MSMA

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ii

MMaannuuaallSSaalliirraannMMeessrraaAAllaamm

MMSSMMAA))

Pusat Penyelidikan Kejuruteraan Sungai dan Saliran Bandar

(REDAC)

Kampus Kejuruteraan, Universiti Sains Malaysia

Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang.

Tel: 04-5941035 Fax: 04-5941036

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Concept and Design Requirement of MSMA

Urban Stormwater Management Short Course

1

Time

Dis

charge

Post DevelopmentUncontrolled Runoff

Pre-DevelopmentUncontrolled Runoff

Post-Development

Controlled Runoff byDetention

1.0 Design Standard

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

2.0 General

Urbanization results in the growth and spread of impervious areas and a diversification of urban landuse practice

with respects to the hydrologic and environmental terms. Landuse changes from rural to urban industrial areascause local runoff impacts on receiving water flow, quality, and ecology. Apart from erosion and sedimentationproblems associated with development, it has become increasingly apparent that stormwater runoff contributes toreceiving waters a significant part of total loads of such pollutants as nutrients (including phosphorus and nitrogen),heavy metals, oil and grease, bacteria, etc.

New, comprehensive, and integrated SWM strategies are now needed to be in line with the governments drive toarchive a sustainable developed nation status in the early 21stcentury. Such new strategies will incorporate interalia,runoff source control, management and delayed disposal on a catchment wide, proactive, and multi-functional basis.This should result in flood reduction, water quality improvement, and ecological enhancement in downstreamreceiving waters. To some extent, it should also contribute to improved urban amenity through the application ofwetlands, landscape for recreation, potential beneficial reuse of stormwater (especially as a non-potable supplysource), and recharge of depleted urban groundwater aquifers to enhance stream base flow during dry seasons.

Stormwater management has development to the point where there are now two fundamental different approachesto controlling the quality, and to some extant, the quality of stormwater runoff. In addition to the traditionalconveyance-oriented approach, a potential effective and preferable approach to stormwater management is thestorage-oriented approach. The function of this approach is to provide for the temporary storage of stormwaterrunoff at or near its point of origin with subsequent slow release to the downstream stormwater system or receivingwater (detention), or infiltration into the surrounding soil (retention).

Detention and retention facilities can reduce the peak and volume of runoff from a given catchment (Figure 18.1),which can reduce the frequency and extent of downstream flooding. Detention/retention facilities have been used toreduce the costs of large stormwater drainage system by reducing the size required for such systems in downstreamareas.

The reduced post-development runoff hydrograph is typically designed so that the peak flow is equal to or less thanthe pre-development peak flow rate. Additionally, in some instances, the volume of the post-development runoffhydrograph is required to be reduced to the same volume as the pre-development runoff hydrograph. This latterrequirement will necessitate the use of retention facilities to retain the differences in volume between the post andpre-development hydrograph.

Figure 18.1 Hydrograph Schematic

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Park Pond

Infiltration Basin

Artificial Recharge

(Todd, 1980)

Storage Reservoir

(Hall, et al., 1993)

Car Park DetentionInfiltration Trench

(CIRIA, 1996)

On-site

Regional

Community

LEVELS

Figure 18.2 Detention/Retention Storage Classifications

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(c) Combined StoragesWith combined storages, a proportion of the total storage is provided as below-ground storage, whilst theremainder of the storage is provided as above-ground storage.

Underground Tank Pipe Package

Rooftop

Car Parking andDriveway Areas

Landscaped Area

Surface Tank

Figure 18.3 Typical OSD Storage Facilities

3.4 Retention Facilities

These facilities encourage the disposal of stormwater at its source of runoff. This is done by having a portion of the

stormwater infiltrate or percolate into the soil. The advantages often cited for the use of local disposal include:1. recharge of groundwater2. reduction in the settlement of the land surface in areas of groundwater depletion3. control of saline water intrusion4. preservation and/or enhancement of natural vegetation5. reduction of pollution transported to the receiving waters6. reduction of downstream flow peaks7. reduction of basement flooding in underground drainage systems8. smaller storm drains at a lesser cost

3.5 On-Site and Community Retention

The main types of retention/infiltration techniques are:(i) Infiltration Trench (Figure 18.5)(ii) Soakaway Pit (Figure 18.6)(iii) Porous Pavement (Figure 18.7)(iv) Infiltration Basin (Figure 18.8)

Figure 18.5 Infiltration Trench

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Figure 18.6 Soakaway Pit Figure 18.7 Porous Pavement

Figure 18.8 Infiltration Basin

4.0 General Design Considerations

Detention on development sites has been seen as the solution to problems of established areas where additionaldevelopment or redevelopment is occurring. Generally, it is not possible, either physically or financially, toprogressively enlarge drainage systems as redevelopments that increase impervious areas and runoff rates andvolumes occur.

Regulations, which put the responsibility on developers to restrict flows, are therefore attractive to drainageauthorities. Flows can be limited by the use of various OSD facilities. The design procedures are based on theRational Method.

Simplified hydrographs are combined with an assumed outlet relationship to determine a critical volume of water tobe stored. Often several cases are considered, to allow for different storm durations. A storage is then to beprovided for this critical volume.

Permissible site discharge (PSD) and site storage requirement (SSR) are used for an OSD development. There are

two basic approaches that may be used for determining the required PSD and SSR as follows:

(a) Site-based MethodsThe PSD and SSR values to be applied to a particular development site are determined by hydrologicanalysis of the development site only, without any consideration of the effect of site discharges on thedownstream catchment. The PSD is the estimated peak flow for the site prior to development for aselected design storm. The only concern is that post-development site discharges are reduced to pre-development levels. PSD values may be determined using either the Rational Method or a hydrographestimation method (refer Chapter 14, MSMA).

Site-based methods do not consider the effects of post-development discharges on the downstreamcatchment since it is assumed that reducing discharges to pre-development levels is sufficient to prevent

increases in downstream flooding.

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(b) Catchment-based Methods

The PSD and SSR values are determined from an analysis of a total catchment instead of a single site.Catchment modelling is undertaken to determine the maximum values of PSD and SSR for a selected designstorm that will not cause flooding at any location within the catchment. These are general values that maybe applied to any site within the catchment.

OSD storages may be analysed using any hydrograph estimation technique, but the Rational Method is the mostpopular. Rational Method hydrograph techniques are acceptable for OSD as development sites are relatively smalland any errors introduced will most likely be minor. The effort involved with more sophisticated computer modellingtechniques is not normally warranted. The Swinburne Method recommended in Section 7.0 or Chapter 19 (MSMA) isbased on the Rational Method.

5.0 Site Selection

For undeveloped sites, the decision of whether or not to include OSD to control site discharges should be made asearly as possible in the concept planning stagefor developing the site. It is far easier to integrate OSD facilities intoa site arrangement as part of the total development concept than to attempt to retrofit them after the form andextent of buildings, driveways, and landscaping have been designed or constructed. This approach will give thedesigner the most flexibility for design and will generally allow opportunities for developing innovative and/or morecost-effective design solutions.

For developed sites, the location and level of existing structures and services can severely restrict opportunities forproviding satisfactory OSD systems. It may not be practical, due to factors such as cost or public safety, to providethe amount of storage necessary to limit post-development peak flows to the amounts required. In such cases,consideration should be given to increasing the limit on post-development peak flows to match the maximumamount of storage available.

6.0 Flow Control Requirements

6.1 Design Storm

The design storm for discharge from an OSD storage, termed the discharge design storm, shall be the minor systemdesign ARI of the municipal drainage system to which the storage is connected (refer Table 4.1, MSMA). The designstorm for calculating the required storage volume, termed the storage design storm, shall be 10 year ARI.

6.2 Permissible Site Discharge (PSD)

The PSD is the maximum allowable post-development discharge from a site for the selected discharge design stormand is estimated on the basis that flows within the downstream stormwater drainage system will not be increased.

6.3 Site Storage Requirement (SSR)

The SSR is the total amount of storage required to ensure that the required PSD is not exceeded and the OSD facilitydoes not overflow during the storage design storm ARI.

6.4 Site Coverage

Where possible, the site drainage system and grading should be designed to direct runoff from the entire site to theOSD system. Sometimes this will not be feasible due to ground levels, the level of the receiving drainage system, orother circumstances. In these cases, as much runoff from impervious areas as possible should be drained to theOSD system.

6.5 Frequency Staged Storage

Generally the most challenging task in designing OSD systems is locating and distributing the storage(s) in the faceof the following competing demands:

making sure the system costs no more than necessary

creating storages that are aesthetically pleasing and complementary to the architectural design avoiding unnecessary maintenance problems for future property owners minimising any personal inconvenience for property owners or residents

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These demands can be balanced by providing storage in accordance with a frequency staged storage approach.Under this approach, a proportion of the required storage for a given ARI is provided as below-ground storage,whilst the remainder of the required storage, up to the design storm ARI, is provided as above-ground storage.Recommended storage proportions for designing a composite above and below-ground storage system using afrequency staged storage approach are provided in Table 19.1. A typical composite storage system is illustrated inFigure 19.1. Refer to Table 19.1 for recommended maximum ponding depths in the above-ground storagecomponent.

Table 19.1 Relative Proportions for Composite Storage Systems

Proportion of Total Storage (%)

Storage Area Below-Ground Storage

Component

Above-Ground Storage

Component

Pedestrian areas 60 40

Private Courtyards 60 40

Parking areas and driveways50 50

Landscaped areas 25 75

Paved outdoor recreation areas15 85

Maximum ponding level forstorage design storm

'Beginning to pond' levelfor above-ground storage

Below-ground storage

Above-ground storage

Freeboardto buildingfloor level

Habitablebuilding

Outlet to public drainage system(preferably free draining, butmay be pumped in some cases)

Figure 19.1 Illustration of a Composite Storage System

6.6 Bypass Flows

An OSD storage is generally designed only to deal with stormwater runoff from the site under consideration. Ifrunoff from outside the site enters the storage, it will fill more quickly, causing a greater nuisance to occupiers and itwill become ineffective in terms of reducing stormwater runoff leaving the site.

Unless the storage is sized to detain runoff from the entire upstream catchment, an overland flow path or a floodwaymust be provided through the site to ensure that all external flows bypass the OSD storage.

The surface area of an overland flow path or a floodway is excluded from the site area for the purpose of calculatingthe site storage requirements. Such areas must be protected from future development within the site by anappropriate covenant or drainage reserve.

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7.0 Determination of PSD and SSR

7.1 OSD Sizing Method

The recommended method for estimating PSD and SSR is the Swinburne Method, developed at the SwinburneUniversity of Technology in Melbourne, Australia. The method uses the Rational Method to calculate site flows, andutilises a non-dimensional triangular site hydrograph as illustrated in Figure 19.3. The site discharges are calculated

using the total catchment time of concentration tcfor the design storm ARI under consideration (Figure 19.2).

The PSD varies with this ratio and may be less than or greater than the peak pre-development site dischargedepending on the position of the site within the catchment. Figure 19.2 illustrates the relationship between tcandtcs.

(i) PSDAs stated in Section 4.1 the discharge design storm for estimating the PSD is the minor system design ARIof the municipal stormwater system to which the site is or will be connected.

The following general equation is used to calculate the PSD for the site in litres per second. The factors aand bare different for above-ground and below-ground storages due to differences in storage geometryand outflow characteristics.

2

42 baa

PSD

= (19.1)

For above-ground storage :

++

= csc

a

pc

c

a ttQ

Qt

t

Qa 25.075.0333.04 (19.1a)

paQQb 4= (19.1b)

For below-ground storage :

++

=

csca

p

cc

a ttQ

Qt

t

Qa 65.035.0333.0548.8 (19.1c)

paQQb 548.8= (19.1d)

where,tc = peak flow time of concentration from the top of the catchment to a designated outlet or point of concern

(minutes)tcs = peak flow time of concentration from the top of the catchment to the development site (minutes)Qa = the peak post-development flow from the site for the discharge design storm with a duration equal to tc

(l/s)Qp = the peak pre-development flow from the site for the discharge design storm with a duration equal to tc (l/s)

Figure 19.2 Relationship Between tcand tcs for the Swinburne Method

Catchment in which

development site

is located

Development site

Time of concentration tcforcatchment Time of concentration tcsfrom top of

catchment to point where flows fromdevelopment site join main drainagesystem for catchment

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SSR

PSD

tc0

Assumed inflowhydrograph

Assumed outflowhydrograph

tf

Qa

Figure 19.3 Swinburne Method Assumptions (tf= time for storage to fill)

(ii) SSR

The storage design stormfor estimating the SSR is 10 year ARI. In sizing the volume of the storage facility,the method assumes a triangular inflow hydrograph and an outflow hydrograph shape related to the type ofstorage adopted. These simplifications are acceptable providing the site catchment is small.

Typically, the critical storm duration that produces the largest required storage volume is different from thetime of concentration used for peak flow estimation. Therefore, storage volumes must be determined for arange of storm durations to find the maximum storage required as indicated in Figure 19.4 (MSMA, 2000).

S

torageVolume

(m3)

Storm Duration (minutes)

Critica

lDuration

Maximum storage

X

X

XX

X XX

X

X

Figure 19.4 Typical Relationship of Storage Volume to Storm Duration

The following general equation is used to calculate the SSR for the site in cubic metres. Different factors for cand dare applied for above-ground and below-ground storages to account for differences in storage geometryand outflow characteristics.

( )dcQtSSR dd = 06.0 (19.2)

For above-ground storage :

=

dQ

PSDPSDc 459.01875.0 (19.2a)

dQ

PSDd

2

214.0= (19.2b)

For below-ground storage :

=

dQ

PSDPSDc 392.01675.0 (19.2c)

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10

dQ

PSDd

2

117.0= (19.2d)

where,td = selected storm duration (minutes)Qd = the peak post-development flow from the site for a storm duration equal to td (l/s)

7.2 OSD Sizing Procedure

A simplified design procedure for determining the required volume of detention storage is as follows:1. Select storage type(s) to be used within the site, i.e. separate above and/or below-ground storage(s), or a

composite above and below-ground storage.2. Determine the area of the site that will be drained to the OSD storage system. As much of the site as possible

should drain to the storage system.3. Determine the amount of impervious and pervious areas draining to the OSD storage system.4. Determine the times of concentration, tcand tcs.5. Calculate the pre and post-development flows, Qpand Qa, for the area draining to the storage for the discharge

design storm with time of concentration tc.6. Determine the required PSD for the site using Equation 19.1 for the discharge design storm.

7. Determine the required SSR for the site using Equation 19.2 for the storage design storm over a range ofdurations to determine the maximum value. For composite storages, apportion the required SSR in accordancewith Table 19.1.

Note: For composite storages, use the PSD and SSR equation factors relating to the largest storage component. Ifthese are equal, use the above-ground storage factors.

8.0 General Considerations

8.1 Drainage System

The stormwater drainage system (including gutters, pipes, open drains, and overland flow paths) for the site must:

be able to convey all runoff to the OSD storage, up to and including the storage design storm, with time ofconcentration tc ensure that the OSD storage is bypassed by all runoff from neighbouring properties and any part of the site

not being directed to the OSD storage facility

The outlet from the OSD facility must be designed to ensure that outflow discharges: do not cause adverse effects on downstream properties by concentrating flow can be achieved with low maintenance

The OSD outlet should be designed to be independent of downstream flow conditions under all design circumstanceswherever possible (i.e. not outlet controlled). If this is not possible, the outlet should be sized to account fordrowned or partly drowned outlet conditions (refer Section 6.1).

8.2 Multiple Storages

In terms of construction and recurrent maintenance costs, it is preferable to provide fewer larger storages than alarger number of smaller storages. Multiple storages should be carefully treated when preparing a detailed design.The storages need to be designed separately with the catchment draining to each storage defined.

The outlet pipe from a storage needs to be connected downstream of the primary outlet structure of any otherstorage, i.e. storages should act independently of each other and not be connected in series.

8.3 Site Grading

Sites should be graded according to the following general guidelines: grade the site for surface drainage such that no serious consequences will occur if the property drainage

system fails. avoid filling the site with stormwater inlets that are not needed.

direct as much of the site as possible to the OSD storage.

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8.4 Floor Levels

The site drainage system must ensure that: all habitable floor levels for new and existing dwellings are a minimum 200 mm above the storage

maximum water surface level for the storage design storm ARI garage floor levels are a minimum 100 mm above the storage design storm ARI maximum water surface

level

A similar freeboard should be provided for flowpaths adjacent to habitable buildings and garages.

8.5 Aesthetics

The designer should try to ensure that OSD storages and discharge control structures blend in with and enhance theoverall site design concept by applying the following general guidelines:

when OSD storage is provided in a garden area, avoid placing the discharge control structure in the centrewhere it will be an eyesore. Where possible, grade the floor of the storage such that the discharge controlstructure is located unobtrusively, e.g. in a corner next to shrubbery or some garden furniture

If space permits, try to retain some informality in garden areas used for storage. Rectangular steep-sidedbasins unbroken by any features maximise the volume, but may detract from the appearance of thelandscaping

8.6 Construction Tolerances

OSD systems is important in protecting downstream areas from flooding. Every effort should be made to avoid, or atleast minimise, construction errors. The design should allow for the potential reduction in the storage volume due tocommon post-construction activities such as landscaping, top dressing and garden furniture. It is recognised thatachieving precise levels and dimensions may not always be possible in practice. It is therefore considered that anOSD system will meet the design intent where the:

storage volume is at least 95% of the specified volume design outflow is within plus or minus 5% of the PSD

8.7 Signs

A permanent advisory sign for each OSD storage facility provided should be securely fixed at a pertinent and clearly

visible location stating the intent of the facility. An example of an advisory sign is shown in Figure 19.5 (MSMA,2000).

WARNINGON-SITE DETENTION AREA

STORMWATER LEVEL MAY

RISE IN THIS AREA

DURING HEAVY RAIN

Figure 19.5 Typical OSD Advisory Sign (UPRCT, 1999)

9.0 Above-Ground Storage

The following guidelines allow the designer maximum flexibility when integrating the storage into the site layout.

Colours:Triangle and WARNING RedWater Blue

Figure and other lettering Black

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9.1 Maximum Storage Depths

Maximum storage depths in above-ground storages should not exceed the values provided in Table 19.2 (MSMA,2000).

Table 19.2 Recommended Maximum Storage Depths for Different Classes of Above-Ground Storage

Storage Classes Maximum Storage Depth

Pedestrian areas 50 mm

Parking areas and driveways 150 mm

Landscaped areas 600 mm

Private courtyards 600 mm

Flat roofs 300 mm

Paved outdoor recreation areas 100 mm

7.2 Landscaped Areas

The minimum design requirements for storage systems provided in landscaped areas which offer a wide range ofpossibilities for providing above-ground storage and can enhance the aesthetics of a site are:

maximum ponding depths shall not exceed the limits recommended in Table 2 under design conditions

calculated storage volumes shall be increased by 20% to compensate for construction inaccuracies and the

inevitable loss of storage due to the build up of vegetation growth over time

the minimum ground surface slope shall be 2% to promote free surface drainage and minimise the possibility of

pools of water remaining after the area has drained

side slopes should be a maximum 1(V):4(H) where possible. If steep or vertical sides (e.g. retaining walls) are

unavoidable, due consideration should be given to safety aspects, such as the need for fencing, both when the

storage is full and empty

subsoil drainage around the outlet should be provided to prevent the ground becoming saturated during

prolonged wet weather where the storage is to be located in an area where frequent ponding could create maintenance problems or

inconvenience to property owners, a frequency staged storage approach should be adopted as recommended in

Table 19.1. If this is not practicable, the first 10-20% of the storage should be provided in an area able to

tolerate frequent inundation, e.g. a paved outdoor entertainment area, a permanent water feature, or a rock

garden

landscaping should be designed such that loose materials such as mulch and bark etc. will not wash into and

block storage outlets

retaining walls shall be designed to be structurally adequate for the hydrostatic loads caused by a full storage

9.3 Impervious Areas

Car parks, driveways, paved storage yards, and other paved surfaces may be used for stormwater detention. Theminimum design requirements for storage systems provided in impervious areas shall be as follows:

to avoid damage to vehicles, depths of ponding on driveways and car parks shall not exceed the limits

recommended in Table 19.2 under design conditions

transverse paving slopes within storages areas shall not be less than 0.7%

if the storage is to be provided in a commonly used area where ponding will cause inconvenience (e.g. a car

park or pedestrian area), a frequency staged storage approach should be adopted as recommended in

Table 19.1. If this is not practical, the first 10-20% of the storage should be provided in a non-sensitive area on

the site

9.4 Flat Roofs

Rooftop storage may be provided on buildings with flat roofs. Stormwater can be detained up to the maximumdepth recommended in Table 19.2 by installing flow restrictors on roof drains.

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Flat roofs used for detention will have a substantial live load component. It is therefore essential that the structuraldesign of the roof is adequate to sustain increased loadings from ponded stormwater. The latest structural codesand standards should be checked before finalising plans. Roofs must also be sealed to prevent leakage.

A typical flow restrictor on a roof drain is shown in Figure 19.6 (MSMA, 2000).

Figure 19.6 Typical Roof Storage Flow Restrictor

9.5 Surface Tanks

Surface tanks are normally provided on residential lots for rainwater harvesting. These tanks collect rainwater fromthe rooftops of buildings and store it for later domestic use. Surface tanks may also be used solely for on-sitedetention, or utilised in combination with storage provided for rainwater harvesting as illustrated in Figure 19.7.

Since surface tanks will only provide detention volume for rooftops of buildings, other forms of detention storage(such as landscaped storage or pipe packages) must also be provided if flows from the whole site are to be reduced.

Building

Roof drainage system

Secondary

outlet

Screen

OSDstorage

Storage forre-use

Primaryoutlet

Figure 19.7 Typical Multi-Purpose Surface Tank

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10.0 Below-Ground Storage

Providing a small proportion of the required storage volume underground can often enhance a development bylimiting the frequency of inundation of an above-ground storage area. In difficult topography, the only feasiblesolution may be to provide all or most of the required storage volume below-ground. However, it should berecognised that below-ground storages:

are more expensive to construct than above-ground storage systems

are difficult to inspect for silt and debris accumulation can be difficult to maintain can be dangerous to work in and may be unsafe for property owners to maintain

When preparing a design for below-ground storage, designers should be aware of any statutory requirements forworking in confined spaces. Where practicable, the design should minimise the need for personnel to enter thestorage space for routine inspection and maintenance.

8.1 Underground Tanks

(a) Basic ConfigurationTypical below-ground storage tanks are either circular or rectangular in plan and/or cross-section (Figure 19.8) but,due to their structural nature, can be configured into almost any geometrical plan shape.

STORAGE TANKOutlet pipe

Access ladder

Access and overflow grate

Trash screen

Inlet pipes

Figure 19.8 Typical Below-ground Storage Tank

(b) Structural AdequacyStorage tanks must be structurally sound and be constructed from durable materials that are not subject todeterioration by corrosion or aggressive soil conditions. Tanks must be designed to withstand the expected live anddead loads on the structure, including external and internal hydrostatic loadings. Buoyancy should also be checked,especially for lightweight tanks, to ensure that the tank will not lift under high groundwater conditions.

(c) Horizontal PlanSite geometry will dictate how the installation is configured in plan. A rectangular shape offers certain cost andmaintenance advantages, but space availability will sometimes dictate a variation from a standard rectangular plan.

It may be necessary on some site to design irregularly shaped tanks. In such cases, construction and maintenancecosts will normally be higher.

(d) Bottom SlopeTo permit easy access to all parts of the storage for maintenance, the floor slope of the tank should not be greaterthan 10%. The lower limit for this slope is 2%, which is needed for good drainage of the tank floor.

(e) VentilationIt is very important to provide ventilation for below-ground storage systems to minimise odour problems. Ventilationmay be provided through the tank access opening(s) or by separate ventilation pipe risers. Although the inflow andoutflow pipes can provide some ventilation of the storage tank, their contribution is unreliable and should not beconsidered in the design. Also, the ventilation openings should be designed to prevent air from being trappedbetween the roof of the storage and the water surface.

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(f) Overflow ProvisionAn overflow system must be provided to allow the tank to surcharge in a controlled manner if the capacity of thetank is exceeded due to a blockage of the outlet pipe or a storm larger than the storage design ARI. As illustrated inFigure 19.8, an overflow can be provided by installing a grated access cover on the tank.

(g) Access OpeningsBelow-ground storage tanks should be provided with openings to allow access by maintenance personnel and

equipment. An access opening should be located directly above the outlet for cleaning when the storage tank is fulland the outlet is clogged. A permanently installed ladder or step iron arrangement must be provided below eachaccess opening if the tank is deeper than 1200 mm.

10.2 Pipe Packages

(a) Basic ConfigurationApipe packageis a below-ground storage consisting of one or more parallel rows of buried pipes connected by acommon inlet and outlet chamber.

The size of a pipe package is determined by the storage volume requirements and the physical availability of spaceon the site. A pipe package does not need to be installed in a straight line along its entire length, it can change

direction anywhere along its length to fit any site space limitations. A typical pipe package, shown in Figure 19.9, isequipped with a flow regulator installed in the outlet chamber and an overflow spillway located at either the inlet oroutlet chamber.

(b) Minimum Pipe Size and Longitudinal GradeTo facilitate inspection and cleaning, the minimum pipe size shall be 900 mm diameter.Pipes should be laid at a minimum longitudinal grade of 2% to avoid standing pockets of water which can occur dueto lack of precision during construction.

(c) Low Flow ProvisionAlthough sediment will settle out inside pipe packages, the extent of deposition can be reduced by installing one ofthe pipes lower than the others as shown in Figure 19.9. To keep the other pipes from filling during low flows, thedifference in level between the low flow pipe and other pipes needs to be sufficient to keep the low flows confinedwholly within the low flow pipe. Confining low flows to one pipe will help the system to become self-cleansing.

(d) Inlet ChamberThe site drainage system is connected to the pipe package through an inlet chamber at the upstream end. Thechamber must be large enough to permit easy access to all of the pipes by maintenance personnel and equipment.

SECTION A-A

900

Low flow pipe

150mm

PLAN

A

AInlet chamber

Outlet chamber with flowregulator and overflow grate

4 x 900 mm diameterstorage pipes

150 mm diameteroutlet pipe

225 mm diameterinlet pipe

Figure 19.9 Basic Layout of a Pipe Package Storage (Stahre and Urbonas, 1990)

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Flow regulating devices for below-ground storages are typically located within the storage facility. In this type ofarrangement, the flow regulator should be located at, or near, the bottom of the storage facility. In some cases,where the topography does not permit emptying of the storage facility by gravity, pumping will be required toregulate the flow rate.

Figure 19.11 shows the indicative location of the primary outlet flow regulator in a typical above and below-groundstorage.

Below-ground storage

Above-ground storage

Locate flow regulatorover storage primaryoutlet

DCP

Figure 19.11 Primary Outlet Flow Regulator

(c) Protection from BlockageIt is essential that all OSD storages are protected from potential blockage by installing trash screens around theprimary outlet (refer to Section 11.6).

11.2 Flow Regulating Devices

(i) OrificeThe simplest flow regulating device is an orifice. The orifice shall be cut into a plate and then securely fixed over the

outlet pipe by at least four bolts or similar (one at each corner) such that it can be readily removed for maintenanceor replacement (refer to Figure 19.12). The orifice must be tooled to the exact dimensions as calculated, with theedges smooth and sharp (not rounded). The minimum orifice diameter shall be 25 mm to minimise the potential forblockage.

Circular hole withsharp edges machinedto 0.5 mm accuracy

3 mm thick corrosionresistant steel plate

150 mm

200 mm

200mm Do

Figure 19.12 Typical Orifice Plate Details (UPRCT,1999)

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(ii) Flow Restricting PipeThe main advantage of using a flow restricting pipe as a storage outlet is that it is difficult to modify the hydrauliccapacity of the pipe, unlike an orifice which can be easily removed. As illustrated in Figure 19.13, the net flowrestricting effect of the pipe is mostly a function of the pipe length and pipe roughness characteristics.

Another advantage is that the required flow reduction may be achieved using a larger diameter opening than anorifice, which considerably reduces the possibility of blockage of the outlet. The pipe must be set at a slope less

than the hydraulic friction slope, but steep enough to maintain a minimum velocity of 1.0 m/s in the pipe in order tokeep any silt carried by the water from settling out within the pipe.

(iii) Discharge Control Pit (DCP)A DCP (Figure 19.20) is typically used to house a flow regulator for an above-ground storage. The DCP provides alink between the storage and the connection to the municipal stormwater drainage system.

To facilitate access and ease of maintenance, the minimum internal dimensions (width and breadth) of a DCP shall

be as follows. These dimensions can be increased to allow greater screen sizes or improve access.

up to 600 mm deep: 600 mm x 600 mm greater than 600 mm deep: 900 mm x 900 mm

The following minimum dimensions will achieve predictable hydraulic characteristics:

minimum design head = 2 Do(from centre of orifice to top of overflow) minimum screen clearance = 1.5 Do(from orifice to upstream face of screen) minimum floor clearance = 1.5 Do(from centreline of orifice to bottom of pit)

Note: Dois the diameter of the orifice

ye

Q

TotalEnergyLineHydraulicGra

deLineD

Flowrestrictingpipe

Trash

screen

DCP orStoragefacility

S.L

ys

L

Figure 19.13 Flow Regulation with an Outlet Pipe (Stahre and Urbonas, 1990)

Outletpipe

Compactedgranularbase

Seepage holes

Galvanised grate

Overflow weirTop of bund wall

DETENTIONSTORAGE

Meshedscreen

Orificeplate

H

Inletpipe

Figure 19.20 Typical DCP (After UPRCT,1999)

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11.3 Trash Screens

All primary outlets must be protected by an internal screen. The screen is required to: protect the outlet from blockage create static conditions around an outlet which helps to achieve predictable discharge coefficients retain litter and debris which would otherwise degrade downstream waterways

11.4 Drowned Outlets

Even when care has been taken to ensure that the outlet pipe from a storage is large enough, the assumption offree discharge may not be valid if the outlet is drowned by the downstream drainage system.

An OSD system is designed to control flows in all storms up to and including the storage design storm ARI, while thedownstream drainage system is often only able to cater for smaller storms (typically 2 year to 5 year ARI) withoutsurcharging. The effects of this surcharging on the storage outlet are shown in Figure 19.22.

(a) DISCHARGE INDEPENDENT OF DOWNSTREAM DRAINAGEThe storage is sufficiently above the downstream waterlevel to remain a free discharge outlet

(b) DISCHARGE DEPENDENT ON DOWNSTREAM DRAINAGEThe outlet to the storage is submerged for some part of the storm.

As the water level in the street open drain rises, the discharge fromthe storage is reduced and the amount of water stored increased.

An assessment should be made to determine if this effect is significant

(c) DISCHARGE DEPENDENT ON DOWNSTREAM DRAINAGE AND STORAGE BELOW SURCHARGED WATER LEVELThe oulet to the storage is affected by downstream water levels over a wide range of storm events

HGL

HGL

HGL

Figure 19.22 Effects of Downstream Drainage on a Storage Outlet (After UPRCT, 1999)

12.0 Secondary Outlets

A suitable overflow arrangement must be provided to cater for rarer storms than the OSD facilities were designed for,

or in the event of a blockage anywhere in the site drainage system.

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The most commonly used arrangement for an above-ground storage is a broad-crested weir which, with moststorages, can be designed to pass the entire overflow discharge with only a few centimetres depth of water over theweir. This is particularly desirable for car park storages to minimise the potential for water damage to parkedvehicles.

The overflow weir must be constructed from durable, non-erodible materials to ensure the discharge capacity of theoverflow is maintained and not changed over time. The most commonly used materials are concrete, pavers or

brickwork.

For a below-ground storage, it is common for the access chamber or manhole to be designed as the overflow system.If this is not practicable, an overflow pipe may be provided at the top of the storage to discharge to a safe pointdownstream.

It is essential that the access opening or overflow pipe has sufficient capacity to pass the storage design storm flow.An access point must be sized for the dimensions required to pass this flow or the dimensions required for ease ofaccess, whichever is larger. A grating is normally placed on the access chamber to allow the storage to overflow.The grating can also serve as a ventilation point to reduce the likelihood of odours in the storage.

As far as possible, all overflows shall be directed away from buildings and adjacent properties. Overflows should bedirected to a flow path through the site and conveyed to a suitable point downstream where they can be combinedwith any uncontrolled discharge from the site.

If the site drainage system becomes blocked, any resulting overflow from an OSD storage should cause flooding in anoticeable location so that the malfunction is likely to be investigated and remedied.

Some typical examples of secondary outlets for above and below-ground storages are illustrated in Figure 19.23.

Opendrain

SECTION C-C

DCP

Overflow down driveway (shallow vee-shapedor trapezoidal driveway cross-section)

Overflow from access chambergrate directed down driveway

SECTION B-B

Undergroundtank

Dwelling

Imperviousarea storageon driveway

Undergroundtank

Landscapedstorage area

Garage

Street

B

B

C

C

Secondaryoutlet

Secondaryoutlet

Secondaryoutlet

AA

PLAN

Overflow throughrectangular broad-crestedweir slot in retaining wall

SECTION A-A

Figure 19.23 Examples of Secondary Outlets

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13.0 Operation and Maintenance

13.1 General

OSD systems are intended to regulate flows over the entire life of the development. This cannot be achieved

without some regular periodical maintenance to ensure OSD facilities are kept in good working order and operate as

designed. The designers task is to minimise the frequency of maintenance and make the job as simple as possible.

The following considerations will assist in this regard, however, they will not always be feasible due to site

constraints:

locate access points to below-ground storages away from heavily trafficked areas and use light duty covers

that can be easily lifted by one person. Manholes in the entrance driveway to a large development can

discourage property owners from regularly inspecting and maintaining the system

locate the DCP for an above-ground storage in an accessible location. A slight regrading of an above-

ground storage floor will often allow a DCP to be moved from a private courtyard into a common open

space area. Common areas are more readily accessible for inspection and maintenance

all DCPs and manholes throughout the site should be fitted with a standard lifting/keying system. This

should assist future property owners to replace missing keys

use circular lids for access openings in pits and manholes wherever possible as they are often easier to

remove and will not drop into the storage when being removed or replaced

use a guide channel inside a storage or DCP to fix the screen in place and put a handle on the screen toassist removal. The guide channel prevents debris from being forced between the wall of the pit and thescreen and allows the screen to be easily removed and replaced in the correct position

For safety, all maintenance access to storages must conform to any statutory requirements for working in confinedspaces. Step irons or access ladders shall be installed where the depth of a below-ground storage or DCP is1200 mm or greater.

All inlet pits and manholes shall be fitted with removable covers and/or grates to permit maintenance, having regardto the need to prevent the covers or grates being removed by children. Grates should have openings that restrictthe entry of debris likely to cause blockages.

To minimise the risk of debris blocking grates or outlets, inlet pits should be located on driveways, walkways, orother impervious areas wherever possible.

For below-ground storages, it is advisable to make provisions for fresh water to wash down the walls of the storageand flush out accumulated sediment and other deposits.

The optimal solution will generally be a system where the property owner, bodycorporate, or responsible authority isable to carry out routine maintenance. Where the property owner or occupier cannot maintain the structure, thismust be clearly identified in the maintenance schedule.

13.2 Maintenance Schedule

A maintenance schedule should be prepared and included in the detailed design submission. The schedule is a setof operating instructions for future property owners and/or occupiers. It should be clearly and simply set out and

include the following type of information.

(i) Who should do the maintenance?(ii)What must be done?(iii) How often should it be done?

The frequencies of both inspections and maintenance will be highly dependant on the nature of the development,location of the storage, and the occurrence of major storms. Suggested frequencies are provided in Table 19.4.

Table 19.4 Suggested Frequencies for Inspection and Maintenance

inspect system every 3 months and after heavy rainfall

Residential lots clean system as required, generally at least every 6 months

inspect system every 2 months and after heavy rainfallCommercial and Industrial lots clean system as required, generally at least every 4 months

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14.0 Design Procedures

General procedures for both the preliminary and detailed design of OSD storage systems are given as follows:

Inspect development site toidentify drainage constraints

Undertake site survey andprepare contour plan

Discuss site layout withArchitect/builder/developer

PreparePreliminary Drainage Plan

Review architectural/building,landscape plans

SubmitPreliminary Drainage Plan

with Land Sub-divisionApplication

external flows entering the site catchment area of any external flows potential discharge points

potential storage areas

location and levels of public drainage system sufficient surface levels to characterise site

extending into adjoining lots if necessary any other constraints (e.g. services and drainage reserves)

estimate storage volume required estimate external flows entering the site

establish building and site layout

select type and location of suitable storage(s)

determine areas unable to drain to storages estimate storage levels and assess available discharge points identify emergency spillway types and locations identify overland flow paths for external flows and storage overflows

check other plans prepared for the development for anyanomalies or conflicts with the Preliminary Drainage Plan

Figure 19.24 Preliminary Design Procedure for OSD Storage Systems

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Obtain copies of approved plansand conditions

Select discharge control deviceand finalise

storage volumes

Design storage systems

select discharge control device for each storage establish level of outlet(s) and ensure free outfall if possible finalise required storage volume(s)

distribute final storage volume(s) to minimise nuisanceponding conditions to property owners

check underground storages for access and ease of maintenance ensure sufficient weir capacity for storage overflows

approved Preliminary Drainage Plan

development/subdivision consent conditions landscape and architectural/building plans

Prepare calculation sheetsand maintenance schedule

Review design

Submit

Detailed Drainage Planwith Building Plan Application

prepare calculation sheets for each storage system

prepare maintenance schedule outlining necesarymaintenance practices

review other plans prepared for the development for any anomaliesor conflicts with the Detailed Drainage Plan

check all stormwater-related development consentconditions have been satisfied

Design drainageconveyance system

Prepare detaileddesign drawings

ensure storage design ARI flows are conveyed to storage for allareas designed to drain to storages

check overland flowpaths have adequate capacity to ensureexternal flows bypass on-site storages

undertake structural design of system elements as necessary prepare plans of sufficient standard and detail to allow

builders/plumbers to construct system specify construction materials

Figure 19.25 Detailed Design Procedure for OSD Storage Systems

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1.0Design Acceptance Criteria1.1. Design Rainfall(Chapter 4, Volume 2, MSMA)

A major/minor system approach shall be adopted for the planning and design of urban stormwater systems.

The minor system is intended to collect and convey runoff from relatively frequent storm events to minimize

inconvenience and nuisance flooding. The major system is intended to safely convey runoff not collected by theminor drainage system to waterways or rivers. The major systems must protect the community from theconsequences of large, reasonable rare events, which could cause severe flood damage, injury and even loss of life.The definition of major/minor system does not refer to size of the drains. Event ARIs to be adopted for the planningand design of minor and major stormwater systems shall be in accordance with Table 4.1 (MSMA, 2000)

Table 4.1 Design Storm ARIs for Urban Stormwater Systems

Average Recurrence Interval (ARI) of Design Storm (year)

QuantityType of Development(See Note 1)

Minor SystemMajor System

(See Note 2 and 3)

Quality

Open Space, Parks andAgricultural Land in urban areas

Residential: Low density Medium density High Density

Commercial, Business andIndustrial Other than CDB

Commercial, Business, Industrialin Central Business District(CDB) areas of Large Cities

1

2510

5

10

up to 100

up to 100up to 100up to 100

up to 100

up to 100

3 month ARI (for all typesof development)

Notes: (1) If a development falls under two categories then the higher of the applicable storm ARIs from theTable shall be adopted.

(2) The required size of trunk drains within the major drainage system, varies. According to currentpractices the trunk drains are provided for the areas larger than 40 ha. Proceeding downstream in thedrainage system, a point may be reached where it becomes necessary to increase the size of the trunkdrain in order to limit the magnitude of gap flows as described in Section 4.6.2.

(3) Ideally, the selection of design storm ARI should also be on the basis of economic efficiency. Inpractice, however, economic efficiency is typically replaced by the concept of the level of protection.In the case where the design storm for higher ARI would be impractical, then the selection ofappropriate ARI should be adjusted to optimise the ratio cost to benefit or social factors. Consequentlylower ARI should be adopted for the major system, with consultation and approval from Local Authority.However, the consequences of the higher ARI shall be investigated and made known. Even though thestormwater system for the existing developed condition shall be designed for a lower ARI storm, theland should be reserved for higher ARI, so that the system can be upgraded when the area is built upin the future.

(4) Habitable floor levels of buildings shall be above the 100 year ARI flood level.

(5) In calculating the discharge from the design storm, allowance shall be made for any reduction indischarge due to quantity control (detention or retention) measures installed as described in Section4.5.

1.2 Major and Minor Systems

The design objectives of the major and minor systems are described in Table 11.1 (MSMA, 2000). Design conceptsfor the major and minor systems are diagrammatically shown in Figure 11.2 (MSMA, 2000).

The minor system is designed to convey runoff from a minor storm, which occurs relatively frequently, andwould otherwise cause inconvenience and nuisance flooding.

The minor system typically comprises a network of kerbs, gutters, inlets, open drains and pipes. The major system, on the other hand, comprises the many planned and unplanned drainage routes, which

convey runoff from a major stormto waterways and rivers.

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Major Flood

Open Drain (or Pipe)

Habitable Floor Levelof Building

Local Road

Padestrian Safety (Wading)Requirements Apply in Major Flood

Freeboard in

Major Flood

Highway is Trafficable inMajor Flood

MAJOR SYSTEM

Minor Flood

MINOR SYSTEM

Inlet

The major system is expected to protect the community from the consequences of large, reasonably rareevents, which could cause severe flood damage, injury and even loss of life.

Table 11.1 Major and Minor System Design Objectives

Major System Minor System

Reduced injury and loss of life Improved aestheticsReduced disruption to normal business activities Reduction in minor traffic accidents

Reduced damage to infrastructure services Reduced health hazards (mosquitoes, flies)

Reduced emergency services costs Reduced personal inconvenience

Reduced flood damage Reduced roadway maintenance

Reduced loss of production -

Reduced clean-up costs -

Increased feeling of security -

Increased land values -

Improved aesthetics and recreational opportunities -

Source: after Argue (1986)

Figure 11.2 Major and Minor System Design Concepts

1.3 On-site and Community Systems

On-site facilities are primarily minor drainage structures provided on individual housing, industrial and infrastructuresites. They are usually built and maintained by private parties/developers. For quantity design they are based onpeak inflow estimates using the Rational Method with design storms between 2 year and 10 year ARI.

Community facilities are major drainage structures provided to cater for larger areas, which can combine differentlanduse areas. They are usually built and maintained by the regulatory authority. For quantity design they arebased on peak inflow estimates using preferably the Hydrograph Method with larger design storms, up to100 year ARI in some instances, depending on the downstream protection requirement (Figure 11.4).

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Figure 11.4 General Design Concept for Multilevel Stormwater System

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2.0Design Storm2.1 Polynomial Approximation of IDF Curves

Polynomial expressions in the form of Equation 1 have been fitted to the published IDF curves for the 35 maincities/towns in Malaysia.

32 ))(ln())(ln()ln()ln( tdtctbaItR +++= (13.2)where,RIt = the average rainfall intensity (mm/hr) for ARI and duration tR = average return interval (years)t = duration (minutes)ato d are fitting constants dependent on ARI which are given in Appendix 13.A (MSMA,2000).

2.2 IDF Values for Short Duration Storms

It is recommended that Equation 1 be used to derive design rainfall intensities for durations down to a lower limit of30 minutes.

For duration between 5 and 30 minutes, the design rainfall depth Pdfor a short duration d(minutes) is given by,

)( 306030 PPFPP Dd = (13.3)

where P30, P60are the 30-minute and 60-minute duration rainfall depths respectively, obtained from the publisheddesign curves. FDis the adjustment factor for storm duration.

The rainfall intensity for short duration storms is given by,

d

PI d= (13.4)

where Pd(mm) is rainfall depth in mm and dis duration in hours.

The value of FDis obtained from Table 13.3 as a function of2P24h, the 2-year ARI 24-hour rainfall depth. Values of

2P24hfor Peninsular Malaysia are given in Figure 13.3 (MSMA, 2000).

Table 13.3 Values of FDfor Equation 13.32P24h(mm)Duration

West Coast East Coast

(minutes) 100 120 150 180 All

5 2.08 1.85 1.62 1.40 1.39

10 1.28 1.13 0.99 0.86 1.03

15 0.80 0.72 0.62 0.54 0.74

20 0.47 0.42 0.36 0.32 0.48

30 0.00 0.00 0.00 0.00 0.00

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2.3 IDF Values for Frequent Storms

Water quality studies, in particular, require data on IDF values for relatively small, frequent storms. The followingpreliminary equations are recommended for calculating the 1, 3, 6-month and 1 year ARI rainfall intensities in thedesign storm, for all durations:

DD II 2083.04.0 = (13.5a)

DD II 225.05.0 = (13.5b)

DD II 25.06.0 = (13.5c)

DD II 218.0 = (13.5d)

where, 0.083ID,0.25ID,

0.5IDand1IDare the required 1, 3, 6-month and 1-year ARI rainfall intensities for any duration

D, and 2IDis the 2-year ARI rainfall intensity for the same duration D, obtained from IDF curves.

2.4 Areal Reduction Factor

It is important to understand that IDF curves give the rainfall intensity at a point. Storm spatial characteristics are

important for larger catchments. In general, the larger the catchment and the shorter the rainfall duration, the lessuniformly the rainfall is distributed over the catchment.

The areal reduction is expressed as a factor less than 1.0. No areal reduction factor is to be used for catchmentareas of up to 10 km2. For large catchments, the design rainfall is calculated with Equation 13.1:

pAc IFI = (13.1)

Where, FA is the areal reduction factor, Ic is the average rainfall over the catchment, and Ip is the point rainfallintensity.

Suggested values of areal reduction factor FAfor Peninsular Malaysia are given in HP No.1-1982. These values arereproduced in Table 13.1 below for catchment areas of up to 200 km2. The values are plotted in Figure 13.1 (MSMA,2000). Intermediate values can be interpolated from this figure.

Table 13.1 Values of Areal Reduction Factors (FA)

Ca tchment

A rea

(km ) 0 .5 1 3 6 24

0 1 .00 1 .00 1 .00 1 .00 1 .00

10 1 .00 1 .00 1 .00 1 .00 1 .00

50 0 .82 0 .88 0 .94 0 .96 0 .97

100 0 .73 0 .82 0 .91 0 .94 0 .96

150 0 .67 0 .78 0 .89 0 .92 0 .95

200 0 .63 0 .75 0 .87 0 .90 0 .93

Storm D ura t ion (ho urs )

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0.20

0.40

0.60

0.80

1.00

10 100 1000Catchment Area (km

2)

Factor,FA

24 hours

6 hours

3 hours

1 hour

0.5 hour

Figure 13.1 Graphical Areal Reduction Factors

2.5 Design Rainfall Temporal Patterns

The temporal distribution of rainfall within the design storm is an important factor that affects the runoff volume,and the magnitude and timing of the peak discharge. Design rainfall temporal patterns are used to represent thetypical variation of rainfall intensities during a typical storm burst. Standardization of temporal patterns allowsstandard design procedures to be adopted in flow calculation.

The recommended patterns in this Manual are based on those from AR&R for durations of one hour or less and fromHP No. 1 (1982) for longer durations.

The standard durations recommended in this Manual for urban stormwater studies are listed in Table 13.4. Theinterim temporal patterns to be used for these standard durations are given in Appendix 13.B (MSMA, 2000).

Table 13.4 Standard Durations for Urban Stormwater Drainage

Standard Duration(minutes)

Number of TimeIntervals

Time Interval(minutes)

10 2 5

15 3 5

30 6 5

60 12 5120 8 15

180 6 30

360 6 60

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3.0Runoff Estimation3.1 Time of Concentration (Chapter 14, Volume2, MSMA)

The time of concentration (tc) is often considered to be the sum of the time travel to an inlet plus the time of travelin the stormwater conveyance system.

Although travel time from individual elements of a system may be very short, the total nominal flow travel time to beadopted for all individual elements within any catchment to its points of entry into the stormwater drainage networkshall not be less than 5 minutes.

For small catchments up to 0.4 hectare in area, it is acceptable to use the minimum times of concentration given inTable 14.3 (MSMA, 2000) instead of performing detailed calculation.

Table 14.3 Minimum Times of Concentration

Drainage Element Minimum tc(minutes)

Roof and property drainage 5

Road inlet 5

Small areas < 0.4 hectare 10Note: The recommended minimum times are based on the minimum duration for which meaningful rain intensity data are available.

The time of concentration (tC) is given by

tC = to + td

Where to= overland flow time and td= flow time in channel, kerbed gutter or pipe.

3.1.1 Overland Flow Time

The Friends formula should be used to estimate overland sheet flow times. It is also given in the form of anomograph in Design Chart 14.1 (MSMA, 2000) for shallow sheet flow over a plane surface.

2/1

3/1..107S

Lnto = (14.1)

Where,

to = overland sheet flow travel time (minutes)

L = overland sheet flow path length (m)

n = Mannings roughness value for the surface

S = slope of overland surface (%)Note: Values for Mannings 'n ' are given in Table 14.2 (MSMA, 2000).

Table 14.2 Values of Mannings 'n' for Overland Flow

Manning nSurface Type

Recommended Range

Concrete/Asphalt** 0.011 0.01-0.013Bare Sand** 0.01 0.01-0.06Bare Clay-Loam**(eroded)

0.02 0.012-0.033

Gravelled Surface** 0.02 0.012-0.03Packed Clay** 0.03 0.02-0.04Short Grass** 0.15 0.10-0.20Light Turf* 0.20 0.15-0.25Lawns* 0.25 0.20-0.30Dense Turf* 0.35 0.30-0.40Pasture* 0.35 0.30-0.40Dense Shrubbery andForest Litter*

0.40 0.35-0.50

* From Crawford and Linsley (1966) obtained by calibration of Stanford Watershed Model.** From Engman (1986) by Kinematic wave and storage analysis of measured rainfall runoff data.

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3.1.2 Roof Drainage Flow Time

The time of flow travel on roofs for residential is generally very small and may be adopted as the minimum time of5 minutes. However, for larger residential, commercial, and industrial developments the travel time may be longerthan 5 minutes in which case it should be estimated using the procedures for pipe and/or channel flow asappropriate.

3.1.3 Kerbed Gutter Flow Time

An approximatekerbed gutter flow time can be estimated from Design Chart 14.2 (MSMA, 2000) or by the followingempirical equation:

S

Ltg

40= (14.3)

Where,

tg = kerbed gutter flow time (minutes)

L = length of kerbed gutter flow (m)

S = longitudinal grade of the kerbed gutter (%)

3.1.4 Channel Flow Time

The Manning's Equation is recommended to calculate flow along a open channel:

V=2/13/21 SR

n (14.4a)

From which,

2/13/2

60

.SR

Lntch = (14.4b)

Where,

V = average velocity (m/s)

n = Manning's roughness coefficient

R = hydraulic radius (m)S = friction slope (m/m)

L = length of reach (m)

tch = travel time in the channel (minutes)

3.1.5 Pipe Flow Time

The time of flow through pipe, tp, is then given by:

V

Ltp = (14.5)

Where,

L = pipe length (m)V = average pipe velocity (m/s)

The velocity V in a pipe running just full can be estimated from pipe flow charts such as those in Chapter 25,Appendix 25.B (MSMA, 2000).

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3.2 Time of Concentration for Natural Catchment

For natural/landscaped catchments and mixed flow paths the time of concentration can be found by use of theBransby-Williams' Equation 14.6 (AR&R, 1987). In these cases the times for overland flow and channel or streamflow are included in the time calculated.

Here the overland flow time including the travel time in natural channels is expressed as:

5/110/1.SALFt cc = (14.6)

Where,

tc = the time of concentration (minute)

Fc = a conversion factor, 58.5 when areaAis in km2,

or 92.5 when area is in ha

L = length of flow path from catchment divide to outlet (km)

A = catchment area (km2or ha)

S = slope of stream flow path (m/km)

3.3 Time of Concentration for Small Catchments

For small catchments up to 0.4 hectare in area, it is acceptable to use the minimum times of concentration given inTable 14.3 instead of performing detailed calculation.

Table 14.3 Minimum Times of Concentration

Drainage Element Minimum tc(minutes)

Roof and property drainage 5

Road inlet 5

Small areas < 0.4 hectare 10

3.4 Rational Method

3.4.1 Rational Formula

The Rational Formula is one of the most frequently used urban hydrology methods in Malaysia to computingstormwater flows from rainfall. It gives satisfactory results for small catchments up to 80 hectares only. The formulais:

360

.. AICQ t

y

y = (14.7)

where,

Qy = yyear ARI peak flow (m3/s)

C = dimensionless runoff coefficientyIt = yyear ARI average rainfall intensity over time of concentration, tc, (mm/hr)

A = drainage area (ha)

Assumptions used in the Rational Method are as follows:

1. The peak flow occurs when the entire catchment is contributing to the flow.2. The rainfall intensity is the same over the entire catchment area.3. The rainfall intensity is uniform over a time duration equal to the time of concentration, tc..4. The ARI of the computed peak flow is the same as that of the rainfall intensity, i.e., a 5 year ARI rainfall

intensity will produce a 5 year ARI peak flow.

A general procedure for estimating peak flow using the Rational Method is shown in Figure 14.2 (MSMA, 2000).

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Select design ARI

Discretise sub-catchment

Estimate time of

concentration, tc

divide sub-catchment into segments of homogeneous

land use or surface slope

estimate overland flow time

select design ARI for both minor and major drainage

systems

Determine average rainfall

intensity, yIt

Determine average rainfall

intensity, yIt

Estimate runoff coefficients

Calculate average runoffcoefficient

calculate yItfor design ARI of yyears and duration t

data for area of interest

use Equation 14.8

calculate peak flow rate fromEquation 14.7

estimate flow times for all other flow components

within the sub-catchment such as kerb gutters, pipe,

and channels, etc.

estimate C values for each segment if there aredifferent land covers

equal to the time of concentration, from IDF

Calculate peak flow rate Qy

for the sub-catchment

Figure 14.2 General Procedure for Estimating Peak Flow for a Single Sub-catchment Using the Rational Method

3.4.2 Runoff Coefficient

The runoff coefficient, C, is a function of the ground cover and a host of other hydrologic abstractions. The runoffcoefficient accounts for the integrated effects of rainfall interception, infiltration, depression storage, and temporarystorage in transit of the peak rate of runoff. It depends on rainfall intensity and duration as well as on thecatchment characteristics. During a rainstorm the actual runoff coefficient increases as the soil become saturated.The greater the rainfall intensity, the lesser the relative effect of rainfall losses on the peak discharge, and thereforethe greater the runoff coefficient.

Recommended runoff coefficient (C)is given in Design Chart 14.3 (urban areas) or Design Chart 14.4 (rural areas) inMSMA (2000), respectively.

Calculate peak flow rate from Equation 14.7

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3.5 Hydrograph Method

For larger catchments, storage and timing effects become significant, and a hydrograph method is needed.Hydrograph methods must be used whenever rainfall spatial and temporal variations or flow routing/storage effectsneed to be considered. Flow routing is important in the design of stormwater detention, water quality facilities, andpump stations, and also in the design of large stormwater drainage systems to more precisely reflect flow peakingconditions in each segment of complex systems.

3.5.1 Time-Area Method

Time-area methods utilise a convolution of the rainfall excess hyetograph with a time-area diagram representing theprogressive area contributions within a catchment in set time increments. Separate hydrographs are generated forthe impervious and pervious surfaces within the catchment. These are combined to estimate the total flow inputs toindividual sub-catchment entries to the urban drain network.

This method assumes that the outflow hydrograph for any storm is characterised by separable subcatchmenttranslation and storage effects. Pure translation of the direct runoff to the outlet via the drainage network isdescribed using the channel travel time, resulting in an outflow hydrograph that ignores catchment storage effects.

To apply the method, the catchment is first divided into a number of time zones separated by isochrones or lines of

equal travel time to the outlet (Figure 14.5b). The areas between isochrones are then determined and plottedagainst the travel time as shown in Figure 14.5c.

The translated inflow hydrograph ordinates qi for any selected design hyetograph (Figure 14.5d) can now bedetermined. Each block of storm in Figure 14.5a should be applied (after deducting losses) to the entire catchment;the runoff from each sub-area reaches the outflow at lagged intervals defined by the time-area histogram. Thesimultaneous arrival of the runoff from areas A1 ,A2,for storms I1 , I2 ,should be determined by properly laggingand adding contributions, or generally:

iiii AIAIAIq .......... 1211 +++= (14.10)

Where,

qi = the flow hydrograph ordinates (m3/s)

Ii = excess rainfall hyetograph ordinates (m/s)Ai = time-area histogram ordinates (m2)

i = number of isochrone area contributing to the outlet

For example, the runoff from storms I1onA3, I2onA2and I3onA1 arrive at the outlet simultaneously, and q3is the

total flow. The total inflow hydrograph (Figure 14.5d) at the outlet can be obtained from Equation 14.10.

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t

(a) Rainfall Histogram (b) Catchment Isochrones

2t

3t

4t

Isochrones

AreaA1

A2

A4

A3

(c) Time-Area Curve (d) Runoff Hydrograph

Runoff(m3/

s)

Time t

tq1

q2

q3

q5

q4

Rain

fall

intensityI

Time t

I1

I2 I3

0

I4

t 2t 3t 4t

t

CumulativeArea

Time t

0 t 2t 3t 4t

Figure 14.5 TimeArea Method

3.5.2 Other Hydrograph Methods

Kinematic Wave Method Non-linear Reservoir Method Rational Method Hydrograph Method

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(a) Uncovered Open Drain

Varies0.5 m minimum1.0 m maximum

5

00maximum

(b) Covered Open Drain

0.5 m minimum1.0 m maximum

Varies

0.5

mm

inimum

1.0

mm

aximum

Varies

Grate orsolid cover

Where there is significant advantage in placing a lined drain on an alignment reserved for another authority, it maybe so placed provided that both the authority responsible for maintenance of the stormwater conveyance and theother authority concerned agree in writing to release the reservation.

Curved alignments are preferred on curved roadways. However, where there are significant advantages, e.g. culs-de-sac or narrow street verges, straight alignments may be acceptable.

3.1.2 Privately Owned Lots

Municipal lined drains shall not be located within privately owned properties. Where lined drains are to be providedat the side or rear of private properties, they shall be placed within a separate drainage reserve in accordance withFigure 26.1(b) (MSMA, 2000)

3.1.3 Public Open Space

The location of lined drains within public land such as open space shall be brought to the attention of the LocalAuthority for consideration. As a guide, unless directed otherwise, lined drains shall be located as close as practicalto the nearest property boundary with due consideration for public safety.

3.2 Lining Materials

Lined drains shall be constructed from materials proven to be structurally sound and durable and have satisfactoryjointing systems.

Lined open drains may be constructed with any of the following materials: plain concrete reinforced concrete stone pitching plastered brickwork precast masonry blocks

Alternative drain materials may be acceptable. Proposals for the use of other materials shall be referred to the LocalAuthority for consideration.

3.3 Geometry

The dimensions of lined open drains have been limited in the interests of public safety and to facilitate ease ofmaintenance. The minimum and maximum permissible cross-sectional dimensions are illustrated in Figure 26.3(MSMA, 2000) and described as follows.

Figure 26.3 Dimension Limits for Open Lined Drains

3.3.1 Depth

The maximum depth for lined open drains shall be in accordance with Table 26.1 (MSMA, 2000)

Table 26.1 Recommended Maximum Depths

Cover Condition Maximum Depth (m)

Without protective covering 0.5With solid or grated cover 1.0

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3.3.2 Width

The width of lined open drains may vary between a minimum width of 0.5 m and a maximum width of 1.0 m.

3.3.3 Side slope

The recommended maximum side slopes for lined open drains is indicated in Table 26.2 (MSMA, 2000)

Table 26.2 Recommended Maximum Side Slopes

Drain Lining Maximum Side Slope

Concrete, brickwork,and blockwork

Vertical

Stone pitching 1.5(H):1(V)

Grassed/Vegetated 2(H):1(V)

3.4 Covers

Open drains in locations open to pedestrian access shall be covered if the depth of the drain exceeds 0.6 m. Thetype of drain cover used will depend on the expected live loadings and whether or not the drain is required to acceptsurface flow. The following types of drain covering are acceptable:

precast reinforced concrete metal grates and solid plates

3.4.1 Precast Reinforced Concrete Covers

Drains not subject to traffic loads or inflow of surface runoff may be covered using precast reinforced concretecovers. Covers should be sized such that the weight is limited to what can be easily lifted by 2 workmen to gainaccess for maintenance.

3.4.2 Metal Grates and Solid Plates

Drains subject to vehicular traffic loads or inflow of surface runoff shall be covered using metal grates or solid plates.Metal covers shall be designed in accordance with the latest edition of relevant Malaysian Standard.

The type of drain cover shall be selected according to the following criteria: subject to traffic loadings Class C subject to traffic loadings Class D

Cast iron covers shall be 'GATIC', or equal.

Covers for lined open drains shall be set at the finished cover levels given in Table 26.3 (MSMA, 2000)

Table 26.3 Cover Levels

Location Cover Level

Paved Areas Flush with finished surfaceFootpaths and street verges Flush with finished surfaceElsewhere 100 mm above surface to allow for topsoiling and grassing

3.5 Freeboard

The depth of an open lined drain shall include a minimum freeboard of 50 mm above the design storm water level inthe drain.

3.6 Velocities and Grades

To prevent sedimentation and vegetative growth, the minimum average flow velocity shall not be less than 0.6 m/s.

The maximum average flow velocity shall not exceed 4 m/s. For flow velocities in excess of 2 m/s, drains shall beprovided with a 1.2 m high handrail fence, or covered with solid or grated covers for the entire length of the drainfor public safety.

3.7 Vehicular Crossings

Driveway entrances to properties and other vehicular crossings shall be structurally designed for a 7 tonne wheelloading.

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3.8 Concrete Works

3.8.1 Concrete Lining Section Thickness

All concrete lining shall be designed to withstand the anticipated hydrodynamic and hydrostatic forces. Theminimum thickness shall not be less than 100 mm.

3.8.2 Concrete Joints

Concrete lined channels shall be constructed of either plain or reinforced concrete (depending on loading conditions)without transverse joints. Expansion/contraction joints shall be installed where new concrete lining is connected to arigid structure or to existing concrete, which is not continuously reinforced. Longitudinal joints, where required, shallbe constructed on the side walls at least 300 mm vertically above the drain invert.

Construction joints are required for all cold joints and where the lining thickness changes.

Reinforcement, if required, shall be continuous through the joint.

All joints shall be designed to prevent differential movement.

3.8.3 Concrete Finish

The surface of the concrete lining may be finished in any of the finishes listed in Design Chart 26.1, MSMA. Thedesigner should check with the Local Authority to determine which finishes are acceptable.

3.8.4 Reinforcement Steel

Steel reinforcement shall have a minimum tensile strength fy= 460 N/mm2. Either deformed bars or wire mesh may

be used depending on load requirements.

Reinforcing steel shall be placed at the centre of the section.Provide additional steel as needed to meet retaining wall structural needs.

3.8.5 Earthwork

The following areas shall be compacted to at least 95% of maximum density as determined by ASTM D698(Standard Proctor):

the top 150 mm of subgrade immediately beneath the drain bottom and side slopes the top 150 mm of earth surface within 1 m of the top edges of the drain all fill material

The subgrade under the drain must be of acceptable strength for the expected loadings, i.e. weight of concrete andwater at maximum flow depth. The following may be used to strengthen or compensate for deficient subgrades:

piling concrete blinding layer geotextiles

3.8.6 Bedding

Provide 100 mm of granular bedding, equivalent in gradation to 20 mm concrete aggregate, under the drain bottomand side slopes.

3.9 Stone Pitching

3.9.1 Stone

The stone used for pitching shall be hard, durable and dense, and not subject to deterioration upon exposure to airand water. Suitable stone is clean rough quarry stone, pit or river cobbles, or a mixture of any of these materials.

Individual pieces shall be approximately cubical or spherical. The maximum stone dimension shall be 250 mm with aminimum dimension between 100 and 150 mm.

3.9.2 Cement Mortar

Cement mortar shall be 1 part ordinary Portland cement to 3 part fine aggregate by volume with sufficient wateradded to produce a suitable consistency for the intended purpose.

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3.9.3 Capping

The top of stone pitching shall be capped with cement mortar to produce an even surface to match the surroundingground level and to provide seating for protective covers if required.

3.10 Bricks and Precast Blocks

Bricks shall be sound, hard, and shall comply with the requirements of Malaysian Standard 76. Precast blocks shallbe constructed in accordance with the Manufacturers specifications.

Cement mortar for brickwork and blockwork shall be the same as that specified for stone pitching.All exposed brickwork surfaces shall be plastered with a 20 mm thickness of plaster consisting of 1 part masonrycement complying with Malaysian Standard 794 to 3 parts sand is volume.

3.11 Weep Holes

Appropriate numbers of weep holes shall be provided in the walls of all open drains relieve hydrostatic pressure.

3.12 Strut Beams

Precast or cast-in-situ struts shall be provided at the top of all stone pitched, brick, and unreinforced precast block

drains that exceed 0.9 m in depth. Strut beams shall be spaced at intervals not exceeding 6 m.

Strut beams shall be 100 mm square in section and shall be reinforced with a single centrally located Y12 bar.

3.13 Maintenance

Lined open drains will require periodical maintenance to remove weed growth, sediment deposits, and debris andlitter accumulation to maintain the designed hydraulic capacity of the drain.

Damaged linings or displaced joints or strut beams should be repaired as soon as practical to prevent furtherdeterioration or failure of sections of the drain. Refer to Section 28.15 (MSMA) for recommendations for inspection.

4.0 Composite Drains

4.1 General

A combination of a grassed section and a lined drain may be provided in locations subject to dry-weather base flowswhich would otherwise damage the invert of a grassed swale, or in areas with highly erodible soils.

The lined drain section is provided at the drain invert to carry dry-weather base flows and minor flows up to arecommended limit of 50% of the 1 month ARI. The grassed section shall be sized to provide additional flowcapacity up to and including the design storm ARI.

The composite drain components shall comply with the relevant design requirements specified for grassed swalesand lined drains.

4.2 Geometry

The preferred shape for a composite drain is shown in Figure 26.4 (MSMA, 2000)

C

Design flow width + freeboard

14 min

Qminor50 mm freeboard

4 min1

Grassed Section

Lined drain

Figure 26.4 Recommended Composite Drain Cross-Section

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C


4 min1 1

4 min

Qminor

(a) ' Vee' Shaped

300mm freeboard

15050

11

4 min14 min

Batter BatterBase

Qminor


(b) Trapezoidal Shaped

C300mm freeboard

5.0 Grassed Swale

5.1 Location

A grassed swale, depression, or minor formalized overland flow path is generally located within parkland, open spaceareas, along pedestrian ways, and along roadways with limited access to adjacent properties.

Grassed swale, should not be provided in urban street verges with adjacent standard density residential andcommercial properties where on-street parking is permitted.

5.2 Alignment

Standardized alignments for grassed swales are provided to limit the negotiations needed when other services areinvolved.

5.2.1 Roadway Reserves

In new development areas, the edge of a grassed swale should generally be located 0.5m from the road reserve orproperty boundary. In existing areas, this alignment may be varied depending on the alignment and depth ofexisting underground services within the road verge. The designer should consult the Local Authority for appropriatealignments in existing areas.

Swales may also be located within road media strips, provided the median is of sufficient width to contain the swaleplus a 1.0 m berm on either side. The swale should be centrally located within the median

5.2.2 Privately Owned Lots

Municipal grassed swales shall not be located within privately owned properties. If swales are to be provided at theside or rear of private properties, they shall be placed within a separate drainage reserve of minimum dimensions inaccordance with Figure 26.1(a).

5.2.3 Public Open Space

The location of swales within public land such as open space should generally conform to natural drainage paths

wherever practical. The designer should consult with the Local Authority for appropriate alignments with dueconsideration for public safety.

5.3 Geometry

The preferred shapes for grassed swales are shown in Figure 26.2 (MSMA, 2000). The flow depth shall not exceed0.9 m.

A vee shaped section will generally be sufficient for most applications; however, a trapezoidal section may be usedfor additional capacity or to limit the depth of the swale.

Figure 26.2 Recommended Grassed Swale Cross-Sections

5.4 Freeboard

The depth of a grassed swale shall include a minimum freeboard of 50 mm above

MSMA

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