(n) Chapter 6 Open Drain Msma
Hydraulic Engineering CEWB222
OPEN DRAINS MSMA CHAPTER 6
Dr. Mohd Hafiz Zawawi Prof. Dr. Ir. Lariyah Mohd Sidek
DRAINAGE SYSTEMS IN MALAYSIA
Separate Drainage System (after O'Loughlin, 1998)
Combined Sewer System (after O'Loughlin, 1998)
Parts of an Malaysian Separate Urban Stormwater Drainage System (after O'Loughlin, 1998)
•property drainage system, •street drainage system, •trunk drainage system, and •receiving waters
Rigid Boundary Channel
Rigid Boundary Channel (Dry Period)
Rigid Boundary Channel Trunk Drain During Dry Period
Rigid Boundary Channel Wet Period
Rigid Boundary Channel Trunk Drain - Wet Period
SURFACE WATER DRAINAGE INTRODUCTION
The project is located in hilly and undulating areas, at Kg. Beting, Kuala Pilah. Planning and achieving sustainable development in such environment is particularly important in regard to drainage, flash flood, erosion and slope stability management. Therefore, the drainage system should be designed adequately, following the guidelines of Manual Saliran Mesra Alam – MSMA (DID, 2000) to prevent the instability of bank, avoid erosion and nuisance flooding.
A minor drainage system was designed to drain the stormwater collected from roofs and properties and attenuated through on-site detention facilities to a stable discharge point where it will not cause overflow, surcharge and erosion. Pipe drainage is suitable mainly for high-density where the land supply is limited and costly. The use of perforated pipe drains was not proposed in the hillside sloping areas due to the risk of increasing soil instability, therefore normal pipe drains were proposed. The drainage system in the hillsides areas was incorporated with the drop structures, cascading drains or energy dissipaters in order to avoid excessive velocities. Alternatively, a pipe system can be successfully used especially in small development, with the head loss being taken up in drop pits and similar structures.
Types Of Minor Drainage Facilities Interceptor Drain:
The interceptor drains are proposed at top of the slopes. These are trapezoidal in shape and similar to the toe drains. These drains are to be constructed with cast in situ concrete with BRC reinforcement.
Berm drains are proposed to collect stormwater from the intermediate slopes. These will be V-shaped concrete sections as shown in the drawings.
Toe drains are proposed at bottom of the slopes to collect and convey the storm runoff safely to the bottom of the hills. These facilities do not have any capability to improve the runoff quality. These are concrete drains of trapezoidal shapes with side slopes of 1:1.
Cascade drains are proposed at the hilly areas to match the terrain and to reduce the flow velocity. These drains will be in combination of concrete precast blocks and cast in situ sidewalls with weep holes without any strut. For the depth shallower than 750 mm, plastered brick walls are proposed to reduce the cost.
Pipe drains are proposed to avoid the problem of space availability and to increase the aesthetics of the surroundings. Perimeter drains consists of perforated RIBLOC SPIROLITE SERIES 2000 HDPE pipe (Type A). Perforated drains are proposed to allow infiltration (control at source) of the stormwaters. The proposed perimeter drains will help reduce the old practices of rapid discharge concept by reducing the velocity before entering into the pipes.
Ecological swale type B
Roadside and Secondary Drain
These drains are provided to collect and convey runoff from the paved road surfaces and perimeter drains. These drains are classified into two categories. The drain Type-B is SPIROLITE SERIES 2000 HDPE nonperforated pipe. These pipes are light-weight, easy to construct, last long and hydraulically efficient. The pipe sizes shall vary between 300 to 900 mm in diameter. The drain Type-C are open channels and do not provide facilities for significant water quality improvement option for the proposed perimeter drains
Vegetated swales are proposed at the areas with mild slopes where enough space is available. The small size of swale was designed to collect the runoff water from the nearby area before its flow into the inlet manholes, which are usually proposed at 12 and 50 m interval of the pipe and open drains, respectively. Perforated HDPE pipes are proposed below the swales to allow infiltration of the stormwater, as shown in the drawing. The swales will be shallow but wide enough to cater for the design floods. The swales have the capabilities to improve runoff quality through filtration. They are expected to trap sediment, heavy metals, hydrocarbons and nutrients from the storm runoff.
These are special types of drains required for the sport fields and mini stadium. The subsoil drainage system for the sport facilities and football field are provided to drain the surface water efficiently and ensure that the facilities are ready to use shortly after rainfall events. The subsoil drainage consists of RIBLOC UPVC pipes sizing from 160 mm to 200 mm. The subsoil drain pipes is designed at 6 m and 10 m intervals for stadium and other field respectively. An adequate gradient is provided from the middle of the field into the nearest outlet drainage system. In general the subsoil drain is surrounded with selected geotextile to avoid the penetration of granular material into the opening of the drain material. RIBLOC pipe system having a very uniform opening at minimum interval along the length of the pipe.
Road Side Drain
Road Side Drain
Types Of Major Drainage Facilities Monsoon Drain
The major drains are provided at the areas downstream from the roadside and secondary drains. According to the conventional practices these are used to be called as Monsoon drains. These drains are proposed to convey larger floods safely to the downstream areas. Open drains with natural sides and precast channel for dry weather flow are provided for monsoon drains. In order to maintain the ecological balance in the drainage system the major drains are proposed to be unlined but protected with reinforced mattress. If the side slopes are not protected with the proposed facilities, erosion will occur due to the high velocities during the floods. The channel sides are proposed to have slopes of 1:3 and to be protected with TRM (Turf Reinforcement Matting) or equivalent, as shown the drawings. At the slope areas energy dissipaters or drop structures will be provided to reduce the velocity of waters.
Not all the drainage outlets could be connected to the proposed regional facilities (lakes), which were large enough to cater room for the excess runoff due to the proposed development. As such, four wet ponds are proposed as community facilities for the control of stormwater at community level. About four percent (4%) of the drainage catchment was allocated for the ponds to cater room for the excess runoff from the respective sub-catchments. Multi level risers and outlet structures are proposed to control discharge from the ponds. The ponds and outlet structures are sized such that post-development flood peak values are less than those of pre-development conditions. The drainage system is expected to discharge better quality of storm runoff compared to the pre-development condition when there was no such facility to improve the storm runoff quality
Other Drainage Facilities Sump
Sumps are provided mainly for the perimeter drains where the gutter from the roofs will meet the drains. It is also provided for the open drains to reduce velocities, trap coarser particles and at the junctions of the drains. The sumps proposed in the open drains are designed different from the conventional practices. The sumps have drop (about 300mm) below the pipes invert which will act as catch basin and trap the coarser particles from the runoff. This is how the proposed facilities will help improve the runoff quality. The sumps are proposed to be concrete structures with gravel pack ( 12 – 50 mm diameter crushed washed stone) at the bottom, as shown in drawing. There is possibility that sumps may be filled with sand during the storms. As such frequent inspections are recommended after the large storms
Culverts are provided to connect the drains across the roads. In order to provide efficient hydraulic performance, reinforced concrete pipe (RCP) culverts of Class ‘Z’ are proposed for the drainage system. Precast box structures are proposed for the culverts larger than 750 mm. Piles are proposed for the stability of the culvert structure. Energy dissipaters are provided at the downstream of the culvert outlets where the slopes are steep and high velocity is expected. Apron walls should be provided around the culvert to protect slopes from the erosion and failure.
INTRODUCTION This chapter provides guidelines for the design of open drainage system, such as lined drains and grassed swales. These facilities, along with stormwater inlets are components of the minor drainage system designed to collect minor flood flows from roads, properties and open space, and convey them to the major drainage system.
It should be noted that fully lined drains are not encouraged anymore in local practice while grass lined ones as encouraged. Developers and designers shall seek approval from the local regulatory authority if such needs arise. Much of procedures and experience that deal with open drainage system have been established in Malaysian practice since late 1970s.
Design Storm Drains and swales should have the capacity to convey the flow up to and including the minor system design ARI.
Design Storm ARIs for Urban Stormwater Systems
Note 1 : Higher of applicable storm ARIs shall be adopted if development falls under two categories Note 2: Size of trunk drains within major drainage system expected to vary, Current practices – trunk drains are provided for areas larger than 40 ha, Downstream size of trunk drain should increase to limit the magnitude of “gap flows” Note 3 : Selection of design storm ARI based on the level of protection in practice, For cases where higher ARI design storm is impractical selection of appropriate ARI should be based on assessment of cost to benefit or social factors, Lowert ARI for major system are to be made with consultation and approval from Local Authority, Consequences of the higher ARI shall be investigated and made known, Land should still be reserved for the higher ARI for future system upgrading Note 4: Habitable floor levels of buildings shall be above the 100 year ARI flood level Note 5: Reduction in discharge due to quantity control (detention or retention) measures to be included
Drainage Reserves Most open drains will be located within road reserve and therefore do not require a separate reserve to allow access for maintenance. However, open drains and swales located outside of road reserves, such as in public walkways and open space areas, should be provided with a drainage reserve In new development areas, the edge of a swale should generally be located 0.5 m from the road reserve or property boundary
STRUCTURAL AND COMPOSITE DRAIN • A lined drain is highly resistant to erosion. This type of drain is expensive to construct, although it should have a very low maintenance cost if properly designed. • A composite drain is combination of a grassed section and a lined drain that may be provided in locations subject to dry-weather base flows which would otherwise damage the invert of a grassed swale, or in areas with highly erodible soils. The composite drain components shall comply with the relevant design requirements specified for grassed swales and lined drains.
Lining Materials Lined drains shall be constructed from materials proven to be structurally sound and durable and have satisfactory jointing systems Lined open drains may be constructed with any of the following materials: • plain concrete; • reinforced concrete; • stone pitching; • plastered brickwork; and • precast masonry blocks.
Design Criteria Geometry • The dimensions of lined open drains have been limited in the interests of public safety and to facilitate ease of maintenance. The minimum and maximum permissible crosssectional dimensions
The dimensions of lined open drains have been limited in the interests of public safety and to facilitate ease of maintenance
0.6 m maximum
Varies 0.5 m minimum 1.2 m maximum
(a) Uncovered Open Drain
0.6 m minimum 1.2 m maximum
Varies 0.5 m minimum 1.2 m maximum
Grate or solid cover
(b) Covered Open Drain
Recommended Composite Drain Cross Section
Depth • The maximum depth for lined open drains shall be in accordance with Table 14.1. A reinforced concrete drain shall be provided for lined open drains that exceed 0.9 m in depth. Cover/Handrail Fence Condition
Maximum Depth (m)
Without protective covering
With solid or grated cover
Width • The width of lined open drains may vary between a minimum width of 0.5 m and a maximum of 1.2 m Side slope Drain Lining Concrete, brickwork and blockwork
Maximum Side Slope Vertical
Grassed/vegetated, rock riprap
Velocities and Longitudinal Slope • To prevent sedimentation and vegetative growth, the minimum average flow velocity for minor drain shall not be less than 0.6 m/s. The maximum flow velocity in open drain should be restricted to a maximum of 2 m/s. However, for flow velocities in excess of 2 m/s and less than 4 m/s, drains shall be provided with a 1.2 m high handrail fence, or covered with metal grates or solid plates for the entire length of the drain for public safety. • As longitudinal slope increase the velocity increases proportionally. Open drains longitudinal slope should be constant and no steeper than 0.2%. Drop structures may be required to reduce the longitudinal slope in order to control flow velocities.
Design Procedure The preliminary sizing estimation procedure for minor drain is given below: • Step 1: Estimate the design discharge, Qminor based on the design minor ARI using suitable methods from those outlined in Chapter 2 (Section 2.3). • Step 2:
Estimate Manning’s n of the lining material.
• Step 3: Select the design cross-section. Determine the depth and the minimum base width for the proposed system. Determine the proposed drain capacity using Manning’s Equation. • Step 4: Compare the estimated drain capacity with the calculated design discharge, Qminor. If the drain capacity is found to be inadequate, then the drain cross section should be modified to increase the capacity. Likewise a reduction in the cross section may also be required if the drain is not to be overdesigned. In the case of any modifications to drain cross section, repeat Step 3.
• Step 5: Calculate the average flow velocity from V = Q/A and check that it is within the maximum and minimum velocity criteria for the open drain. If not, adjust the drain dimensions and return to Step 3. • Step 6: Determine the flow depth, y and check if y is within required limits for the open drain type. If not, adjust the drain dimensions and return to Step 3. • Step 7: Add the required freeboard. If required, calculate the top width of drain for drains with sloping sides. • Step 8:
Calculate the width of the drainage reserve.
Grassed Swales in Malaysia
Advantages – easy to incorporate into landscaping; – good removal of urban pollutants; – reduces runoff rates and volumes; – low capital cost; – maintenance can be incorporated into general landscape management; and – good option for small area retrofits.
Disadvantages • not suitable for steep areas; • limited to small areas; • risks of blockages in connecting pipework/culverts; • sufficient land may not be available for suitable swale designs to be incorporated; and • standing water in vegetated swales can result in potential safety, odour, and mosquito problems.
Design Consideration and Requirements Drainage Area • Grassed swales engineered for enhancing water quality cannot effectively convey large flows. Therefore, swales are generally appropriate for catchments with small, flat impermeable areas. If used in areas with steep slopes, grassed swales must generally run parallel to contours in order to be effective Space Requirement • Grassed swales must be effectively incorporated into landscaping and public open spaces as they demand significant land-take due to their shallow sideslopes. Grassed swales are generally difficult to be incorporated into dense urban developments where limited space may be available
Site Slope • Grassed swales are usually restricted to sites with significant slopes, though careful planning should enable their use in steeper areas by considering the contours of the site (CIRIA, 2007). The longitudinal terrain slope should not exceed 2% as low runoff velocities are required for pollutant removal and to prevent erosion. Longitudinal slopes can be maintained at the desired gradient and water can flow into swales laterally from impermeable areas. Subsurface Soils and Groundwater • Where grassed swales are designed to encourage infiltration, the seasonally high groundwater table must be more than 1 m below the base of the swale. Where infiltration is not required, the seasonally high groundwater level should be below any underdrain provided with the swales (CIRIA, 2007).
Geometry • The preferred shapes for swales are shown in Figure below. The depth shall not exceed 1.2 m. A ‘vee’ or triangular shaped section will generally be sufficient for most applications; however, a trapezoidal or parabolic swale shape may be used for additional capacity or to limit the depth of the swale. Swales with trapezoidal cross sections shall be recommended for ease of construction. A parabolic shape is best for erosion control, but is hard to construct.
• For a trapezoidal shape, the bottom width should be between 0.5 m and 3.0 m. The 0.5 m minimum bottom width allows for construction considerations and ensures a minimum filtering surface for water quality treatment. The 3.0 m maximum bottom width prevents shallow flows from concentrating and potentially eroding channels, thereby maximizing the filtering by vegetation.
• Side slope shall not be steeper than 2(H):1(V) while side slope 4(H):1(V) or flatter is recommended for safety reason. However, side slope of 2(H):1(V) in residential areas are strongly discouraged. The larger the wetted area of the swale, the slower the flow and the more effective it is in removing pollutants.
Recommended Swale Cross Sections
Longitudinal Slope • Slope of swales should normally be between 0.1% (1 in 1000) and no greater than 0.5% (1 in 200). Underdrains may be required for slopes below 0.2% (1 in 500), while drop structures such as rock check dams in the channel may be required for slopes greater than 0.2% to reduce the drainage longitudinal slope such that the design flow velocities do not exceed the permissible limits. Freeboard • The depth of a swale shall include a minimum freeboard of 50 mm above the design stormwater level (based on maximum design flows) in the swale to allow for blockages. Velocities • Maximum acceptable flow rate velocities for conveyance of peak design flow (maximum flood flow design) along the swale shall not exceed the recommended maximum scour velocity for various ground covers and values of soil erodibility, or ideally be less than 2 m/s, unless additional erosion protection is provided.
Underdrain • A swale should have the capacity to convey the peak flows from the design minor ARI without exceeding the maximum permissible velocities. If this is not practical or there is insufficient space for a swale, designer should consider dividing the flow into surface and subsurface conduits Underdrains can also be placed beneath the channel to prevent ponding. • It is important for biofiltration swales to maximise water contact with vegetation and the soil surface. Gravely and coarse sandy soils will not provide water quality treatment unless the bottom of the swale is lined to prevent infiltration. (Note: sites that have relatively coarse soils may be more appropriate for stormwater quantity infiltration purposes after runoff treatment has been accomplished). Therefore, the bed of a biofiltration swale shall consist of a permeable soil layer above the underdrain material.
EXERCISE • REFER TO CHAPTER 14 MSMA 2nd EDITION
MSMA’s Proposed Solutions