)

empirical coefficient (refer discussion below)

specific gravity

slope of revetment

The design wave height (H) for revetment design for use in the Hudson formula (Equation 7-1) should be based on the lesser of either:  

the depth limited maximum wave height (Hb) or

the average of the highest 10% of all wave heights (H1/10) in the design seastate (where H1/10 ~ 1.27Hs (based on CIRIA, 2007))

Often coastal revetments are located in areas where the design sea-state is depth limited. In these situations, the depths are so shallow immediately off-shore from the revetment that wave have already broken. The maximum breaking wave height for flat slopes (of the bathymetry in front of the revetment) can be determined using Equation 7-2 (FHWA, 2008). For non-flat slopes, reference should be made to PPA (2009).

The Hudson formula has performed well in testing for wave heights of 1.5 m or less (refer FHWA, 2008). For greater wave heights, more judgment and specialist input is required. Equation 7-2

where: Hb

7.2.4.2

ds

𝐻𝐻𝑏𝑏 = 0.8𝑑𝑑𝑠𝑠 =

=

maximum breaking wave height (m)

design depth at the toe of the structure

Hudson Coefficient (Kd)

Ideally the KD coefficient should be determined in accordance with physical model tests. However, this is not always possible. Some suggested values, based on PPA (2009), are provided for dry boulder (rock) revetments in Table 7-3. For other types of armor units, refer to PPA (2009). Key parameters in this table are as follows:

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

 



Thickness – refers to the number of units comprising the armor layer. Generally, a minimum thickness of 300 to 500 mm should be adopted Breaking Wave – refers to depth-induced wave breaking on the foreshore in front of the structure at the design sea-state. This is not representative of the influence of the structure itself on breaking waves, but rather the depth immediately off-shore. Maximum Slope – refers to the maximum slope recommended for this type of revetment.

Table 7-3

Armour Unit

7.2.4.3

7.2.4.4

7.2.5

7.2.6

Suggested Hudson Coefficient Values

Thickness

Placement

Breaking Wave (KD)

NonBreaking Wave (KD)

Recommended Maximum Slope (V:H)

Smooth Rounded Rock

2

Random

1.3

2.4

1:2

Rough Angular

2

Random

2

4

1:2

Damage to Revetment

The Hudson Formula (see Equation 7-1) generally assumes a level of damage. The damage level is generally expected to be in the order of 5% of the armor units in the rock face which have moved.

Rock Grading

For dry boulder riprap revetments, a gradation of D85/D15 consistent with Section 5.9 should be adopted.

Height of Structure

The height of the structure should be designed to prevent overtopping for the protection level identified in Section 7.1.2. A freeboard of 0.6 m should be added to design still water level plus wave runup.

Determining Maximum Wave Runup

The maximum wave runup can be determined from Equation 7-3, which is based on the procedure presented in FHWA (2009). The parameters for the equation are presented conceptually in Figure 7-4. Equation 7-3

𝑅𝑅𝑢𝑢 = 1.6𝐻𝐻𝑠𝑠 (𝑟𝑟𝑟𝑟)

With an upper limit of:

𝑅𝑅𝑢𝑢 = 3.2𝑟𝑟𝑟𝑟𝑠𝑠

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

where: Ru

vertical height of runup on slope (m)

Hs

=

significant wave height (m)

ξ

=

dimensionless breaker parameter from Table 7-4.

r

Figure 7-4

Table 7-4

=

coefficient of armor roughness (= 0.55 for riprap)

Overview of Parameters for Wave Runup

Dimensionless Breaker Parameter

Value of the Dimensionless Breaker Parameter (ξ)

Type of Wave (Figure 7-5)

ξ < 0.5

Spilling

0.5 < ξ < 2.5

Plunging

2.5 < ξ < 3.5

Collapsing

ξ > 3.5

Surging

Figure 7-5

7-8

=

Types of Waves

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

7.2.7

Slopes of Revetment

Recommended maximum slopes for revetments are provided in Table 7-3.

For very mild slopes, the Hudson formula can underestimate the armor unit sizing, and care should be taken in designing in mild slopes. Hudson’s formula was derived specifically for regular sloped cross sections. When the cross section differs, then care should be taken a more assessment may be required.

7.2.8

7.2.9

For example, the incorporation of a vertical wall with a revetment can result different reflective and turbulent behavior, which would result in a change to the armor units required. Flanking

As noted in FHWA (2008), flanking occurs when adjacent, unprotected shorelines continue to recede. Erosion at the end of the wall allows wave action to remove the soil from behind the wall starting at the ends, then progressing along the walls it fails. Flanking can be avoided by extending the revetment or wall to meet an existing revetment or a wall or natural rock outcropping, or by using a return wall. A return wall is aligned perpendicular to the shoreline. The length of the return wall should exceed the expected long-term and storm-induced recession of the adjacent shorelines. Overtopping Splash

Overtopping splash at the top of a revetment can lead to failure through exposing and eroding the soil behind the revetment. Where overtopping is expected to occur, a splash apron should be incorporated. The width of this will be dependent on the severity of the expected overtopping. A minimum apron of 2 to 3 m should be adopted (FHWA, 2008). Wave overtopping of revetments and seawalls occurs when runup exceeds the top or crest of the structure. Building seawalls high enough to completely prevent overtopping is often unacceptable because of aesthetics and costs. Two aspects of overtopping of interest to the design engineer are the time-averaged volumetric rate of overtopping (which can be used to size appropriate drainage or management of overtopping flows) and the intensity or force of a single wave overtopping event. Accurately estimating volumetric overtopping rates can be vital to design of seawall crest elevations if inland flooding is caused. Unfortunately, accurately estimating overtopping rates can be very difficult for many situations and input to the design team from a trained coastal engineer is likely appropriate (FHWA, 2008). Safety should also be considered for overtopping. The amount of overtopping can cause potential safety risks for pedestrians and/or cars on the landward side.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

7.2.10

Filter Material

Revetments often incorporate one or more granular underlayers or filter layers and a core. A geotextile can sometimes be placed between the core and the filter layer, particularly where the core is a fine material such as sand. In some cases, the geotextile can be used to replace some of the underlayer. The importance of the filter is to prevent the core from washing through the armor layer.

7.2.11

The underlayer should have a median weight no smaller than one-tenth of the armor layer stones (USACE, 1984).

Toe Protection

A conservative assumption is to adopt the same armor unit size for the toe protection as for the revetment slope. However, the energy acting at the toe is less than that on the slope of the revetment, and therefore it can be more economical to reduce the armor unit size.

CIRIA (2007) presents a methodology for sizing of rock for depth limited scenarios (refer to Section 7.2.4.1) and for sloped breakwaters (refer to Table 7-5). This allows for the sizing of rock for toe protection, relative to:    

h = water depth (m)

ht = depth to the toe (m)

Hs = significant wave height

D50 = toe protection rock diameter, based on a specific gravity of 2.6

Larger rock will be required at the toe for vertical revetments. Refer to PPA (2009). Different options for toe protection are provided in Table 7.5. Table 7-5

7-10

Relationship for Toe Protection ht/h

Hs/D50

0.5

3.3

0.6

4.5

0.7

5.4

0.8

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 7-6

Example of Toe Protection Options

Source: CIRIA, 2007

7.2.12

Protection against Scour

There are a number of ways to protect against scour. An overview of some of these is provided below (after CIRIA, 2007): 





Reduce the forces from reflections. This can be done by designing or making the revetment slope less steep and/ or by using an energy dissipating revetment facing (e.g. angular stones instead of smooth). This is the preferred approach to scour protection.

Isolate the problem area close to the structure by placing a scour-control blanket, which may consist of rockfill, pre-fabricated flexible mats or gabion mattress. This is common where the above cannot be adopted. Improve the quality of the bed foundation material (e.g. by replacing the material or by applying full, partial or local grouting).

Methods for extending the toe protection to protect scour are similar to those identified in Section 5.5. Examples of some options for scour protection are provided in Figure 7-7 and Figure 7-8.

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Figure 7-7

Example Scour Protection using Toe extending to Depth of Anticipated Scour in Moderate Scour Environments

Source: CIRIA, 2007 Figure 7-8

Example Scour Protection using Toe extending to Depth of Anticipated Scour in Severe Scour Environments

Source: CIRIA, 2007

7.2.13

7.2.14

Structural Stability

The structural stability of a revetment can be designed in accordance with PPA (2009).

Aesthetic Considerations

Revetments can impact on the aesthetics of coastal landscapes. Where the structure is intended for recreational areas or areas of social importance, it is recommended that architectural considerations be included in the design.

An example of one such approach is the construction of a sea wall in California, presented in FHWA (2008). In that application, a sea wall was constructed so as to appear like a natural bluff. This is shown in Figure 7-9.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 7-9

Example Sea Wall - Constructed to appear like a natural bluff

Source: FHWA, 2008

7.3

Alternatives There are numerous alternatives to revetments and sea walls for shoreline protection. These include:   

Detached breakwaters (Figure 7-10) Groynes (Figure 7-11) Sub-merged reefs

The design of these types of structures is detailed in CIRIA (2007). Figure 7-10

Example Detached Breakwaters

Source: CIRIA, 2007

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 7-11

Example of Groynes as Shoreline Protection

Source: CIRIA, 2007

7.4

References CIRIA, CUR, CETMEF (2007). The Rock Manual. The Use of Rock in Hydraulic Engineering (2nd edition). C683, CIRIA, London.

Philippine Port Authority [PPA] (2009). Engineering Standards for Port and Harbor Structures, March.

US Army Corp of Engineers [USACE] (1984). Shore Protection Manual, 4th Edition, US Government Printing Office, Washington DC.

US Department of Transportation Federal Highway Administration [FHWA] (2008). Highways in the Coastal Environment, 2nd Edition, Hydraulic Engineering Circular 25, FHA NHI-07-096.

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8

Water Supply

8.1

Overview Water supply design within the Philippines is commonly undertaken by a number of different agencies, such as the Metropolitan Waterworks and Sewerage System for the Metro Manila Area and the Local Water Utilities Administration for the water districts outside Metro Manila.

For the situation where DPWH is involved in water supply, it is recommended that the procedures outlined in the Rural Water Supply Design Manual (WPP, 2012). This document is provided in three volumes:   

8.2

Volume 1 – Design Manual

Volume 2 – Construction Supervision Manual

Volume 3 – Operation and Maintenance Manual

This document covers the key elements of water supply that will normally involve DPWH. The following provides an overview of some of the key considerations for water supply.

Distribution of Water in the Philippines

Rainfall within the Philippines varies significantly both in time and location. Increased rainfall volumes also leads to an increase in surface water due to runoff, and an increase in groundwater recharge through infiltration.

There are four broad rainfall categories for the Philippines, shown in Figure 8-1, namely: 







Type I: Two pronounced seasons: dry from November to April and wet during the rest of the year. These areas are shielded by mountain ranges but are open to rains brought in by southwest monsoons (Habagat) and tropical cyclones. Type II: Characterized by the absence of a dry season but with a very pronounced maximum rain period from November to January. Regions with this climate are located along or very near the eastern coast.

Type III: Seasons are not very pronounced but are relatively dry from November to April and wet during the rest of the year. These areas are partly sheltered from the trade winds but are open to Habagat and are frequented by tropical cyclones. Type IV: Characterized by a more or less even distribution of rainfall throughout the year.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 8-1

Rainfall Distribution in the Philippines

Source: WPP, 2012

8.3

Water Sources

8.3.1

Rainwater

Rainwater, or atmospheric water, is water vapour that has condensed in the atmosphere and fallen to earth as rain, where it becomes runoff.

Rainwater may be harvested through collecting the runoff from impervious surfaces (such as roofs and pavements) during rain events.

Rainwater is typically of a reasonable quality. The greatest risk to rainwater quality is the vessel used to store the collected water. Rainwater tanks need ongoing inspections and maintenance to ensure that the water is not contaminated by microbial or chemical pollutants. Rainfall is not evenly distributed throughout the Philippines, and some regions experience significantly more rainfall than others (refer to Section 8.2).

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

8.3.2

Surface Water

Surface water is exposed to the atmosphere and includes water bodies such as lakes and ponds, rivers and streams, reservoirs, seas and oceans.

Surface water occurs either through the runoff from rains or the surcharge of ground water systems. The primary determinate of the amount of surface water available is the amount of rainfall experienced by the region, and is also affected by the climate, vegetation, geographical and topological characteristics of the catchment area. Surface water has the potential to pick up contaminants as if flows over surfaces. This is particularly true for flow over developed catchment that may pick up pollutants from urban and agricultural land uses. The Rural Water Supply Design Manual - Volume 1(WPP, 2012) recommends that all surface water sources should be assumed to be contaminated and require treatment before domestic use.

8.3.3

Given the expensive nature of water treatment systems, particularly in rural settings (refer to Section 8.4.2), the use of surface water in water supply systems shall be avoided if possible.

Groundwater

Groundwater is water that has filtered through the soil layer from rainfall or surface water to create underground water reservoirs. The upper surface of the groundwater storage is termed the water table.

Groundwater is typically of a good quality due to the filtering effects of the soil having removed microorganisms, sediments and organic matter. However, it may contain dissolved natural salts and substances, some of which may be harmful, so testing is still recommended before use. It may also be affected by contaminated land, and this should be considered if it is identified a potential geohazard in the Preliminary GeoHazard Assessment (Volume 2A: GeoHazards). Groundwater may be extracted using: 





Springs – a point at which ground water naturally flows onto the surface, which occurs when the water table is higher than the terrain. Springs may be intermittent or continuous, and can be developed to improve their ability to service a population. Wells – a hole dug down through the soil to reach the water table, so that the bottom of the well fills with water. Water may be extracted from the base of the well through buckets (raised by hand or via mechanical means) or through pumping.

Infiltration Galleries – are horizontal wells, formed by digging horizontal, perforated pipes into water bearing soils. The pipes collect water from the soil and discharge it to a storage structure.

Details on the siting, construction and use of these systems is provided in Rural Water Supply Design Manual - Volume 1(WPP, 2012)

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

8.3.4

Selection of a Water Source for Water Supply

The primary concern when selecting a water source for a water supply system should be adequacy and reliability. Adequacy requires that the water source be able to supply enough water to meet demand, and reliability requires that the extraction and distribution system implemented is robust and able to consistently deliver the demand volume. Other concerns include water quality, cost and legality.

With regards to adequacy and reliability, the most desirable supplies are (WPP, 2012): 1. An inexhaustible supply which flows by gravity through the distribution

system

2. A gravity source supplemented by storage reservoirs 3. An inexhaustible supply that requires pumping

8.4

4. A supply that requires both storage and pumping

Water Quality

The required water quality will depend on the usage requirements. Potable water for domestic consumption will require a very high water quality. Lower quality may be acceptable for toilet water, irrigation or agricultural uses.

8.4.1

Water Quality Parameters

8.4.1.1

Physical

Physical quality parameters relate to the physical experience of using the water. Although negative physical aspects may not pose a health risk, they may reduce peoples’ willingness to utilize the water source. Physical parameters to be assessed include:     8.4.1.2

Turbidity Color Odor

Taste

Chemical

Chemical pollutants within water sources may occur through natural or anthropogenic means. Urban development and agriculture are common sources of chemical pollutants. To assess a water sources chemical quality, testing should be undertaken on:    

8-4

Hardness

Alkalinity and acidity Dissolved oxygen

Chemical and biological oxygen demand

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

   8.4.1.3

Nutrients (such as nitrogen and phosphorus) Heavy metals

Toxic substances

Microbial

Microbial water quality needs to be strictly monitored, particularly for domestic water sources. Microbial contamination can quickly result in adverse health impacts for users of the water system. Microbial assessments should be undertaken for:    8.4.2

Protazoa

Helminths Bacteria

Water Treatment

Water sources with poor quality may undergo a treatment process to ensure they are suitable for their end use. Treatment processes are expensive, both in their set up and ongoing operation, and ideally water treatment should be avoided by selecting appropriate water sources with good quality water. Where suitable, clean water supplies are not available, water treatment may be appropriate. Treatment options are varied, and will depend on the quality issues that need to be rectified. Common treatment options include:    

Sedimentation Filtration Aeration

Disinfection

Water treatment systems should be designed such that:

  

They are as simple as possible

They minimize mechanical and electronic operated systems They minimize chemical inputs into the system

A discussion on water treatment options and methods is provided in Rural Water Supply Design Manual - Volume 1(WPP, 2012).

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

8.5

Water Demand and Usage

8.5.1

Demand

An assessment of the expected water demand on a supply system should be undertaken as the first step in designing a water supply system. The assessment should also investigated the expected future demands as it may be desirable to size the system based on these future demands in order to avoid additional works in the future. The amount of water demanded from a system is affected by:       

8.5.2

The level of service to be provided

The size of the population using the system

Quantity and quality of the water available in the region Water tariffs

Climatic conditions (rainfall)

Water usage habits of the population

Volume of non-revenue water (leakage, illegal connections, free water)

The Rural Water Supply Design Manual - Volume 1 (WPP, 2012) provides a methodology on predicting system demand.

Service Levels

Water service levels in the Philippines are classified into three types, depending on how the water is provided to consumers: 





8.5.3

Type 1: Point Source – A well or developed spring, without a distribution system. Users come to the point source to collect their water. Best suited to rural areas where affordability is low and houses are not crowded. Typical serves an average of 15 households, within 250m of the point source.

Type 2: Communal Faucet or Stand Post – Still a communal system, where people come to collect water, but this system includes a distribution network. The network delivers water from the source to a number of stand posts located in the township. Each stand post serves 4 to 6 houses, within a radius of 25m. Best suited to urban fringe areas where population densities justify a simple piped system.

Type 3: Individual Connections – A fully piped and distributed network that delivers water to individual households. Best suited to densely populated areas that can be offered the additional costs that this system requires.

Sustainability

Designing a sustainable system will reduce ongoing costs, improves financial viability, provides continuous, suitable services to consumers and reduces stresses placed on the natural water system. Sustainability considerations should be investigated for the following areas: 8-6

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects



  

8.6

Technical - ensure that the proposed system is appropriate to consumer requirements, and that it will be able to be maintained using local workers and supplies. Financial - are consumers willing and able to pay the costs associated with the proposed system. Social are consumers satisfied with the proposed system, and achieving the level of service that they require. Environmental is the proposed extraction suitable for the natural system.

Distribution Network

The distribution network is used to deliver water from the source to the end user. Typically the distribution system will be piped, but may also include open channels. Open systems should be avoided where possible in order to minimize the risk of contamination. If channels are required, they should be designed in accordance with Section 6.3.

Piped systems should be designed using the hydraulic grade line (HGL) methodology, as discussed in Section 4.6.

The distribution network may be classified into two general systems; dead end (also termed branched) or looped. These systems are shown in Figure 8-2.

In a dead end system, the size of the distribution line decreases as the distance from the source increases, as typically more remote pipes have to carry less water. Design is relatively simple as the direction and rate of flow in each pipe can be easily determined. The dead end system is generally cheaper than a looped system. However, a break in a pipe will affect all downstream connections, and velocities and head losses may cause problems during high demand periods. A looped system has increased connectivity between pipes and fewer dead ends. This results in lower pipe velocities and the ability to isolate breaks without impacting other sections of the network. The system however requires additional pipe length, and typically larger pipe sizes throughout the network.

A methodology for piped network design is provided in Rural Water Supply Design Manual - Volume 1 (WPP, 2012).

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Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

Figure 8-2

Source: WPP, 2012

8-8

Distribution System Classification

Design Guidelines, Criteria and Standards: Volume 3 – Water Engineering Projects

8.7

Reservoirs Reservoirs are included in distribution systems in order to:   



Balance the supply and demand in the system

Maintain adequate and relatively uniform pressure in the system

To provide a back-up storage volume to prevent service interruption when working on pipes between the source and the reservoir, or if supply is temporarily reduced To allow uniform operation of pumps

Reservoirs may be constructed either at ground level, or at an elevated position, relative to the location of the consumers. Elevated reservoirs allow for gravity to assist in distributing the water. In hilly areas, water may flow directly to an elevated reservoir. In flatter areas, pumping will be required to lift the water to the elevated position.

8.8

The Rural Water Supply Design Manual - Volume 1 (WPP, 2012) contains a methodology for sizing and designing reservoirs for water supply systems.

Pumping

If possible, water distribution systems should be designed to operate under gravity flow, so that pumping is not required. This reduces the complexity of the system and the required operation and maintenance costs. In low grade regions, or where elevated reservoirs are utilized, pumps may be required to move water through the distribution system and maintain appropriate pressure levels.

8.9

Pump systems should be designed in accordance with Section 6.10 and the relevant manufacturer’s specifications.

References

WPP (Water Partnership Program), 2012, Rural Water Supply Volume 1, Design Manual, The World Bank Office, Manila, Philippines.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

9

Climate Change

9.1

Climate Predictions for the Philippines PAGASA undertook a study on the likely influences of climate change in the Philippines in 2011 (PAGASA, 2011). The study focused on projected changes as a result of climate change to 2020 and 2050, measured relative to a baseline period from 1971 to 2000. The study focused on the following impacts of climate change:   

Projected increases in temperature

Magnitude of changes to long term (e.g. annual) rainfall Frequency of extreme weather events, including:

- Extreme temperature - Number of dry days - Extreme rainfall

The PAGASA (2011) study found the following:  



 



All areas of the Philippines will get warmer, more so in the relatively warmer summer months. Annual mean temperatures (average of maximum and minimum temperatures) in all areas in the country are expected to rise by 0.9 °C to 1.1 °C in 2020 and by 1.8 °C to 2.2 °C in 2050.

In terms of seasonal rainfall change it was found that there was a substantial spatial difference in the projected changes in rainfall in 2020 and 2050 in most parts of the Philippines, with reduction in rainfall in most provinces during the summer season making the usually dry season drier, while rainfall increases are likely in most areas of Luzon and Visayas during the southwest monsoon and the SON seasons, making these seasons still wetter, and thus with likelihood of both droughts and floods in areas where these are projected. The northeast monsoon season rainfall is projected to increase, particularly for areas characterized by Type II climate with potential for flooding enhanced.

During the southwest monsoon season, larger increases in rainfall are expected in provinces in Luzon (0.9% to 63%) and Visayas (2% to 22%) but generally decreasing trends in most of the provinces in Mindanao in 2050. However, projections for extreme events in 2020 and 2050 show that hot temperatures (indicated by the number of days with maximum temperature exceeding 35 °C) will continue to become more frequent, number of dry days (days with less than 2.5 mm of rain) will increase in all parts of the country and heavy daily rainfall (exceeding 300 mm) events will also continue to increase in number in Luzon and Visayas.

A full discussion on likely impacts, and a breakdown on a region by region basis, is provided in PAGASA (2011).

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

The PAGASA (2011) study, like most climate based assessments that has been undertaken, has focused on daily rainfall events. In particular, it has estimated:  

Changes to seasonal rainfall.

Changes in the number of days which have in excess of 300 mm of rainfall.

Seasonal rainfall is useful for impacts to water supplies, but only provides an indication on extreme rainfall events. Similarly, while the number of days with rainfall in excess of 300mm suggests that severe storms will be more frequent, the exact frequency and size of these rainfall events is not reported. This makes it difficult to directly correlate, for example, changes in the frequency of present day 100 year rainfall.

At the time of this report, there were no known detailed studies of sea level rise within the Philippines. The recent Intergovernmental Panel on Climate Change (IPCC, 2013) provides estimates of likely global sea level rise relative to 1986 to 2005. These estimates are provided Table 9-1. The mean values are the range in mean values from the different climate scenarios, while the ranges are the ranges in values between the different models. It is important to note that sea level rise will not be uniform across the world. However, IPCC (2013) notes that it is very likely that sea levels will rise in more than about 95% of the ocean area. Furthermore, around 70% of the coastlines around the world are projected to experience sea level rise within 20% of the global mean sea level change. Table 9-1

Sea Level Rise Predictions (IPCC, 2013) 2046 – 2065

9.1.1

2081 - 2100

Mean

Range

Mean

Range

0.24 – 0.30 m

0.17 to 0.38 m

0.40 – 0.6 m

0.26 – 0.82 m

Impacts of Climate Change

Table 9-2 provides an overview of some of the potential impacts of climate change.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

Table 9-2

Overview of Different Impacts of Climate Change

Climate Change Impact

Impact on Hydrological and Coastal Regime

Changes in Temperature

 

Changes evaporation from lakes and water bodies, reducing storage over time; Changes in evapotranspiration, and hence changes infiltration losses during rainfall events. This may result in a changes in runoff during certain events

Changes in Long Term Rainfall



Changes in long term rainfall affect water supply systems such as water reservoirs and dams and groundwater supplies; Affect the baseflow of rivers, with subsequent environmental and human impacts

 Changes in Extreme Rainfall



Affects flood estimation. Where extreme rainfall events increase, the flood protection of current structures will be reduced. For example, a 100 year flood dike might be reduced to a 50 year flood dike under a future climate change scenario.

Changes in Sea Levels



Increases in sea level rise will reduce the protection provided by coastal structures such as sea walls and revetments; Increases in saline intrusion into groundwater systems, which may affect water supplies and environmental reliance on these systems; Changes in wetland and low lying river systems, with greater saline intrusion. This will have a resulting impact on environment and livelihoods that revolve around these areas.

 

Changes in Typhoon Frequency

9.1.2

9.2

  

The potential increase in typhoon frequency will impact extreme rainfall (as noted above); Frequency of extreme winds and impacts on structures Storm surge, which will be affected both due to the typhoon and further exacerbated by sea level rise.

Uncertainty

It is important to understand that there is significant uncertainty in the estimation of climate change. This is through both the estimation of likely emission scenario through to the complex global weather patterns that are affected by the increases in emissions and natural meteorological changes. Estimating changes in weather, such as changes in the path and track of typhoons, can be extremely difficult. Therefore, it is important to understand this in reviewing estimates from different bodies, and understand that this provides a new level of uncertainty into the design process.

Designing for Climate Change

Historically design has been undertaken on the assumption of a stationary climate. For example, recurrence intervals for rainfall (e.g. 100 year rainfall) are based on analysis of historical rainfall records to determine an appropriate rainfall frequency for design. While there may be seasonality and long term cycles in the historical data sets, the underlying assumption is that the long-term average climate is static. Climate change confounds this with a long term change in the underlying climate. This means, for example, that a 100 year rainfall determined from historical data sets may only be equivalent to a 50 year rainfall in the future.

Furthermore, unless current emission trends change, climate change will continue moving into the future. Therefore, for example, a 100 year storm surge level now might be equivalent to a 50 year storm surge level at 2050 and a 20 year storm surge level by 2010.

9-3

The discussion provided in this section focuses on engineering structures, which is the primary focus of the Guides, and incorporating climate change into design.

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

9.2.1

9.2.2

There are many broader mitigation strategies such as emission reductions, development planning, planned retreats and other strategies that are not covered by this Guide, and should be considered as part of broad planning and adaptation strategies for the Philippines.

Design Life

Understanding the design life of a structure is an important aspect in incorporating climate change into design. Structures with short design lives, generally 20 years or less, are unlikely to be significantly impacted by climate change within their lifespan. Structures with longer design lives will need to have taken into consideration the potential impacts that climate change will have throughout the life of the structure.

Implementation Timeframe

Related to the design life is the implementation timeframe for the structure. The further into the future the works are planned for, the greater the potential impact that climate change will have on the function of that structure.

9.2.3

This is particularly important for the master planning phase of projects, where the implementation of works may be a number of years in the future.

Incorporating Climate Change

In the design of structures, there are two key ways that climate change can be incorporated. This is discussed below: 



Incorporating into Present Design - the design of the structure is upgraded to account for climate change estimates now. This will ensure that the structure is “climate proof” or climate resilient for its entire design life.

Planned Upgrade – the design is undertaken in such a way that it is possible to upgrade the structure in the future. For example, a dike is designed so that it can be raised in the future as the need arises.

The choice of method to be adopted in the design should be based on cost estimates and economic cost benefit analysis. For example, the cost to incorporate an additional 0.5 m on a dike may not represent a significant cost and therefore a planned upgrade may not be appropriate. However, in other cases the social impacts of raising a dike may be too great at present, and therefore a planned upgrade may be the best approach. The advantage of the planned upgrade approach is that the accuracy of the climate predictions is uncertain (refer to Section 9.1.2). Therefore, an estimate of the future climate in 2050 may either be better or worse. A planned upgrade approach allows for a trigger based design to be undertaken. For example, rather than identifying that a sea wall should be raised in 2030, it is identified that the sea wall should be raised when 0.2 m of sea level rise has occurred, for example. This will prevent unnecessary upgrades from occurring in the future, but does require monitoring of these trigger levels. The recommended allowance is identified in Section 9.2.4.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

9.2.4

Suggested Allowance for Climate Change

Climate change is an area of evolving scientific study, and further information and guidance should be sought as a part of the design process. As noted in Section 9.2.3, there are different methods for incorporating Climate Change in a design. The following provides some suggested allowances for climate change that can be considered for design. This is based on the current available information and provides a best estimate. Two approaches are recommended for seal level rise and changes in rainfall:

 

9.2.4.1

General Approach – this is the default approach that can be adopted for all projects. Alternative Approach – where the general approach may result in a significant cost to the infrastructure, then the alternative approach may be adopted.

Changes to Extreme Rainfall

Increases in extreme rainfall events will alter the frequency of existing flood events. For example, a current 100 year flood may be equivalent to a 50 year flood in 2100. However, the key challenge is estimating this change in frequency. The current estimates from PAGASA (2011) suggest that some areas will see increases in extreme daily events, but there is no information how different magnitude events may change. It is quite possible, for example, that more extreme events (such as a 100 year rainfall event) will change by a different proportion to more frequent rainfall events (such as a 5 year rainfall event).

Furthermore, a daily rainfall event is not necessarily representative of shorter duration rainfall events that are more critical for smaller catchments and urban environments, or longer period rainfall events that may be more critical in affecting stability of slopes. In the absence of any other information on rainfall events, it is suggested that a sensitivity analysis be undertaken. This sensitivity analysis should consider increases in rainfall intensities of 10% and 20%, and determine the likely impact on the proposed hydraulic design. In some situations, there will be minimal impacts, while in others the differences in flood levels will be more significant. Judgment, together with discussion with key policy and decision makers, will be required where it is difficult to incorporate potential changes in rainfall into the design based on the discussion in Section 9.2.3. Table 9-3

Suggested Approach for Incorporating Changes to Extreme Rainfall

Approach

9-5

Recommendation

General Approach

Incorporate a 10% increase in rainfall intensity in the design. For example, a 100 year rainfall intensity is increased by 10%.

Alternative Approach

This sensitivity analysis should consider increases in rainfall intensities of 10%, and determine the likely impact on the proposed hydraulic design. In some situations, there will be minimal impacts, while in others the differences in flood levels will be more significant. Where the cost implications are significant, then consideration for a Planned Upgrade approach should be considered, as identified in Section 9.2.3.

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

9.2.4.2

Sea Level Rise

Table 9-1 provides an overview of different sea level rise estimates based on IPCC (2013). In the absence of more up to date information, a sea level rise of 0.3 m might be appropriate for 2050, which would cover a typical design life in the order of 50 years. For design and planning out to 2100, then a potential sea level rise of 0.5 m might be appropriate, given the uncertainty. The suggested approaches are provided in Table 9-4. Table 9-4

Suggested Approach for Incorporating Sea Level Rise

Approach

9.3

Recommendation

General Approach

Allow for a 0.3 m sea level rise in the design.

Alternative Approach

Determine the likely impacts of a 0.3 m sea level rise. Refer to potential for Planned Upgrade as discussed in Section 9.2.3.

References Intergovernmental Panel on Climate Change (2013). Working Group 1 Contribution to the IPCC Fifth Assessment Report (AR5) – Climate Change 2013: The Physical Science Basis, Final Draft, September.

Philippine Atmospheric, Geophysical and Astronomical Services Administration [PAGASA] (2011). Climate Change in the Philippines, February.

9-6

Annex A

Estimating Scour

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

Contents A.

ESTIMATING SCOUR ....................................................................................................................................... 1

A.1 A.1.1 A.1.2 A.1.3 A.2 A.2.1 A.2.2 A.2.3 A.3 A.3.1 A.3.2 A.3.3 A.3.4 A.4 A.5 A.5.1 A.5.2 A.6 A.7 A.8

SCOUR ANALYSIS ................................................................................................................................................... 1 Long-Term Profile and Plan Form Changes ................................................................................................... 1 Contraction ..................................................................................................................................................... 2 Local Scour ...................................................................................................................................................... 3 BRIDGE SCOUR ESTIMATION ................................................................................................................................... 3 Contraction Scour Conditions ......................................................................................................................... 3 Live Bed Contraction Scour Equation ............................................................................................................ 4 Clear Water Contraction Scour Equation ...................................................................................................... 5 LOCAL SCOUR ........................................................................................................................................................ 6 Pier Scour Equation ........................................................................................................................................ 6 Scour at Abutments......................................................................................................................................... 8 Froehlich's Live-Bed Abutment Scour Equation ............................................................................................ 8 HIRE Live-Bed Abutment Scour Equation .................................................................................................... 10 SCOUR AT TRANSVERSE STRUCTURES .................................................................................................................... 10 SCOUR AT LONGITUDINAL STRUCTURES ................................................................................................................. 11 Scour with Flow Parallel to a Vertical Wall ................................................................................................. 11 Scour with Flow Impinging at an Angle on a Vertical Wall ........................................................................ 12 SCOUR AT BENDS ................................................................................................................................................. 12 ESTIMATION OF BED SCOUR AT GROUNDSILLS ........................................................................................................ 13 REFERENCES ....................................................................................................................................................... 14

Tables and Figures Table A1-1

Exponent K1 for Live Bed Contraction Scour ..........................................................................................................5

Table A1-2

Correction Factor, K1, for Pier Nose Shape ...............................................................................................................7

Table A1-4

Increase in Equilibrium Pier Scour Depths, K3, for Bed Condition ................................................................7

Table A1-3 Table A1-5

Figure A1-1

Correction Factor, K2, for Angle of Attack, , of the Flow ....................................................................................7 Abutment Shape Coefficients*.......................................................................................................................................9

Fall Velocity of Sand-sized Particles with Specific Gravity of 2.65 ...............................................................5

Figure A1-2

Abutment Shape ...................................................................................................................................................................9

Figure A1-4

Determination of Length of Embankment (L') Blocking Live Flow for Abutment Scour Estimation

Figure A1-3 Figure A1-5 Figure A1-6

Orientation of Embankment Angle, θ, to the Flow ................................................................................................9

................................................................................................................................................................................................... 10

Definition of Dmxb and Dmnc ..................................................................................................................................... 13 Downstream Erosion at Groundsill ......................................................................................................................... 14

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

A.

A.1

Estimating Scour Reasonable and prudent hydraulic analysis of a bridge design or flood control project like a revetment requires that an assessment be made of the proposed structure’s vulnerability to undermining due to potential scour. Because of the extreme hazard and economic hardships posed by a rapid bridge collapse or other structure collapse, special considerations must be given to selecting appropriate flood magnitudes for use in the analysis. The following discussions provide a description of stream stability and scouring mechanism.

Scour Analysis

Scour is the result of the erosive action of flowing water excavating and carrying away material from the bed and banks of streams. Potential scour can be a significant factor in the analysis of a stream crossing system. The design of a crossing system involves an acceptable balance between a waterway opening that will not create undue damage by backwater or suffer undue damage from scour and a crossing profile sufficiently high to provide the required traffic service. The rates of scour in different materials and under different flow conditions depend on erosive power of the flow, erosion resistance of the material, and a balance between sediment transported into and out of a section.

With erosion-resistant materials, final, worst case, or equilibrium scour may not be reached in any one flood but may develop over a long series of events. The methods currently available do not specifically accommodate cohesive bed materials or time-dependency. Therefore, consider the results of any scour calculations only as an indication of the maximum potential scour. Use judgment to decide whether or not calculated depths are likely for the given site conditions and life expectancy of the bridge. Present applicable technology dictates that bridge scour should be evaluated as interrelated components:

 Long-term profile (aggradation / degradation), plan-form (lateral channel

movement) changes.

 Contraction scour / deposition. A.1.1

 Local scour.

Long-Term Profile and Plan Form Changes

Long-term profile changes can result from stream bed profile changes that occur from aggradation and / or degradation:  Aggradation is the deposition of bedload due to a decrease in stream sediment

transport capacity that results from a reduction in the energy gradient.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

 Degradation is the scouring of bed material due to increased stream sediment

transport capacity that results from an increase in the energy gradient.

Forms of degradation and aggradation impose a permanent future change for the stream bed elevation at a bridge site where they can be identified.

Plan-form changes are morphological changes (e.g., meander migration, bank widening). The lateral movement of meanders can threaten bridge approaches and increase scour by changing flow patterns approaching a bridge opening. Bank widening can cause significant changes in the flow distribution and thus the bridge’s flow contraction ratio.

A.1.2

No reasonable, definitive methods are apparent for accurately estimating longterm natural scour. However, consider the potential for long-term natural scour. Generally, projections based an evaluation of the history of the site or ones similar to the site may suffice. Contraction

Channel contraction scour results from a constriction of the channel that may, in part, be caused by bridge piers in the waterway. Deposition results from an expansion of the channel or the bridge site being positioned immediately downstream of a steeper reach of stream. Highways, bridges and natural channel contractions are the most commonly encountered cause of contraction scour. Contraction scour occurs when the flow area of a stream at flood stage decreases either by a natural contraction or by a bridge. From continuity, a decrease in flow area results in an increase in average velocity and bed shear stress through the contraction, thus increasing erosive forces and removing more bed material from the contracted reach than is transported into the reach. This increase in transport of bed material from the reach lowers the natural bed elevation. As the bed elevation decreases, the flow area increases, and the velocity and shear stress decrease until relative equilibrium is reached, i.e., until the quantity of bed material that is transported into the reach is equal to that removed from the reach.

Depending on the stream flow, contraction scour can be either live-bed or clearwater. Live-bed scour occurs when the bed material upstream of the constriction is in motion. The scour that results at the constriction reflects equilibrium between the sediment transported into the section and that transported away from the section. Under live-bed conditions, scour holes created during the rising stage of a flood often refill during the recession stage. Clear-water scour occurs when the bed material is not in motion. The sediment transported into the contracted section is essentially zero. Clear-water scour occurs when the shear stress induced by the water flow exceeds the critical shear stress of the bed material. Generally, with clear-water scour, no refilling occurs during the recession of the flood due to the lack of sediment supply. During the initial stages of a flood, clear-water scour could occur followed by live-bed scour at higher flood stages.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

A.1.3

Local Scour

Local scour involves the removal of material around piers, abutments, spurs, and embankments.

Local scour is a function of the geometry of these features as they relate to the flow geometry. However, the importance of these geometric variables will vary. Increasing the pier or cofferdam width either through design or debris accumulation will increase the amount of local scour, but only up to a point in subcritical flow streams. After reaching this point, pier scour should not be expected to measurably increase with increased stream velocity or depth.

Armoring occurs because a stream or river is unable, during a particular flood, to move the more coarse material comprising either the bed or, if some bed scour occurs, its underlying material. Scour may occur initially but later become arrested by armoring before the full scour potential is reached for a given flood magnitude. When armoring does occur, the coarser bed material will tend to remain in place or quickly redeposit to form a layer of riprap-like armor on the stream bed or in the scour holes and thus limit further scour for a particular discharge. This armoring effect can decrease scour hole depths that were predicted based on formulae developed for sand or other fine-material channels for a particular flood magnitude. When a larger flood occurs than used to define the probable scour hole depths, scour will probably penetrate deeper until armoring again occurs at some lower threshold.

A.2

A.2.1

If armoring of the stream bed occurs, there may be a tendency for the stream to widen its banks to maintain a continuity of sediment transport. This could result in a more unstable, braided regime. Such instabilities may pose serious problems for bridges and flood control structures because they encourage further, difficultto-assess plan-form changes. Also, the effect of bank widening is to spread the approach flow distribution that, in turn, results in a more severe bridge opening contraction.

Bridge Scour Estimation

The following procedure for scour estimation is generally based on Evaluating Scour at Bridges (HEC18, 2001), and has been condensed and simplified. HEC18 (2001) should be consulted for further information and background.

Contraction Scour Conditions

One way to appraise whether clear-water scour or live-bed scour is occurring is through the critical velocity Vc for incipient motion of the D50 size of the bed material. This can be compared with the computed average velocity at the upstream approach. This approach is reasonable as long as the subject portion of channel is not heavily vegetated. If Vc < V (where V is cross sectional average velocity), the bed material is most likely in motion, and you can consider live-bed scour. If Vc > V the bed material probably is not in motion and you may assume

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

clear-water scour. The critical velocity is calculated using the equation by Laursen (1963): Equation A1-1

where: Vc

=

D50

=

y1

A.2.2

0.33 𝑉𝑉𝐶𝐶 = 6.19𝑦𝑦10.166 𝐷𝐷50

=

critical velocity above which material will be transported, (m/s)

average depth of flow in the main channel at approach section, (m) bed particle size in a mixture of which 50% are smaller, (m)

Live Bed Contraction Scour Equation

The average live bed contraction scour depth uses the modified version of Laursen’s live-bed scour equation (based on HEC18): Equation A1-2

𝑄𝑄2 0.857 𝑊𝑊1 𝑘𝑘1 𝑦𝑦2 = 𝑦𝑦1 ( ) ( ) 𝑄𝑄1 𝑊𝑊2

where:

𝑦𝑦𝑠𝑠 = 𝑦𝑦2 − 𝑦𝑦0

ys

=

average depth of contraction scour (m)

y1

=

average depth in the main channel or floodplain at the approach section (m)

y2

yo

Q1 Q2

=

=

=

average depth after scour in the contracted section (m). This is taken as the section inside bridge at the upstream end.

average depth in the main channel or floodplain at the contracted cross section before scour (m) flow in the main channel or floodplain at approach section (m3/s)

=

flow in the main channel or floodplain at the contracted cross section (m3/s)

=

bottom width of main channel or floodplain at the contracted cross section less pier widths (m)

Wl

=

K1

=

an exponent determined using the Table A1-1.

ω

=

fall velocity of bed material (m/s) based on Figure A1-1

W2

V*

=

bottom width of the main channel or floodplain at the approach section (m)

shear velocity in upstream cross section (m/s) = (gy1S1)0.5

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

S1

Table A1-1

=

slope of energy grade line of main channel (m/m)

Exponent K1 for Live Bed Contraction Scour

V*/

K1

2.0

0.69

Mostly suspended bed material discharge

Figure A1-1

Mode of Bed Material Transport

Fall Velocity of Sand-sized Particles with Specific Gravity of 2.65

Source: HEC18

A.2.3

Clear Water Contraction Scour Equation

The average depth in the contracted cross section including clear-water contraction scour is expressed in following: Equation A1-3

] 2⁄ 3 2 𝐷𝐷𝑚𝑚 𝑊𝑊

3/7

ys

=

y2 –y0 (average contraction scour depth)

y2

=

average depth in the contracted section after contraction scour, (m)

where: yo A-5

𝑦𝑦2 = [

0.025𝑄𝑄 2

=

average existing depth in the contracted section, (m)

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

Q

=

discharge through the bridge or on the set-back overbank area at the bridge associated with the width W, (m)

D50

=

median particle size diameter (m) (a suggested minimum for cohesive soils is 0.004 in. or 0.1 mm)

W

Dm

A.3 A.3.1

=

=

bottom width of the contracted section less pier widths, (m)

diameter of smallest non-transportable particle in the bed material (1.25 D50) in the contracted section (m)

During a flood, bridges over streams with coarse bed material are often subjected to clear-water scour at low discharges, live-bed scour at the higher discharges, and then clear-water scour on the falling stages. Clear-water scour reaches its maximum over a longer period of time than live-bed scour because clear-water scour occurs mainly in coarse bed material streams. In fact, local clear-water scour may not reach a maximum until after several floods. Maximum local clear-water pier scour is about 10% greater than the equilibrium local live-bed pier scour.

Local Scour

Pier Scour Equation

Either live-bed or clear-water scour may occur at piers. In both cases, it is estimated using CSU equation, which assumes live-bed scour in non-cohesive bed material. Equation A1-4

where:

𝑦𝑦𝑠𝑠 = 2𝐾𝐾1 𝐾𝐾2 𝐾𝐾3 𝐾𝐾4 𝑎𝑎0.65 𝑦𝑦10.35 𝐹𝐹𝐹𝐹10.43

ys

=

depth of pier scour (m)

K1

=

correction factor for pier nose shape (see ‘Correction Factor K1 for Pier Nose Shape’ table below)

y1

K2

K3

=

=

correction factor for bed condition (see ‘Correction Factor K3 for Bed Condition’ table below)

=

pier width (m)

=

Fr1

=

v1 g

correction factor for angle of attack (see ‘Correction Factor K2 for Angle of Flow Attack’ table below)

=

K4

a

flow depth directly upstream of pier (m)

=

=

correction factor for armoring of bed material (The value varies only for a bed material D50 in excess of 60 mm) Froude Number of flow directly upstream of pier

mean velocity of flow directly upstream of the pier (m/s) gravitational constant (9.81 m/s2)

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

Table A1-2

Correction Factor, K1, for Pier Nose Shape

Shape of Pier Nose

K1

Square nose

1.1

Round nose

1.0

Circular cylinder

1.0

Group of cylinders

1.0

Sharp nose

0.9

Table A1-3

Correction Factor, K2, for Angle of Attack, , of the Flow

Angle

L/a=4

L/a=8

L/a=12

0

1.0

1.0

1.0

15

1.5

2.0

2.5

30

2.0

2.75

3.5

45

2.3

3.3

4.3

90

2.5

3.9

5.0

Angle = skew angle of flow; L = length of pier (in direction of flow), m

Increase in Equilibrium Pier Scour Depths, K3, for Bed Condition

Table A1-4

Bed Condition

Dune Height m

K3

Clear-Water Scour

N/A

1.1

Plane bed and Anti-dune flow

N/A

1.1

3> H  0.6

1.1

9> H  3

1.2 to 1.1

H9

1.3

Small dunes Medium dunes Large dunes

The correction factor K4 decreases scour depths for armoring of the scour hole for bed materials that have a D50 equal to or larger than 2.0 mm and D95 equal to or larger than 20 mm. The correction factor then is as follows: If D50 < 2 mm or D95 < 20 mm, then k4 = 1

If D50 > or equal 2 mm and D95 >or equal 20 mm, then K4 = 0.4 (Vr)0.15 where: Vr

=

(V1 – VicD50) / (VcD50 – VicD50) > 0

VicDx

=

approach velocity (m/s) required to initiate scour at the pier for the grain size Dx (m)

And

VicDx VcDx

A-7

=

=

0.645 (Dx/a)0.053 VcDx

critical velocity (m/s) for incipient motion for grain size Dx (m)

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

VcDX

=

6.19 y10.166 Dx0.333

y1

=

depth of flow just upstream of the pier, excluding local scour, (m)

where:

V1

Dx

A.3.2

A.3.3

=

=

velocity of the approach flow just upstream of the pier, (m/s) grain size for which x percent of bed material is finer, (m)

For complex pier foundations such pile groups, pile groups and pile caps, pile groups, pile caps and solid piers exposed to flow, detailed scour estimation is referred to in Evaluating Scour at Bridges (HEC18, 2001).

Scour at Abutments

As a check on the potential depth of scour to aid in the design of the foundation and placement of rock riprap and / or guide banks, Froehlich's (1989) live-bed scour equation or the HIRE equation in FHA (2001[2]) can be used.

Froehlich's Live-Bed Abutment Scour Equation

Froehlich’s (1989) equation for abutment scour is as follows: Equation A1-5

where:

𝐿𝐿′ 𝑦𝑦𝑠𝑠 = 2.27𝑦𝑦𝑎𝑎 𝐾𝐾1 𝐾𝐾2 ( ) 𝑦𝑦𝑎𝑎

0.43

𝐹𝐹𝐹𝐹 0.61 + 1

ys

=

scour depth, m

K2

=

coefficient for angle of embankment to flow

K1

=

coefficient for abutment shape (Table A1-5)

K2

=

θ >90

if embankment points upstream

θ 25). Where this is not the case, Equation A1-8 may be adopted. Equation A1-7

where: ys

=

equilibrium depth of scour, m

a

=

structure length projecting normal to the flow, m

y1

Fr

A.5

A.5.1

𝑦𝑦𝑠𝑠 = 4𝑦𝑦1 𝐹𝐹𝑟𝑟0.33 = =

average upstream flow depth in the main channel or on the overbank, outside the influence of the structure, m upstream Froude Number outside the influence of the structure Equation A1-8

𝑎𝑎 0.4 𝑦𝑦𝑠𝑠 = 1.1𝑦𝑦1 ( ) 𝐹𝐹𝑟𝑟0.33 𝑦𝑦1

Scour at Longitudinal Structures

The following provides relationships for estimating scour for longitudinal structures, such as sheet pile walls. For the specific case of estimating scour along a vertical wall with an unconstrained valley width, reference should be made to Section 4.3.4 of HEC23 (2009).

Scour with Flow Parallel to a Vertical Wall

Scour occurs at longitudinal structures when the flow is parallel to a wall due to the change in friction presented by the wall.

HEC23 (2009) presents a methodology for calculating the scour, as provided in Equation A1-9. Equation A1-9

where: ys

=

equilibrium depth of scour, m

Fr

=

upstream Froude Number

y1

A-11

𝑦𝑦𝑠𝑠 = 𝑦𝑦1 (0.73 + 0.14𝜋𝜋𝐹𝐹𝑟𝑟2 ) =

average flow depth in the main channel, m

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

A.5.2

Scour with Flow Impinging at an Angle on a Vertical Wall

When flow arrives at an angle to the vertical wall, there is additional scour that can occur. HEC23 (2009) provides an estimation of the scour depth for this scenario and this is shown in Equation A1-10. Equation A1-10

𝑦𝑦𝑠𝑠 = 𝑦𝑦1 [(0.73 + 0.14𝜋𝜋𝐹𝐹𝑟𝑟2 )𝑐𝑐𝑐𝑐𝑐𝑐∅ + 4𝐹𝐹𝑟𝑟0.33 𝑠𝑠𝑠𝑠𝑠𝑠∅]

Where the parameters are the same as those for Equation A1-7 and:

A.6

θ = angle between the impinging flow direction and the vertical wall

Scour at Bends

Flow behaviour around a bend will result in higher scour on the outside of the bend than that of the inside of the bend. In the design of revetments and dikes, it is important to understand the maximum depth of scour that is expected at the outside of the bend.

HEC23 (2009) provides a method for estimating the expected depth on the outer part of the bend. This is the long term expected depth based on the bend characteristics. It can be used to subsequently determine the maximum scour that is expected. Equation A1-11

where:

𝑅𝑅𝑐𝑐 𝑊𝑊 𝐷𝐷𝑚𝑚𝑚𝑚𝑚𝑚 = 1.8 − 0.051 ( ) + 0.0084 ( ) 𝐷𝐷𝑚𝑚𝑚𝑚𝑚𝑚 𝑊𝑊 𝐷𝐷𝑚𝑚𝑚𝑚𝑚𝑚

Rc

=

centreline radius of the bend, m

Dmxb

=

maximum water depth in the bend, m

W

Dmnc

=

=

width of the bend, m

average water depth in the crossing upstream of the bend, m

A-12

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

Figure A1-5

Definition of Dmxb and Dmnc

Source : HEC23, 2009

A.7

Estimation of Bed Scour at Groundsills For the purposes of estimating the downstream scour channel beds resulting from groundsills, it has been assumed that:  

Flow is occurring in unsubmerged flow conditions (a conservative assumption) That the drop is vertical

A sketch of a typical vertical drop groundsill, with unsubmerged flow occurring, is shown in Figure A1-5. The scour depth, ds, may be found from the equation: where: ds

=

local scour depth for a free overfall, measured from the streambed downstream of the drop (m)

Ht

=

total drop in head, measured from the upstream to downstream energy grade line (m)

q

dm Ku

A-13

𝑑𝑑𝑠𝑠 = (𝐾𝐾𝑢𝑢 𝐻𝐻𝑡𝑡0.225 𝑞𝑞 0.54 ) − 𝑑𝑑𝑚𝑚

= = =

discharge per unit width (m3/s/m) tailwater depth (m) 1.9

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

The subscripts ‘u’ and ‘d’ refer to upstream and downstream of the channel drop respectively.

Note that the estimated depth of scour is independent of the grain size of the bed material in the above equation. It is assumed that the bed will scour regardless, but that the rate of scour will vary depending on the bed material. The check dam must be designed structurally to withstand the forces of water and soil assuming that the scour hole is as deep as estimated from the above equation, in order to ensure successful operation over the design life of the groundsill. Figure A1-6

Downstream Erosion at Groundsill

Source: FHA, 2009

A.8

References Federal Highway Administration, 2001. Evaluating Scour At Bridges Hydraulic Engineering Circular No. 18, U.S. Department of Transportation, Washington.

Federal Highway Administration, 2001 [2], River Engineering for Highway Encroachments - Highways in the River Environment, FHWA NHI 01-004, Federal Highway Administration, Hydraulic Series No. 6, Washington, D.C.

US Department of Transportation – Federal Highway Administration (2009) [HEC23]. Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance, 3rd Edition, Volume 1 and 2, Hydraulic Engineering Circular 23, September, Publication Number: FHWA-NHI-09-112. Froehlich, D.C., 1989, Abutment Scour Prediction, Presentation, Transportation Research Board, Washington, D.C.

A-14

Annex B

Sediment Transport Concepts

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

B.1

Overview A brief introduction to key sediment transport concepts is provided as the topic is complex and there are many different approaches to analysis. This section is largely based on FHA (2012). Sediment transport involves complex processes that interact to produce the existing channel form and future channel adjustments. The amount of material transported or deposited in a stream under a given set of conditions is the result of the interaction of two groups of variables that influence:





Quantity and Quality of Sediment - The variables depend on the geology and topography of watershed; magnitude, intensity, duration, distribution, and season of rainfall; soil moisture conditions; vegetal cover; cultivation and grazing; surface erosion and bank cutting

Capacity of the Stream to Transport the Sediment - The variables depend on hydraulic properties of the stream channel which are fluid and flow related properties including: slope, roughness, hydraulic radius, discharge, velocity, velocity distribution, turbulence, tractive force, viscosity and density of the fluid sediment mixture, and size and gradation of the sediment.

These variables are not all independent and, in some cases, their effect is not definitely known. The variables which control the amount of sediment brought down to the stream are subject to wide variation, not only between streams but at a given point of a single stream. The quantitative analysis of any particular case is extremely difficult. It is practicable to measure the sediment discharge over a long period of time and record the results, and from these records to determine a soil loss from the area.

The variables that deal with the capacity of the stream to transport solids are subject to mathematical analysis as these variables are closely related to the hydraulic variables which control the capacity of the stream to carry water.

Many aspects of hydrology play a role in sediment transport analyses including peak flow rates, individual flood hydrographs, and the duration of flow. The entire range of flow may be significant because even though the highest flows have the highest rates of sediment transport, lower flows may have significantly longer durations and produce the greatest cumulative sediment transport.

As channels respond and adjust to changes in flow and sediment supply, changing watershed conditions often result in changes in channel geometry. Channel geometry, bed material, and vegetation determine hydraulic variables (such as velocity and depth), which in turn control sediment transport capacity. Therefore, sediment transport and channel stability depend not only on the specific physical processes, but also the history of natural and human-induced factors in the watershed.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

B.2

Sediment Continuity Sediment transport capacity is primarily a function of sediment size and the hydraulic properties of the channel. When the transport capacity of the flow equals sediment supply from upstream, a state of equilibrium exists.

Application of the sediment continuity concept to a channel reach illustrates the relationship between sediment supply and transport capacity. During a given time period the amount of sediment coming into the reach minus the amount leaving the downstream end of the reach equals the change in the amount of sediment stored in that reach. The sediment inflow to a given reach is defined by the sediment supply from the watershed and channel (upstream of the study reach plus lateral input directly to the study reach). The transport capacity of the channel within the given reach defines the sediment outflow.

When the sediment supply is less than the transport capacity, erosion (degradation) will occur in the reach so that the transport capacity at the outlet is satisfied, unless controls exist that limit erosion. Conversely, when the sediment supply is greater than the transport capacity, deposition (aggradation) will occur in the reach.

Controls that limit erosion may either be human induced or natural. Humaninduced controls included bank protection works, grade control structures, and stabilized bridge or culvert crossings. Natural controls can be geologic, such as outcrops, or the presence of significant coarse sediment material in the channel. The presence of coarse material can result in the formation of a surface armour layer of larger sediments.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

B.3

Sediment Properties Knowledge of the properties of sediment particles is important, as they indicate the behaviour of the particles in their interaction with the flow. Important sediment properties are discussed below. Particle Size

Of the various sediment properties, physical size has by far the greatest significance and other parameters such as fall velocity tend to be related to physical size. In general, sediments have been classified into boulders, cobbles, gravels, sands, silts, and clays on the basis of their nominal or sieve diameters. Particle Shape

Shape refers to the overall geometrical form of a particle. Sphericity is defined as the ratio of the surface area of a sphere of the same volume as the particle to the actual surface area of the particle. Roundness is defined as the ratio of the average radius of curvature of the corners and edges of a particle to the radius of a circle inscribed in the maximum projected area of the particle. Because of simplicity and effectiveness of correlation with the behaviour of particles in the flow, the most commonly used parameter to describe particle shape is the Corey shape factor, Sp, (Refer FHA 2001). Fall Velocity

The prime indicator of the interaction of sediments in suspension within the flow is the fall velocity of sediment particles. The fall velocity of a particle is defined as the velocity of that particle falling alone in quiescent, distilled water of infinite extent. Sediment Size Distribution

Several methods of obtaining sediment size distribution are available. Each method for size distribution analysis is appropriate for only a particular range of particle sizes. Specific Weight

Specific weight is weight per unit volume and is expressed in grams per cubic centimeter. Porosity

The porosity of granular materials is the ratio of the volume of void space to the total volume of an undisturbed sample. Angle of Repose

The angle of repose is the maximum slope angle upon which non-cohesive material will reside without moving. It is a measure of the inter-granular friction of the material and is different for dry versus submerged conditions.

B-3

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

B.4

Sediment Transport Concepts

B.4.1

Initiation of Motion The initiation or ceasing of motion of sediment particles is involved in many geomorphic and hydraulic problems including stream stability and scour at highway bridges, sediment transport, erosion, slope stability, stable channel design, and design of riprap. These problems can only be addressed when the threshold of sediment motion is fully understood. Beginning of motion can be related to when the shear stress exceeds the critical shear stress of the grains of sediment. Shear stress on the bed of the channel can be estimated as: Equation B4-1

τ0=γyS0

where: τ0

=

γ

=

y

S0

shear stress (Pa)

=

flow depth (m)

=

bed slope

specific weight of water (N/m3)

The critical shear stress for a particle can be estimated as: Equation B4-2

τc=ks Ds (γs-γ)

where: τc

=

critical shear stress for beginning of motion (Pa)

Ds

=

particle size (m)

ks

γs

=

=

Shields parameter

specific weight of particle (N/m3)

The Shields parameter ranges from 0.03 to 0.10 for natural sediments and depends on particle shape, angularity, gradation and imbrication. The use of 0.047 is common for sand sizes. When the shear stress of the flow exceeds the critical shear stress of the particle, the channel bed begins to mobilize and bed material is transported downstream. Particle motion begins as sliding and rolling of individual particles along the bed. It is important to recognize that the Shields equation is not a sediment transport equation because it does not provide any estimate of the amount of sediment in motion. It is also important to note that only the shear stress acting on the particles, or grain friction, should be used in applying this relationship.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

B.4.2

Modes of Sediment Transport Once the critical shear stress is exceeded, bed material begins to move (roll, slide) along the bed surface. This material is referred to as bed load or contact load because it is in almost continuous contact with the bed. For small amounts of positive excess shear stress this is the only mode of bed material transport. As excess shear stress increases, turbulence begins to suspend some of the particles. The turbulence acts to mix the particles in the water column and gravity causes the particles to settle. Therefore, bed material can also be transported downstream as suspended bed material load. The two types of bed material load are illustrated in Figure B-1. Figure B-1

Suspended and Bed Load

Source: FHA, 2001

The suspended bed material load depends on the interaction between gravity and turbulence. Because gravity is causing particles to settle, they are concentrated near the bed. Turbulence mixes the particles in the water column and, depending B-5

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

on the size and specific weight of the particles, relatively few particles may reach the surface.

B.4.3

Larger particles have greater fall velocities and therefore for a given level of turbulence large particles will remain close to the bed. Finer particles are mixed higher into the flow and have higher concentrations. Extremely fine particles have nearly uniform concentrations, primarily silts and clays, and have very small fall velocities. They are defined as wash load, which are derived primarily from upland erosion and bank erosion of floodplain materials. Wash load material is not found in appreciable quantities in the channel bed. In summary, bed material is transported in contact with the bed (bed load) and in suspension (suspended bed material load). The total sediment load transported by the channel also includes wash load, which is supplied to the channel rather than derived from the bed. In coarse bed channels, such as cobble-bed and boulder-bed streams, sand may act as wash load because it is not found in appreciable quantities in the bed and because the supply is far less than the channel capacity to transport this size.

Effects of Bed Forms at Stream Crossings

In sand-bed streams, sand material is easily eroded and is continually being moved and shaped by the flow. The interaction between the flow of the water-sediment mixture and the sand-bed creates different bed configurations which change the resistance to flow, velocity, water surface elevation, and sediment transport. At high flows, most sand-bed stream channels shift from a dune bed to a transition or a plane bed configuration. The resistance to flow is then decreased by one-half to one-third of that preceding the shift in bed form. The increase in velocity and corresponding decrease in depth may increase scour around bridge piers, abutments, spurs or guide banks and may increase the required size of riprap.

B.4.4

Another effect of bed forms on highway crossings is that with dunes on the bed, there is a fluctuating pattern of scour on the bed. Methods for computing bed-form geometry can be found in Julien and Klaassen (1995) and Karim (1999). With a dune bed, the Manning n could be more than twice as large as a plane bed. A change from a dune bed to a plane bed, or the reverse, can have an appreciable effect on depth and velocity. In the design of a bridge or a stream stability or scour countermeasure, it is good engineering practice to assume a dune bed (large n value) when establishing the water surface elevations, and a plane bed (low n value) for calculations involving velocity.

Sediment Transport Equations

Equations for predicting bed material sediment transport differ depending on the mode of sediment transport. ASCE (2008) includes 16 bed load equations. The Meyer-Peter and Müller (1948) equation is considered to be a classic bed load equation (Refer FHWA 2001). The HDS 6 manual (FHWA 2001) includes 20 sediment transport equations and discusses their applicability to various grain sizes. The HEC-RAS Reference Manual (USACE 2010) and the SAM reference manual (USACE 2002) include information on the range of data (particle size, specific gravity, velocity, depth, slope, channel width and temperature) used to B-6

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

develop many of the sediment transport equations used for sand and gravel sizes. Any equation that is considered for use should be evaluated for applicability to the specific conditions.

B.4.4.1

An overview of three methods to estimate the bed material sediment transport is presented as derived primarily from FHWA (2001). Upstream sediment input to the study area should be considered. Detailed information and key assumptions for these methods should be studied in FHWA (2001) before use of the method as following as estimation of sediment transport is a complex. Meyer-Peter & Muller Equation

A simplified version of the Meyer-Peter & Muller equation is widely used. This equation was based on experiments with sand particles of uniform sizes, sand particles of mixed sizes and density, and natural gravel, lignite and barite. Care should be taken when applying this equation to other conditions.

The Meyer-Peter and Muller formula is often written in the form: Equation B4-3

qb = K(𝜏𝜏−𝜏𝜏

where:

𝑐𝑐 )

3/2

3/2

1 K= [ ] 1/3 𝛾𝛾𝑠𝑠− 𝛾𝛾 2/3 𝛾𝛾 𝐵𝐵 (𝑔𝑔) ( 𝛾𝛾 ) 𝑠𝑠

≅

12.9

𝛾𝛾𝑠𝑠 √𝜌𝜌

Qb 𝐾𝐾𝐵𝐵 3/2 𝜏𝜏 = ( ) ( ) 𝛾𝛾𝛾𝛾𝑜𝑜 𝑆𝑆𝑓𝑓 Q 𝐾𝐾𝑟𝑟

qB

=

𝜏𝜏 = 𝐵𝐵 ′ (𝛾𝛾𝑠𝑠 − 𝛾𝛾)𝐷𝐷𝑚𝑚

metric-tons/day/meter (Tons/day/foot)

Qb

=

water discharge quantity determining the bed-load transport, m3/s (cfs)

D90, Dm

=

particle size, mm (both SI and English units)

Q

=

total water discharge m3/s (cfs)

The quantity Dm is the effective diameter of the sediment given by: Dm =

Σ| p| DS| 100

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

where: p|

=

Percentage by weight of that fraction of the bed material geometric mean size, DS| V KB f' =√ b Kr 8 √gRSf

Where fb’, the Darcy-Weisbach bed friction bed friction factor for the grain roughness, fb’ is determined from the Nikuradse pipe friction data with pipe diameter equal to four times the hydraulic radius and K6 = D90. If the boundary is hydraulically rough, (VxD90/ν≥100), Kr is given by: 𝐾𝐾𝑟𝑟 =

26

𝐷𝐷901/6

The ratio Qb/Q for rectangular channels is given by: Qb = Q

And for trapezoidal channels is: Qb = Q

1+

2y

1

n

1+ ( Wo ) ( nw ) 1

2yo (1+H25 ) W

1/2

n

b

( nw ) b

2

3/2

The term Nb is the Manning’s roughness coefficient for the bed of rectangular channels: 2/3 2yo nw 3/2 Nb =n {1+ (1- ( ) )} w n And for trapezoidal channels

where:

2/3

nw 3/2 [1- ( ) ]} n

n, nb, nw =

roughness coefficients of the total stream, of the bed, and of the banks, respectively

W

bottom width

H5

B-8

1/2

2yo (1+H52 ) nb =n {1+ w

= =

horizontal side slope related to one unit vertically

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

B.4.4.2

Einstein Method

The Einstein suspended load equation is described in detail in FHWA (2001). The complex method is best suited to computer models and calculations. It is a solution to the general suspended load equation: Equation B4-4 yo

where:

a

q6

=

suspended load discharge per unit width

V

=

velocity at height y above the bed

γs C

B.4.4.3

qs = γs ∫ vcdy

=

=

weigh per unit volume of suspended sediment volumetric concentric at height y above the bed

It applies logarithmic velocity distribution and solves a number of equations for different grain sizes. Further details are available in FHWA (2001).

Colby Method

Colby’s method is a graphical method for estimating total load, and provides a reasonable method for hand calculations, and in particular, is useful to cross check other methods.

Colby developed four graphical relationships which are shown in the following figures to determine the bed sediment discharge. The curves were derived based on a large amount of streams and flume data, and were guided by the Einstein bedload function. However, a significant amount of the curves were extrapolated, as indicated by the dashed lines in the following graphs. Please note that all figures are in English Units, and will require conversion to metric units.

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

Figure B- 2

B-10

Relation Of Discharge of Sands to Mean Velocity for Six Median Sizes of Bed Sands, Four Depths of Flow, and a Water Temperature of 60°F

Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

Figure B- 3

Colby's Correction Curves for Temperature and Fine Sediment

The general process for applying this method is:

1. Uncorrected sediment discharge qn for the given V, yo, and D50 can be found from Figure B-2 by first reading qn knowing V and D50 for two depths that bracket the desired depth and then interpolating on a logarithmic graph of depth versus qn to get the bed sediment discharge per unit width. 2. Two correction factors k1 and k2 shown in Figure B-2 account for the effect of water temperature and fine suspended sediment on the bed sediment discharge. If the bed sediment size falls outside the 0.20 mm to 0.30 mm range, the factor k3 from Figure B-2 is applied to correct for the effect of sediment size.

3. Unit bed sediment discharge qT corrected for the effect of water temperature, presence of fine suspended sediment and sediment size is given by the equation: Equation B4- 5

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Design Guidelines, Criteria and Standards: Volume 3 – Water Projects Design

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References American Society of Civil Engineers [ASCE] (2008). Sedimentation Engineering Processes, Measurements, Modeling, and Practice, M.H. Garcia (ed.), ASCE Manuals and Reports on Engineering Practice No. 110. Julien, P.Y. (2010). University Press.

Erosion and Sedimentation, second edition, Cambridge

Simons, D.B. and F. Senturk (1992). Resources Pub.

Sediment Transport Technology, Water

U.S. Army Corps of Engineers (2002). SAM Hydraulic Design Package for Channels, Coastal and Hydraulics Laboratory, U.S. Army Engineer Research and Development Center. US Department of Transportation Federal Highways Division [FHWA] (2001). River Engineering for Highway Encroachments – Highways in the River Environment, Hydraulic Design Series No. 6. US Department of Transportation Federal Highways Division [FHWA] (2012). Stream Stability at Highway Structures, 4th Edition, Hydraulic Engineering Circular No. 20, April.

Yang, C.T. (2003). Sediment Transport: Theory and Practice, Krieger Publishing Company.

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