(n) Chapter 6 Open Drain Msma

7/24/2019 (n) Chapter 6 Open Drain Msma

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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.htm


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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



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Wet Period



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Trunk Drain - Wet Period



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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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Documents

(n) Chapter 6 Open Drain Msma