Flood and Lahar Control

Flood and Lahar Control  ALAS, NORVIE P. QUIJANO, JAMES GOODW QUIJANO, GOODWILL ILL TEJADA, TEJAD A, ALLAN CEDR CEDRIC IC P. TOREJA, ROMARK  CE-4202

Flood Control

Flood Control  Flood - an overflowin overflowing g of a large large amount of water  beyond its normal confines, especially over what is normally dry land.  Flood Control - refer referss to all metho methods ds used used to reduce or prevent the detrimental effects of fl floo ood d wa watter ers. s.

Types of flooding  Areal – when floods happen on flat or low-lying areas and when  water is supplied by rainfall or snowmelt more rapidly than it can eith ei ther er in infi filt ltra rate te or ru run n of off. f.  Riverine - occurs when rivers rivers burst burst their their banks banks as a result result of sustained or intense rainfall. Flooding can be worsened by melting snow.  Estuarine and Coastal Coastal - is commonly commonly caused caused by a combination combination of sea tidal surges cause caused d by by winds and low barome barometric tric press pressure, ure, and they may be exacerbated by high upstream river flow.

Types of flooding Urban Flooding - is the the inundation inundation of land land or propert property y in a built environment, particularly in more densely populated areas, caused  by rainfall overwhelming the capacity of drainage systems, systems, such as sto storm rm sewer sewers. s. Catastrophic - is usually usually assoc associated iated with major major infrast infrastructur ructuree failures such as the collapse of a dam, but they may also be caused by drainage channel modification from a lan lands dsli lide de,, ea eart rthq hqua uake ke or vo volc lcan anic ic erupti eruption on..

 AREAL FLOODING

ESTUARINE/COASTAL FLOODING FLOODING

RIVERINE FLOODING

URBAN FLOODING

CATASTROPHIC FLOODING

Lahar Control

Introduction

Definitions Volcanic Debris Flow (Lahar) – A type of mudflow or debris debris flow composed of slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along the river valley. Lahars are extremely destructive: they can flow tens of meters per second, be 140 meters (460ft) deep, and destroy and structures in their path.

Volcanic Debris Flow (Lahar)

Definitions Volcanic Debris Flow (Lahar) – Any mass movement involving volcanically volcanically derived, sand-size material or finer. – This may be total mass moved. – More commonly this serves as a matrix for the movement of larger volcanic clasts. – Water rich

Definitions Volcanic Debris Flow (Lahar) • Thus range from slumped deposits of ash and hyaloclasti hyaloc lastites tes to megabrec megabreccias. cias. • Deposits may be relatively local (few 100 m’s in length and few meters thick) • To regional (10’s of km’s long and > 100 m in thickness)

Formation • During eruptions • Reworking of volcanic debris during times of quiescence or after  volcano has become extinct • Ways of formation formation:: – Heavy rains – Melting snow or ice (Jokulhaups) – Walls of crater lake lake collapse – Hydrotherm Hydrothermal al alteration- clay-rich rocks – Caldera collapse and mass wasting of walls – Earthquakes – Pyroclastic flows mix with water

Mass Movement • Once under way controlling factors for flow length and speed are: – Grain size – Grain size distribution-electrochemical forces-clay and silt-size – Solid to water water ratioratio- lamin laminar ar flow Thicker near source Follows valleys and depressions, fans out away from  valleys Can cause erosion by undercutting banks of streams or those of steep steep slopes- adds to sediment sediment load (bulking up)

Since mid-June 1991, when violent eruptions triggered Mount Pinatubo's first lahars in 500 years, a system to monitor and warn of lahars has been in operation. Radiotelemetered rain gauges provide data on rainfall in lahar source regions, acoustic flow monitors on stream banks detect ground vibration as lahars pass, and manned  watchpoints further confirm that lahars are rushing rushing down Pinatubo's slopes. This system has enabled warnings to  be sounded for most but not all major lahars lahars at Pinatubo, saving hundreds of lives. Physical preventative measures  by the Philippine government were were not adequate to stop over 20 feet (6.1 m) of mud from flooding many villages around Mount Pinatubo from 1992 through 1998.

TITAN2D is a free software application developed by the Geophysical Mass Flow Group at the State University of New York (SUNY) at Buffalo. TITAN2D  was developed for the purpose of simulating simulating granular granular flows (primarily geological mass flows such as debris avalanches and landslides) over digital elevation models (DEM)s of natural terrain. The code is designed to help scientists and civil protection authorities assess the risk of, and mitigate, hazards due to dry debris flows and avalanches. TITAN2D combines numerical simulations simulations of a flow with digital elevation data of natural terrain supported through a Geographical Information System (GIS) interface such as GRASS.

Types of Lahar & Flood Control Structures

Types of Lahar & Flood Control Structures •

Check Dams

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Retaining Walls

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Bunding

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Sausage Groynes

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Gabion/Mattress Groynes

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Paved Drains

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Reservoirs

Types of Lahar & Flood Control Structures  A dam is a barrier that impounds impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees (also known as dikes) are used to manage or prevent water flow into specific land regions.

Types of Lahar & Flood Control Structures Check Dams •

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These are small gravity dams, usually constructed with rocks and mortar or concrete, of variable height and  width. This type of structures are located in small or medium sized gullies to stabilize riverbed slopes and prevent soil erosion. Check dams, protect gullies from being eroded by rainfall and runoff impact.

CHECK DAMS

Types of Lahar & Flood Control Structures  Retaining Walls •

These are rock/concrete block structures built on steep slopes anywhere in the watershed, where the erosion of the base foundation threatens lands and/or homes.

RETAINING WALLS

Types of Lahar & Flood Control Structures  Bunding •

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Bunding is the general name used in Jamaica for flexible flexible structures of variable thickness and length, composed of galvanized wire mesh, stone, wild-cane and riverbed materials. Bunding is used to prevent bank bank erosion and landslips and to protect agricultural lands from being flooded.

Types of Lahar & Flood Control Structures  Sausage Groynes Groynes •

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These are long, cylindrical, slightly flexible structures of  variable thickness, composed of wire wire and rocks. Groynes are placed Groynes placed at the foot of banks along along small, slow-moving streams, where there are signs of undermining and threats to permanent structures. Sausage groynes Sausage groynes are designed designed to stop such such action by allowing the bank to collapse to an angle of repose in such a manner as to form a moderate slope, which encourages the growth of vegetation.

Types of Lahar & Flood Control Structures Gabion/Mattress Groynes •

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These are long, flexible structures of variable thickness, composed of wire and rocks. They are placed on the shaped banks of large, fastmoving streams where severe erosion is occurring and many pose a danger to permanent structures. Mattress groynes are designed designed to prevent the further erosion of the riverbank. They trap soil particles to allow a build-up of soil; thereby encouraging the growth of  vegetation.

GABION WALLS

Types of Lahar & Flood Control Structures  Paved Drains made of of clay clay or earth earth fill fill mater material ial are are  1. Levees - are made  being used with some some structural modifications and have proven quite effective. The slopes of an earth fill levee should be no more than 2:1.

 2. Flood Walls Walls of reinforced reinforced concrete - require very little space and are often used to protect cities and developed areas. They are costly to construct, but involve minimal maintenance costs.

Types of Lahar & Flood Control Structures  Paved Drains  3. Channel Alterations Alterations - reduce floods by deepening and widening the channel by cutting meanders. Sometimes these works can have undesirable effects, by aiding the sediment transport process. Care must be taken when channel alternatives are considered.  4. Detention Ponds - ponds placed on and off-side, can minimize the damage to downstream structures by reducing peak flows. They should be considered in the design of downstream protective structures.

PAVED DRAINS

Types of Lahar & Flood Control Structures  Reservoirs •

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Reservoirs are one of the most direct methods of flood control through storing surface runoff; thus, attenuating flood waves and storing flood water to be redistributed  without exceeding downstream downstream flood conditions. For flood control, it is ideal to maintain the reservoir at the lowest level possible for storage. On the other hand, keeping the reservoir at a high level provides the ability to maintain low flows and hydropower production in droughts.

RESERVOIR