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Dr. Mohd Hafiz Zawawi
Prof. Dr. Ir. Lariyah Mohd Sidek
Hydraulic Engineering
CEWB222
OPEN DRAINS
MSMACHAPTER 6
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DRAINAGE SYSTEMS IN MALAYSIA
Separate
Drainage
System (after
O'Loughlin,
1998)
Combined Sewer System (after
O'Loughlin, 1998)
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Parts of an Malaysian Separate Urban Stormwater
Drainage System (after O'Loughl in, 1998)
property drainage
system,
street drainagesystem,
trunk drainage
system, and
receiving waters
http://www.ee.uts.edu.au/~simonb/Switch%20site/Education/stormwater%20drainage/Property%20Drainage.htmhttp://www.ee.uts.edu.au/~simonb/Switch%20site/Education/stormwater%20drainage/Street%20Drainage.htmhttp://www.ee.uts.edu.au/~simonb/Switch%20site/Education/stormwater%20drainage/Trunk%20Drainage.htmhttp://www.ee.uts.edu.au/~simonb/Switch%20site/Education/stormwater%20drainage/Receiving%20Waters.htmhttp://www.ee.uts.edu.au/~simonb/Switch%20site/Education/stormwater%20drainage/Receiving%20Waters.htmhttp://www.ee.uts.edu.au/~simonb/Switch%20site/Education/stormwater%20drainage/Trunk%20Drainage.htmhttp://www.ee.uts.edu.au/~simonb/Switch%20site/Education/stormwater%20drainage/Street%20Drainage.htmhttp://www.ee.uts.edu.au/~simonb/Switch%20site/Education/stormwater%20drainage/Property%20Drainage.htm7/24/2019 (n) Chapter 6 Open Drain Msma
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Rigid Boundary Channel
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Rigid Boundary Channel(Dry Period)
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Trunk Drain During Dry Period
Rigid Boundary Channel
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Wet Period
Rigid Boundary Channel
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Trunk Drain - Wet Period
Rigid Boundary Channel
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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 drainagesystem 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 Drains A minor drainage system was designed to drain the
stormwater collected from roofs and properties and
attenuated through on-site detention facilities to a stabledischarge 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.
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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
Drain:
Berm drains are proposed to collect stormwater from the
intermediate slopes. These will be V-shaped concrete
sections as shown in the drawings.
Toe Drain: 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
Drain:
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.
Perimeter
Drain
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 thepipes.
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Perimeter Drains
Perimeter swale Perimeter swale Perimeter swale
Ecological swale type B
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Road Side Drain
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Road Side Drain
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Swales
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Swales
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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. Opendrains 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.
Community
Ponds
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 levelrisers 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
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Monsoon Drain
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Monsoon Drain
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Detention Pond
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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 actas 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
Culvert 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 providedaround the culvert to protect slopes from the erosion and failure.
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Sump
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Sump
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Culvert
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Design Criteria
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INTRODUCTIONThis 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.
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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
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g y
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 practicestrunk 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 sti ll 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
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Drainage Reserves
Most open drains will be located within roadreserve and therefore do not require a separate
reserve to allow access for maintenance.
However, open drains and swales located outside ofroad reserves, such as in public walkways and openspace areas, should be provided with a drainage
reserve
In new development areas, the edge of a swaleshould generally be located 0.5 m from the road
reserve or property boundary
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STRUCTURAL AND COMPOSITE DRAIN
A lined drain is highly resistant to erosion. This type ofdrain is expensive to construct, although it should havea very low maintenance cost if properly designed.
A composite drain is combination of a grassed sectionand a lined drain that may be provided in locationssubject to dry-weather base flows which wouldotherwise damage the invert of a grassed swale, or inareas with highly erodible soils. The composite drain
components shall comply with the relevant designrequirements specified for grassed swales and lineddrains.
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Lining Materials
Lined drains shall be constructed from materials provento be structurally sound and durable and have satisfactory
jointing systems
Lined open drains may be constructed with any of thefollowing materials:
plain concrete;
reinforced concrete;
stone pitching;
plastered brickwork; and
precast masonry blocks.
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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 cross-
sectional dimensions
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The dimensions of lined open drains have been limited in the
interests of public safety and to facilitate ease of maintenance
(a) Uncovered Open Drain (b) Covered Open Drain
0.5 m minimum1.2 m maximum
Varies
0.6mm
inimum
1.2mm
axim
um
Varies
Grate orsolid cover
Varies0.5 m minimum1.2 m maximum
0.6m
maximum
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Recommended Composite Drain Cross
Section
h
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Depth
The maximum depth for lined open drains
shall be in accordance with Table 14.1. Areinforced concrete drain shall be provided for
lined open drains that exceed 0.9 m in depth.
Cover/Handrail Fence ConditionMaximum Depth
(m)
Without protective covering 0.6
With solid or grated cover 1.2
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Width
The width of lined open drains may vary
between a minimum width of 0.5 m and amaximum of 1.2 m
Side slope
Drain Lining Maximum Side Slope
Concrete, brickwork and blockwork Vertical
Stone pitching 1.5(H):1(V)
Grassed/vegetated, rock riprap 2(H):1(V)
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Velocities and Longitudinal Slope
To prevent sedimentation and vegetative growth, theminimum average flow velocity for minor drain shall not beless than 0.6 m/s. The maximum flow velocity in open drainshould be restricted to a maximum of 2 m/s. However, forflow 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, orcovered with metal grates or solid plates for the entirelength of the drain for public safety.
As longitudinal slope increase the velocity increasesproportionally. Open drains longitudinal slope should beconstant and no steeper than 0.2%. Drop structures may berequired to reduce the longitudinal slope in order to controlflow velocities.
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Design ProcedureThe preliminary sizing estimation procedure for minor drain is givenbelow:
Step 1: Estimate the design discharge, Qminorbased on thedesign minor ARI using suitable methods from those outlined inChapter 2 (Section 2.3).
Step 2: Estimate Mannings nof the lining material.
Step 3: Select the design cross-section. Determine the depthand the minimum base width for the proposed system.Determine the proposed drain capacity using Mannings Equation.
Step 4: Compare the estimated drain capacity with thecalculated design discharge, Qminor. If the drain capacity is foundto be inadequate, then the drain cross section should be modifiedto increase the capacity. Likewise a reduction in the cross section
may also be required if the drain is not to be overdesigned. In thecase of any modifications to drain cross section, repeat Step 3.
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Step 5: Calculate the average flow velocity fromV= Q/Aand check that it is within the maximum andminimum velocity criteria for the open drain. If not,adjust the drain dimensions and return to Step 3.
Step 6: Determine the flow depth, yand check if yis
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 slopingsides.
Step 8: Calculate the width of the drainage reserve.
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Swales
Grassed Swales in Malaysia
Ad
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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.
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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 resultin potential safety, odour, and mosquitoproblems.
i id i d
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Design Consideration and
RequirementsDrainage 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 side-
slopes. Grassed swales are generally difficult to be incorporated into dense
urban developments where limited space may be available
l
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Site Slope
Grassed swales are usually restricted to sites with significantslopes, 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% aslow runoff velocities are required for pollutant removal and toprevent erosion. Longitudinal slopes can be maintained at thedesired 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 mbelow the base of the swale. Where infiltration is notrequired, the seasonally high groundwater level should bebelow any underdrain provided with the swales (CIRIA, 2007).
G
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Geometry
The preferred shapes for swales are shown inFigure below. The depth shall not exceed 1.2 m. Avee or triangular shaped section will generallybe sufficient for most applications; however, atrapezoidal or parabolic swale shape may be usedfor additional capacity or to limit the depth of theswale. Swales with trapezoidal cross sections shallbe recommended for ease of construction. Aparabolic shape is best for erosion control, but ishard to construct.
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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 aminimum filtering surface for water quality
treatment. The 3.0 m maximum bottom width
prevents shallow flows from concentratingand potentially eroding channels, thereby
maximizing the filtering by vegetation.
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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 arestrongly discouraged. The larger the wetted
area of the swale, the slower the flow and the
more effective it is in removing pollutants.
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Recommended Swale Cross Sections
Longitudinal Slope
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g p
Slope of swales should normally be between 0.1% (1 in 1000) andno greater than 0.5% (1 in 200). Underdrains may be required forslopes below 0.2% (1 in 500), while drop structures such as rockcheck dams in the channel may be required for slopes greater than
0.2% to reduce the drainage longitudinal slope such that the designflow 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 designflows) 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 notexceed the recommended maximum scour velocity for variousground covers and values of soil erodibility, or ideally be less than 2m/s, unless additional erosion protection is provided.
U d d i
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Underdrain
A swale should have the capacity to convey the peak flows from thedesign minor ARI without exceeding the maximum permissiblevelocities. If this is not practical or there is insufficient space for aswale, designer should consider dividing the flow into surface andsubsurface conduits Underdrains can also be placed beneath thechannel to prevent ponding.
It is important for biofiltration swales to maximise water contactwith vegetation and the soil surface. Gravely and coarse sandy soilswill not provide water quality treatment unless the bottom of theswale is lined to prevent infiltration. (Note: sites that have
relatively coarse soils may be more appropriate for stormwaterquantity infiltration purposes after runoff treatment has beenaccomplished). Therefore, the bed of a biofiltration swale shallconsist of a permeable soil layer above the underdrain material.
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EXERCISE
REFER TO CHAPTER 14 MSMA 2ndEDITION
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MSMAs Proposed Solutions
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