MSMAread.pdf

Manual Saliran Mesra Alam (MSMA)

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

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 areas cause local runoff impacts on receiving water flow, quality, and ecology. Apart from erosion and sedimentation problems associated with development, it has become increasingly apparent that stormwater runoff contributes to receiving 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 government’s drive to archive a sustainable developed nation status in the early 21st century. 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 downstream receiving waters. To some extent, it should also contribute to improved urban amenity through the application of wetlands, landscape for recreation, potential beneficial reuse of stormwater (especially as a non-potable supply source), 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 approaches to controlling the quality, and to some extant, the quality of stormwater runoff. In addition to the traditional conveyance-oriented approach, a potential effective and preferable approach to stormwater management is the storage-oriented approach. The function of this approach is to provide for the temporary storage of stormwater runoff at or near its point of origin with subsequent slow release to the downstream stormwater system or receiving water (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 to reduce the costs of large stormwater drainage system by reducing the size required for such systems in downstream areas. The reduced post-development runoff hydrograph is typically designed so that the peak flow is equal to or less than the pre-development peak flow rate. Additionally, in some instances, the volume of the post-development runoff hydrograph is required to be reduced to the same volume as the pre-development runoff hydrograph. This latter requirement will necessitate the use of retention facilities to retain the differences in volume between the post and pre-development hydrograph.

Post Development Uncontrolled Runoff

Discharge

Pre-Development Uncontrolled Runoff Post-Development Controlled Runoff by Detention

Time Figure 18.1 Hydrograph Schematic

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3.0 ON-SITE DETENTION 3.1 Principles of Quantity Control Stormwater quantity control facilities can be classified by function as: (i) detention, or (ii) retention facilities. The detention concept is most often employed in urban stormwater drainage systems to limit the peak outflow rate. The primary function of detention facilities is to reduce peak discharge by the temporary storage and gradual release of stormwater runoff by way of an outlet control structure or other release mechanism. Retention facilities are commonly sized to provide only a reduction in the volume of stormwater runoff generated from an urban area. However, peak flow reduction can also be achieved in minor storm events if the storage volume is large enough to capture the peak flow before the storage is filled, i.e. the time to fill the basin is longer than the time to peak of the inflow hydrograph. Detention and retention storages may be classified on the basis of their location and size as follows (Figure 18.2):

(i) on-site storage : small storages constructed on individual residential, commercial, and industrial lots (ii) community storage : storage facilities constructed in public open space areas, or in conjunction with public recreation and sporting facilities

(iii) regional storage : large community storage facilities constructed at the lower end of catchments prior to discharge to receiving waters Facilities can also be categorised as:

(i) on-line storage : a facility that intercepts flow directly within a conveyance system. (ii) off-line storage : the diversion of flow from a conveyance system into a separate storage facility. (iii) conveyance storage : This is an often-neglected form of storage, because it is dynamic and requires channel storage routing analysis to identify. 3.2 Detention Facilities The most common type of storage facilities used for controlling peak flow are ‘dry facilities’, which release all the runoff temporarily detained during a storm. Other facilities which are becoming more commonly used are detention ‘ponds’, which incorporate a permanent pool of water for water quality control as well as provision for the temporary storage and release of runoff for flood control.

3.3 On-Site Detention On-site detention (OSD) may be provided as above-ground storages, below-ground storages, or a combination of both. The common types of above and below-ground storages used are illustrated in Figure 18.3.

(a)

Above-ground Storages The main advantages of above-ground storages are they can generally be easily incorporated into the site by slight regrading or modification to the design of surface features and are relatively inexpensive compared to below-ground storages. The above-ground storages include: (i) (ii) (iii) (iv)

(b)

Landscaped areas (such as lawns and garden beds) Impervious areas (such as ar parks, driveways, paved storage yards) Flat Roofs Surface Tanks

Below-ground Storages The main advantages of below-ground storages are they are out of sight, occupy less physical space, and will not cause any inconvenience with ponding of water that could result using above-ground storage. The examples of below-ground storage are: (i) Underground Tanks (ii) Pipe Packages

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Infiltration Trench (CIRIA, 1996)

Car Park Detention

Park Pond

LEVELS On-site Community Regional Infiltration Basin Storage Reservoir (Hall, et al., 1993)

Artificial Recharge (Todd, 1980)

Figure 18.2 Detention/Retention Storage Classifications

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(c)

Combined Storages With combined storages, a proportion of the total storage is provided as below-ground storage, whilst the remainder of the storage is provided as above-ground storage.

Rooftop Surface Tank

Car Parking and Driveway Areas Landscaped Area

Underground Tank

Pipe Package

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 groundwater 2. reduction in the settlement of the land surface in areas of groundwater depletion 3. control of saline water intrusion 4. preservation and/or enhancement of natural vegetation 5. reduction of pollution transported to the receiving waters 6. reduction of downstream flow peaks 7. reduction of basement flooding in underground drainage systems 8. 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 additional development or redevelopment is occurring. Generally, it is not possible, either physically or financially, to progressively enlarge drainage systems as redevelopments that increase impervious areas and runoff rates and volumes occur. Regulations, which put the responsibility on developers to restrict flows, are therefore attractive to drainage authorities. Flows can be limited by the use of various OSD facilities. The design procedures are based on the Rational Method. Simplified hydrographs are combined with an assumed outlet relationship to determine a critical volume of water to be stored. Often several cases are considered, to allow for different storm durations. A storage is then to be provided 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 Methods The PSD and SSR values to be applied to a particular development site are determined by hydrologic analysis of the development site only, without any consideration of the effect of site discharges on the downstream catchment. The PSD is the estimated peak flow for the site prior to development for a selected design storm. The only concern is that post-development site discharges are reduced to predevelopment levels. PSD values may be determined using either the Rational Method or a hydrograph estimation method (refer Chapter 14, MSMA). Site-based methods do not consider the effects of post-development discharges on the downstream catchment 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 design storm that will not cause flooding at any location within the catchment. These are general values that may be applied to any site within the catchment.

OSD storages may be analysed using any hydrograph estimation technique, but the Rational Method is the most popular. Rational Method hydrograph techniques are acceptable for OSD as development sites are relatively small and any errors introduced will most likely be minor. The effort involved with more sophisticated computer modelling techniques is not normally warranted. The Swinburne Method recommended in Section 7.0 or Chapter 19 (MSMA) is based 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 as early as possible in the concept planning stage for developing the site. It is far easier to integrate OSD facilities into a site arrangement as part of the total development concept than to attempt to retrofit them after the form and extent of buildings, driveways, and landscaping have been designed or constructed. This approach will give the designer the most flexibility for design and will generally allow opportunities for developing innovative and/or more cost-effective design solutions.

For developed sites, the location and level of existing structures and services can severely restrict opportunities for providing satisfactory OSD systems. It may not be practical, due to factors such as cost or public safety, to provide the 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 maximum amount 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 system design ARI of the municipal drainage system to which the storage is connected (refer Table 4.1, MSMA). The design storm 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 storm and 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 facility does 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 the OSD system. Sometimes this will not be feasible due to ground levels, the level of the receiving drainage system, or other circumstances. In these cases, as much runoff from impervious areas as possible should be drained to the OSD system. 6.5

Frequency Staged Storage

Generally the most challenging task in designing OSD systems is locating and distributing the storage(s) in the face of 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 a frequency staged storage approach are provided in Table 19.1. A typical composite storage system is illustrated in Figure 19.1. Refer to Table 19.1 for recommended maximum ponding depths in the above-ground storage component.

Table 19.1 Relative Proportions for Composite Storage Systems Proportion of Total Storage (%) Below-Ground Storage Component

Above-Ground Storage Component

Pedestrian areas

60

40

Private Courtyards

60

40

50

50

25

75

15

85

Storage Area

Parking areas and driveways Landscaped areas Paved outdoor recreation areas

Habitable building

Freeboard to building floor level

Maximum ponding level for storage design storm

Above-ground storage

'Beginning to pond' level for above-ground storage

Below-ground storage

Outlet to public drainage system (preferably free draining, but may 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. If runoff from outside the site enters the storage, it will fill more quickly, causing a greater nuisance to occupiers and it will 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 floodway must 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 calculating the site storage requirements. Such areas must be protected from future development within the site by an appropriate 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 Swinburne University of Technology in Melbourne, Australia. The method uses the Rational Method to calculate site flows, and utilises a non-dimensional triangular site hydrograph as illustrated in Figure 19.3. The site discharges are calculated using the total catchment time of concentration tc for 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 discharge depending on the position of the site within the catchment. Figure 19.2 illustrates the relationship between t c and t cs. (i)

PSD As stated in Section 4.1 the discharge design storm for estimating the PSD is the minor system design ARI of 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 a and b are different for above-ground and below-ground storages due to differences in storage geometry and outflow characteristics.

PSD =

a−

a 2 − 4b 2

(19.1)

For above-ground storage : Qp ⎞ ⎛ Q ⎞⎛ a = ⎜⎜ 4 a ⎟⎟ ⎜⎜ 0.333 tc + 0.75 tc + 0.25 tcs ⎟⎟ Qa ⎝ tc ⎠ ⎝ ⎠ b = 4 Qa Q p

(19.1a) (19.1b)

For below-ground storage : Qp ⎞ ⎛ Q ⎞⎛ + 0.35 t c + 0.65 t cs ⎟⎟ a = ⎜⎜ 8.548 a ⎟⎟ ⎜⎜ 0.333 t c tc ⎠ ⎝ Qa ⎝ ⎠ b = 8.548 Qa Q p

(19.1c) (19.1d)

where, t c = peak flow time of concentration from the top of the catchment to a designated outlet or point of concern (minutes) t cs = peak flow time of concentration from the top of the catchment to the development site (minutes) Q a = the peak post-development flow from the site for the discharge design storm with a duration equal to t c (l/s) Q p = the peak pre-development flow from the site for the discharge design storm with a duration equal to t c (l/s)

Time of concentration tc for catchment

Time of concentration tcs from top of catchment to point where flows from development site join main drainage system for catchment

Catchment in which development site

Development site

is located

Figure 19.2 Relationship Between t c and t cs for the Swinburne Method

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Assumed inflow hydrograph

Qa

Assumed outflow hydrograph SSR PSD

tf

0

tc

Figure 19.3 Swinburne Method Assumptions (t f = time for storage to fill) (ii)

SSR The storage design storm for 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 of storage adopted. These simplifications are acceptable providing the site catchment is small.

Maximum storage X X X X Critical Duration

Storage Volume (m3 )

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

X

X

X X X

Storm Duration (minutes) 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 c and d are applied for above-ground and below-ground storages to account for differences in storage geometry and outflow characteristics. SSR = 0.06 t d ( Qd − c − d

)

(19.2)

⎛ PSD c = 0.875 PSD ⎜⎜ 1 − 0.459 Qd ⎝

⎞ ⎟ ⎟ ⎠

(19.2a)

PSD 2 Qd

(19.2b)

⎛ PSD ⎞ ⎟ c = 0.675 PSD ⎜⎜ 1 − 0.392 Qd ⎟⎠ ⎝

(19.2c)

For above-ground storage :

d = 0.214

For below-ground storage :

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d = 0.117

PSD 2 Qd

(19.2d)

where, t d = selected storm duration (minutes) Q d = the peak post-development flow from the site for a storm duration equal to t d (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, t c and t cs . 5. Calculate the pre and post-development flows, Q p and Q a , for the area draining to the storage for the discharge design storm with time of concentration t c . 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 of durations to determine the maximum value. For composite storages, apportion the required SSR in accordance with Table 19.1. Note: For composite storages, use the PSD and SSR equation factors relating to the largest storage component. If these 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 of concentration t c • 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 circumstances wherever possible (i.e. not outlet controlled). If this is not possible, the outlet should be sized to account for drowned 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 a larger 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 other storage, 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 the overall 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 centre where it will be an eyesore. Where possible, grade the floor of the storage such that the discharge control structure 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-sided basins unbroken by any features maximise the volume, but may detract from the appearance of the landscaping 8.6

Construction Tolerances

OSD systems is important in protecting downstream areas from flooding. Every effort should be made to avoid, or at least minimise, construction errors. The design should allow for the potential reduction in the storage volume due to common post-construction activities such as landscaping, top dressing and garden furniture. It is recognised that achieving precise levels and dimensions may not always be possible in practice. It is therefore considered that an OSD 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).

WARNING

Colours: Triangle and “WARNING” Water Figure and other lettering

Red Blue Black

ON-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.

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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 of possibilities 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. The minimum 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 maximum depth 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 structural design of the roof is adequate to sustain increased loadings from ponded stormwater. The latest structural codes and 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 from the rooftops of buildings and store it for later domestic use. Surface tanks may also be used solely for on-site detention, 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. Roof drainage system Screen Secondary outlet

OSD storage

Primary outlet Building

Storage for re-use

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 by limiting the frequency of inundation of an above-ground storage area. In difficult topography, the only feasible solution may be to provide all or most of the required storage volume below-ground. However, it should be recognised 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 for working in confined spaces. Where practicable, the design should minimise the need for personnel to enter the storage space for routine inspection and maintenance.

8.1 Underground Tanks (a) Basic Configuration Typical 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.

Access and overflow grate

Inlet pipes

Access ladder STORAGE TANK

Outlet pipe

Trash screen

Figure 19.8 Typical Below-ground Storage Tank (b) Structural Adequacy Storage tanks must be structurally sound and be constructed from durable materials that are not subject to deterioration by corrosion or aggressive soil conditions. Tanks must be designed to withstand the expected live and dead 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 Plan Site geometry will dictate how the installation is configured in plan. A rectangular shape offers certain cost and maintenance 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 maintenance costs will normally be higher. (d) Bottom Slope To permit easy access to all parts of the storage for maintenance, the floor slope of the tank should not be greater than 10%. The lower limit for this slope is 2%, which is needed for good drainage of the tank floor. (e) Ventilation It is very important to provide ventilation for below-ground storage systems to minimise odour problems. Ventilation may be provided through the tank access opening(s) or by separate ventilation pipe risers. Although the inflow and outflow pipes can provide some ventilation of the storage tank, their contribution is unreliable and should not be considered in the design. Also, the ventilation openings should be designed to prevent air from being trapped between the roof of the storage and the water surface.

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(f) Overflow Provision An overflow system must be provided to allow the tank to surcharge in a controlled manner if the capacity of the tank is exceeded due to a blockage of the outlet pipe or a storm larger than the storage design ARI. As illustrated in Figure 19.8, an overflow can be provided by installing a grated access cover on the tank. (g) Access Openings Below-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 full and the outlet is clogged. A permanently installed ladder or step iron arrangement must be provided below each access opening if the tank is deeper than 1200 mm.

10.2

Pipe Packages

(a) Basic Configuration A pipe package is a below-ground storage consisting of one or more parallel rows of buried pipes connected by a common inlet and outlet chamber. The size of a pipe package is determined by the storage volume requirements and the physical availability of space on 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, is equipped with a flow regulator installed in the outlet chamber and an overflow spillway located at either the inlet or outlet chamber. (b) Minimum Pipe Size and Longitudinal Grade To 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 due to lack of precision during construction. (c) Low Flow Provision Although sediment will settle out inside pipe packages, the extent of deposition can be reduced by installing one of the pipes lower than the others as shown in Figure 19.9. To keep the other pipes from filling during low flows, the difference in level between the low flow pipe and other pipes needs to be sufficient to keep the low flows confined wholly within the low flow pipe. Confining low flows to one pipe will help the system to become self-cleansing. (d) Inlet Chamber The site drainage system is connected to the pipe package through an inlet chamber at the upstream end. The chamber must be large enough to permit easy access to all of the pipes by maintenance personnel and equipment.

150 mm diameter outlet pipe

4 x 900 mm diameter storage pipes A

Outlet chamber with flow regulator and overflow grate

225 mm diameter inlet pipe

A

Inlet chamber

PLAN

Low flow pipe 150 mm

0 90

SECTION A-A

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

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Concept and Design Requirement of MSMA

(e) Outlet Chamber At the downstream end, the pipe package is connected to the municipal stormwater drainage system through an outlet chamber. The chamber must also be large enough for maintenance access. Flow through the outlet chamber may be controlled by one of the primary outlet devices discussed in Section 11.0. (f) Overflow Provision To prevent water from surcharging at stormwater inlets or manholes upstream during storms larger than the storage design storm or if the primary outlet becomes blocked, a secondary outlet overflow system must be installed at either the inlet or outlet chamber (refer to Figure 19.10). (g) Access Openings Access openings are required at both chambers to facilitate normal cleaning and maintenance of a pipe package. Such openings provide access for personnel and cleaning equipment, and ventilation and lighting. If more than three parallel pipes are used, two openings should be installed in each chamber. The maximum distance between access openings shall not exceed 30 m. Therefore, on long pipe packages, additional access openings along each of the pipes may be required. 11.0

Primary Outlets

11.1

General Design Considerations

(a) Flow Regulation Flow detention is provided by a storage volume that is released by some types of flow regulating device. It is the flow regulator that determines how efficiently the storage volume will be utilised. Obviously, the flow regulator has to be in balance with the available storage volume for the range of runoff events it is designed to control.

Secondary outlet (overflow spillway) Sealed outlet chamber Primary outlet Pipe package

(a) Secondary outlet at inlet chamber Secondary outlet (overflow spillway)

Primary outlet Pipe package

(b) Secondary outlet at outlet chamber

Figure 19.10 Pipe Package Secondary Outlet (After ATV, 1978)

(b) Location of the Flow Regulator Flow regulating devices for above-ground storages are typically housed in an outlet structure, called a discharge control pit (DCP), which is an important component of the storage facility. It not only controls the release rate, but also determines the maximum depth and volume within the storage.

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Concept and Design Requirement of MSMA

Flow regulating devices for below-ground storages are typically located within the storage facility. In this type of arrangement, 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 to regulate the flow rate. Figure 19.11 shows the indicative location of the primary outlet flow regulator in a typical above and below-ground storage.

Above-ground storage DCP Locate flow regulator over storage primary outlet

Below-ground storage Figure 19.11 Primary Outlet Flow Regulator

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

11.2

Flow Regulating Devices

(i) Orifice

The 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 maintenance or replacement (refer to Figure 19.12). The orifice must be tooled to the exact dimensions as calculated, with the edges smooth and sharp (not rounded). The minimum orifice diameter shall be 25 mm to minimise the potential for blockage.

200 mm

Circular hole with sharp edges machined to 0.5 mm accuracy

3 mm thick corrosion resistant steel plate

Do

150 mm 200 mm

Figure 19.12 Typical Orifice Plate Details (UPRCT,1999)

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Concept and Design Requirement of MSMA

(ii) Flow Restricting Pipe

The main advantage of using a flow restricting pipe as a storage outlet is that it is difficult to modify the hydraulic capacity of the pipe, unlike an orifice which can be easily removed. As illustrated in Figure 19.13, the net flow restricting 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 an orifice, 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 to keep 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 a link 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: greater than 600 mm deep:

600 mm x 600 mm 900 mm x 900 mm

The following minimum dimensions will achieve predictable hydraulic characteristics: • minimum design head = 2 D o (from centre of orifice to top of overflow) • minimum screen clearance = 1.5 D o (from orifice to upstream face of screen) • minimum floor clearance = 1.5 D o (from centreline of orifice to bottom of pit)

Note : D o is the diameter of the orifice

ys

DCP or Storage facility Trash screen

D

Tota l Ene rg y L Hydr aulic ine Grad e Lin e

Flow restrict ing pipe

S.L

ye Q

L

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

Top of bund wall

Galvanised grate

DETENTION STORAGE

Meshed screen

Outlet pipe

Inlet pipe Orifice plate

Compacted granular base

Seepage holes

Figure 19.20 Typical DCP (After UPRCT,1999)

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Concept and Design Requirement of MSMA

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 of free 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 the downstream drainage system is often only able to cater for smaller storms (typically 2 year to 5 year ARI) without surcharging. The effects of this surcharging on the storage outlet are shown in Figure 19.22.

HGL

(a) DISCHARGE INDEPENDENT OF DOWNSTREAM DRAINAGE The storage is sufficiently above the downstream water level to remain a free discharge outlet

HGL

(b) DISCHARGE DEPENDENT ON DOWNSTREAM DRAINAGE The outlet to the storage is submerged for some part of the storm. As the water level in the street open drain rises, the discharge from the storage is reduced and the amount of water stored increased. An assessment should be made to determine if this effect is significant

HGL

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

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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Concept and Design Requirement of MSMA

The most commonly used arrangement for an above-ground storage is a broad-crested weir which, with most storages, can be designed to pass the entire overflow discharge with only a few centimetres depth of water over the weir. This is particularly desirable for car park storages to minimise the potential for water damage to parked vehicles. The overflow weir must be constructed from durable, non-erodible materials to ensure the discharge capacity of the overflow 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 point downstream. 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 of access, 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 be directed to a flow path through the site and conveyed to a suitable point downstream where they can be combined with any uncontrolled discharge from the site. If the site drainage system becomes blocked, any resulting overflow from an OSD storage should cause flooding in a noticeable 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.

Overflow through rectangular broad-crested weir slot in retaining wall

Garage C

Dwelling C B

Open drain A

A

B

Impervious area storage on driveway Secondary outlet

SECTION A-A

Overflow from access chamber grate directed down driveway

Underground tank

Underground tank

Secondary outlet

SECTION B-B Secondary outlet

Street

Landscaped storage area Overflow down driveway (shallow vee-shaped or trapezoidal driveway cross-section)

PLAN

DCP SECTION C-C

Figure 19.23 Examples of Secondary Outlets

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Concept and Design Requirement of MSMA

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 designer’s 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 aboveground 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 to assist removal. The guide channel prevents debris from being forced between the wall of the pit and the screen 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 confined spaces. Step irons or access ladders shall be installed where the depth of a below-ground storage or DCP is 1200 mm or greater. All inlet pits and manholes shall be fitted with removable covers and/or grates to permit maintenance, having regard to the need to prevent the covers or grates being removed by children. Grates should have openings that restrict the 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, or other impervious areas wherever possible. For below-ground storages, it is advisable to make provisions for fresh water to wash down the walls of the storage and flush out accumulated sediment and other deposits. The optimal solution will generally be a system where the property owner, bodycorporate, or responsible authority is able to carry out routine maintenance. Where the property owner or occupier cannot maintain the structure, this must 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 set of 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 Residential lots

Commercial and Industrial lots

•

inspect system every 3 months and after heavy rainfall

•

clean system as required, generally at least every 6 months

•

inspect system every 2 months and after heavy rainfall

•

clean system as required, generally at least every 4 months

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Concept and Design Requirement of MSMA

14.0

Design Procedures

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

Inspect development site to identify drainage constraints

Undertake site survey and prepare contour plan

Discuss site layout with Architect/builder/developer

Prepare Preliminary Drainage Plan

Review architectural/building, landscape plans

y y y y

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

y y y

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)

y y y

estimate storage volume required estimate external flows entering the site establish building and site layout

y y y y y

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

y

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

Submit Preliminary Drainage Plan with Land Sub-division Application

Figure 19.24 Preliminary Design Procedure for OSD Storage Systems

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Concept and Design Requirement of MSMA

Obtain copies of approved plans and conditions

y y y

approved Preliminary Drainage Plan development/subdivision consent conditions landscape and architectural/building plans

Select discharge control device and finalise storage volumes

y y y

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

y

distribute final storage volume(s) to minimise nuisance ponding conditions to property owners check underground storages for access and ease of maintenance ensure sufficient weir capacity for storage overflows

Design storage systems

y y

Design drainage conveyance system

Prepare detailed design drawings

y y

y y y

Prepare calculation sheets and maintenance schedule

Review design

ensure storage design ARI flows are conveyed to storage for all areas designed to drain to storages check overland flowpaths have adequate capacity to ensure external 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

y y

prepare calculation sheets for each storage system prepare maintenance schedule outlining necesary maintenance practices

y

review other plans prepared for the development for any anomalies or conflicts with the Detailed Drainage Plan check all stormwater-related development consent conditions have been satisfied

y

Submit Detailed Drainage Plan with Building Plan Application

Figure 19.25 Detailed Design Procedure for OSD Storage Systems

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Runoff Estimation

1.0 Design Acceptance Criteria 1.1. 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 the minor drainage system to waterways or rivers. The major systems must protect the community from the consequences 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 planning and 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) Type of Development Quantity (See Note 1) Quality Major System Minor System (See Note 2 and 3) up to 100 3 month ARI (for all types Open Space, Parks and 1 of development) Agricultural Land in urban areas Residential: • Low density • Medium density • High Density

2 5 10

up to 100 up to 100 up to 100

Commercial, Business and Industrial – Other than CDB

5

up to 100

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

10

up to 100

Notes:

(1) If a development falls under two categories then the higher of the applicable storm ARIs from the Table shall be adopted. (2) The required size of trunk drains within the major drainage system, varies. According to current practices the trunk drains are provided for the areas larger than 40 ha. Proceeding downstream in the drainage system, a point may be reached where it becomes necessary to increase the size of the trunk drain 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. In practice, 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 of appropriate ARI should be adjusted to optimise the ratio cost to benefit or social factors. Consequently lower 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 the stormwater system for the existing developed condition shall be designed for a lower ARI storm, the land should be reserved for higher ARI, so that the system can be upgraded when the area is built up in 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 in discharge due to quantity control (detention or retention) measures installed as described in Section 4.5.

1.2 Major and Minor Systems The design objectives of the major and minor systems are described in Table 11.1 (MSMA, 2000). Design concepts for 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, and would 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 storm to waterways and rivers.

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Runoff Estimation

•

The major system is expected to protect the community from the consequences of large, reasonably rare events, 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 aesthetics Reduced 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)

MAJOR SYSTEM

Habitable Floor Level of Building

Highway is Trafficable in Major Flood Inlet

Freeboard in Major Flood

Major Flood

Minor Flood Open Drain (or Pipe)

MINOR SYSTEM

Figure 11.2

Local Road Padestrian Safety (Wading) Requirements Apply in Major Flood

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 infrastructure sites. They are usually built and maintained by private parties/developers. For quantity design they are based on peak 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 different landuse areas. They are usually built and maintained by the regulatory authority. For quantity design they are based on peak inflow estimates using preferably the Hydrograph Method with larger design storms, up to 100 year ARI in some instances, depending on the downstream protection requirement (Figure 11.4).

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Runoff Estimation

Figure 11.4 General Design Concept for Multilevel Stormwater System

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Runoff Estimation

2.0 Design Storm 2.1 Polynomial Approximation of IDF Curves Polynomial expressions in the form of Equation 1 have been fitted to the published IDF curves for the 35 main cities/towns in Malaysia.

ln( RI t ) = a + b ln(t ) + c(ln(t )) 2 + d (ln(t ))3

(13.2)

where, R It = the average rainfall intensity (mm/hr) for ARI and duration t R = average return interval (years) t = duration (minutes) a to 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 of 30 minutes. For duration between 5 and 30 minutes, the design rainfall depth Pd for a short duration d (minutes) is given by,

Pd = P30 − FD ( P60 − P30 )

(13.3)

where P30, P60 are the 30-minute and 60-minute duration rainfall depths respectively, obtained from the published design curves. FD is the adjustment factor for storm duration. The rainfall intensity for short duration storms is given by,

I=

Pd d

(13.4)

where Pd (mm) is rainfall depth in mm and d is duration in hours. The value of FD is obtained from Table 13.3 as a function of 2P24h, the 2-year ARI 24-hour rainfall depth. Values of P24h for Peninsular Malaysia are given in Figure 13.3 (MSMA, 2000).

2

Table 13.3 Duration

Values of FD for Equation 13.3 2 P24h (mm) West Coast

(minutes) 5 10 15 20 30

≤ 100 2.08 1.28 0.80 0.47 0.00

120 1.85 1.13 0.72 0.42 0.00

150 1.62 0.99 0.62 0.36 0.00

East Coast ≥ 180 1.40 0.86 0.54 0.32 0.00

All 1.39 1.03 0.74 0.48 0.00

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Runoff Estimation

2.3 IDF Values for Frequent Storms Water quality studies, in particular, require data on IDF values for relatively small, frequent storms. The following preliminary equations are recommended for calculating the 1, 3, 6-month and 1 year ARI rainfall intensities in the design storm, for all durations: 0.083

I D = 0.4 × 2I D

(13.5a)

0.25

I D = 0 .5 × I D

(13.5b)

0 .5

I D = 0 .6 × I D

(13.5c)

I D = 0.8 × 2I D

(13.5d)

1

2

2

where, 0.083ID ,0.25ID , 0.5ID and 1ID are the required 1, 3, 6-month and 1-year ARI rainfall intensities for any duration D, and 2ID is 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 less uniformly 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 catchment areas of up to 10 km2. For large catchments, the design rainfall is calculated with Equation 13.1:

I c = FA × I p

(13.1)

Where, FA is the areal reduction factor, Ic is the average rainfall over the catchment, and Ip is the point rainfall intensity. Suggested values of areal reduction factor FA for Peninsular Malaysia are given in HP No.1-1982. These values are reproduced 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) C a tc h m e n t S to rm D u ra tio n ( h o u rs ) A re a (k m 2) 0 10 50 100 150 200

0 .5 1 .0 0 1 .0 0 0 .8 2 0 .7 3 0 .6 7 0 .6 3

1 1 .0 0 1 .0 0 0 .8 8 0 .8 2 0 .7 8 0 .7 5

3 1 .0 0 1 .0 0 0 .9 4 0 .9 1 0 .8 9 0 .8 7

6 1 .0 0 1 .0 0 0 .9 6 0 .9 4 0 .9 2 0 .9 0

24 1 .0 0 1 .0 0 0 .9 7 0 .9 6 0 .9 5 0 .9 3

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Runoff Estimation

1.00

Factor, F A

0.80

0.60 24 hours 6 hours 3 hours

0.40

1 hour 0.5 hour

0.20 10

100 2 Catchment Area (km ) Figure 13.1

1000

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 the typical variation of rainfall intensities during a typical storm burst. Standardization of temporal patterns allows standard 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 from HP No. 1 (1982) for longer durations. The standard durations recommended in this Manual for urban stormwater studies are listed in Table 13.4. The interim 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 Number of Time Time Interval (minutes) Intervals (minutes) 10 2 5 15 3 5 30 6 5 60 12 5 120 8 15 180 6 30 360 6 60

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Runoff Estimation

3.0 Runoff Estimation 3.1 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 travel in the stormwater conveyance system. Although travel time from individual elements of a system may be very short, the total nominal flow travel time to be adopted for all individual elements within any catchment to its points of entry into the stormwater drainage network shall 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 in Table 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 10 Note: 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 Friend’s formula should be used to estimate overland sheet flow times. It is also given in the form of a nomograph in Design Chart 14.1 (MSMA, 2000) for shallow sheet flow over a plane surface.

to =

107. n . L1/ 3 S 1/ 2

(14.1)

Where,

to = L = n = S = Note:

* **

overland sheet flow travel time (minutes) overland sheet flow path length (m) Manning’s roughness value for the surface slope of overland surface (%) Values for Manning’s 'n ' are given in Table 14.2 (MSMA, 2000). Table 14.2 Values of Manning’s 'n' for Overland Flow Surface Type Manning n Recommended Range Concrete/Asphalt** 0.011 0.01-0.013 Bare Sand** 0.01 0.01-0.06 Bare Clay-Loam** 0.02 0.012-0.033 (eroded) Gravelled Surface** 0.02 0.012-0.03 Packed Clay** 0.03 0.02-0.04 Short Grass** 0.15 0.10-0.20 Light Turf* 0.20 0.15-0.25 Lawns* 0.25 0.20-0.30 Dense Turf* 0.35 0.30-0.40 Pasture* 0.35 0.30-0.40 Dense Shrubbery and 0.40 0.35-0.50 Forest Litter* 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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Runoff Estimation

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 of 5 minutes. However, for larger residential, commercial, and industrial developments the travel time may be longer than 5 minutes in which case it should be estimated using the procedures for pipe and/or channel flow as appropriate. 3.1.3 Kerbed Gutter Flow Time An approximate kerbed gutter flow time can be estimated from Design Chart 14.2 (MSMA, 2000) or by the following empirical equation:

tg =

L

(14.3)

40 S

Where,

t g = 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= From which,

1 2 / 3 1/ 2 R S n

tch = Where, = = = = = =

V n R S L tch

n . L 2 / 3 1/ 2 R S 60

(14.4a)

(14.4b)

average velocity (m/s) Manning's roughness coefficient hydraulic radius (m) friction slope (m/m) length of reach (m) travel time in the channel (minutes)

3.1.5 Pipe Flow Time The time of flow through pipe, t p, is then given by:

tp = Where, = pipe length (m) = average pipe velocity (m/s)

L V

(14.5)

L V

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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Runoff Estimation

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 the Bransby-Williams' Equation 14.6 (AR&R, 1987). In these cases the times for overland flow and channel or stream flow are included in the time calculated. Here the overland flow time including the travel time in natural channels is expressed as:

tc =

F .L A S 1/ 5 c 1 / 10

(14.6)

Where, t c = the time of concentration (minute) Fc = a conversion factor, 58.5 when area A is in km2, or 92.5 when area is in ha L = length of flow path from catchment divide to outlet (km) A = catchment area (km2 or 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 in Table 14.3 instead of performing detailed calculation. Table 14.3 Minimum Times of Concentration Minimum t c (minutes) Drainage Element 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 computing stormwater flows from rainfall. It gives satisfactory results for small catchments up to 80 hectares only. The formula is:

C . yI t . A Qy = 360 where, = = = =

Qy C y It A

(14.7)

y year ARI peak flow (m3/s) dimensionless runoff coefficient

y year ARI average rainfall intensity over time of concentration, t c , (mm/hr) drainage area (ha)

Assumptions used in the Rational Method are as follows: 1. 2. 3. 4.

The peak flow occurs when the entire catchment is contributing to the flow. The rainfall intensity is the same over the entire catchment area. The rainfall intensity is uniform over a time duration equal to the time of concentration, tc.. 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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Runoff Estimation

Select design ARI

y

select design ARI for both minor and major drainage systems

y

divide sub-catchment into segments of homogeneous land use or surface slope

y y

estimate overland flow time estimate flow times for all other flow components within the sub-catchment such as kerb gutters, pipe, and channels, etc.

y

calculate yIt for design ARI of y years and duration t equal to the time of concentration, from IDF data for area of interest

y

estimate C values for each segment if there are different land covers

Discretise sub-catchment

Estimate time of concentration, tc

Determine average rainfall Determine average rainfall yI intensity, t

intensity, yIt

Estimate runoff coefficients

Calculate average runoff coefficient

Calculate peak flow rate Qy for the sub-catchment

y

y

use Equation 14.8

calculate peak flow rate from Calculate peak flow rate from Equation 14.7 Equation 14.7

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 runoff coefficient accounts for the integrated effects of rainfall interception, infiltration, depression storage, and temporary storage in transit of the peak rate of runoff. It depends on rainfall intensity and duration as well as on the catchment 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 therefore the greater the runoff coefficient. Recommended runoff coefficient (C) is given in Design Chart 14.3 (urban areas) or Design Chart 14.4 (rural areas) in MSMA (2000), respectively. Urban Stormwater Management Short Course

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Runoff Estimation

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 effects need to be considered. Flow routing is important in the design of stormwater detention, water quality facilities, and pump stations, and also in the design of large stormwater drainage systems to more precisely reflect flow peaking conditions 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 the progressive area contributions within a catchment in set time increments. Separate hydrographs are generated for the impervious and pervious surfaces within the catchment. These are combined to estimate the total flow inputs to individual sub-catchment entries to the urban drain network. This method assumes that the outflow hydrograph for any storm is characterised by separable subcatchment translation and storage effects. Pure translation of the direct runoff to the outlet via the drainage network is described 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 plotted against 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 be determined. 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. The simultaneous arrival of the runoff from areas A1 , A2,…for storms I1 , I2 ,…should be determined by properly lagging and adding contributions, or generally:

qi = I i . A1 + I i −1 . A2 + ....... + I1 . A i

(14.10)

Where, = the flow hydrograph ordinates (m3/s) = excess rainfall hyetograph ordinates (m/s) = time-area histogram ordinates (m2) = number of isochrone area contributing to the outlet

qi Ii Ai i

For example, the runoff from storms I1 on A3, I2 on A2 and I3 on A1 arrive at the outlet simultaneously, and q3 is the total flow. The total inflow hydrograph (Figure 14.5d) at the outlet can be obtained from Equation 14.10.

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Rainfall intensity I

Runoff Estimation

4 Δt

Δt Isochrones

Δt

2 Δt

3 Δt

Area A1

t

0

I4

Δt

I1

A4

3 Δt

I3

2Δ

I2

4 Δt

Time t

A2

(b) Catchment Isochrones

Cumulative Area

Runoff (m3/s)

(a) Rainfall Histogram

A3

q2 q3 q4 q1

0

Δt

2 Δt

3 Δt

Δt

q5

4 Δt Time t

Time t (c) Time-Area Curve

(d) Runoff Hydrograph

Figure 14.5

Time–Area Method

3.5.2 Other Hydrograph Methods • • •

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

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1.0 General

1.0.1

New direction in the stormwater management has imposed the control of both runoff quantity and quality.

1.0.2

The establishment of a storage-oriented approach for controlling runoff quantity from development sites.

1.0.3

A major and minor system approach shall be adopted for the planning and design of drainage system.

1.1 Minor Drainage System

1.1.1

The minor system shall be designed to collect and convey runoff from relatively frequent storm events to minimize inconvenience and nuisance flooding.

1.1.2

The example of minor system are includes pipe drains and open drains.

1.2 Major Drainage System

1.2.1

The major system shall be designed to safely convey runoff in excess of minor drainage system to waterways or rivers. The major system shall protect the development area from the consequence of large, reasonably rare events, which could cause severe flood damage.

1.2.2

The example of major drainage system is engineered waterways.

2.0 Open Drains (Chapter 26, Volume 10 MSMA) 2.1 Drainage Reserves

Open drains located within a road reserve do not require a separate reserve to allow access for maintenance. Drains located outside of road reserves, such as in public walkways and open space areas, should be provided with a drainage with a drainage reserve in accordance with Figure 26.1 (MSMA, 2000) Drainage Reserve 0.5 m min

Design flow width + freeboard

(a)

Grassed Swale

0.5 m min

Drainage Reserve 1.5 m minimum

(b)

1.0 m

Lined Open Drain

Figure 26.1 Reserve Width for Open Drains 3.0 Lined Drains 3.1 Locations and Alignments

Standardised locations for lined drains are provided to limit the negotiations needed when other services are involved. 3.1.1 Roadway Reserves The outer edge of a lined drain should be located 0.5 m from the property boundary on the high side of road reserves to permit relatively short connections to service adjacent properties. Lined drains may also be located within road median strips. The Local Authority should be consulted for standard alignments of public utility services within street verges.

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Where there is significant advantage in placing a lined drain on an alignment reserved for another authority, it may be so placed provided that both the authority responsible for maintenance of the stormwater conveyance and the other authority concerned agree in writing to release the reservation. Curved alignments are preferred on curved roadways. However, where there are significant advantages, e.g. culsde-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 provided at the side or rear of private properties, they shall be placed within a separate drainage reserve in accordance with Figure 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 Local Authority for consideration. As a guide, unless directed otherwise, lined drains shall be located as close as practical to 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 satisfactory jointing 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 Local Authority for consideration. 3.3 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 are illustrated in Figure 26.3 (MSMA, 2000) and described as follows. Varies

(a)

Uncovered Open Drain

0.5 m minimum 1.0 m maximum

0.5 m minimum 1.0 m maximum

Varies

500 maximum

Varies 0.5 m minimum 1.0 m maximum

Grate or solid cover

(b)

Covered Open Drain

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 0.5 With 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, Vertical and blockwork 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. The type of drain cover used will depend on the expected live loadings and whether or not the drain is required to accept surface 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 concrete covers. Covers should be sized such that the weight is limited to what can be easily lifted by 2 workmen to gain access 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 Paved Areas Footpaths and street verges Elsewhere

Cover Level Flush with finished surface Flush with finished surface 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 in the 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 be provided with a 1.2 m high handrail fence, or covered with solid or grated covers for the entire length of the drain for public safety. 3.7 Vehicular Crossings

Driveway entrances to properties and other vehicular crossings shall be structurally designed for a 7 tonne wheel loading. Urban Stormwater Management Short Course

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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. minimum thickness shall not be less than 100 mm.

The

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 a rigid structure or to existing concrete, which is not continuously reinforced. Longitudinal joints, where required, shall be 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. The designer 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 and water 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 bottom and 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 air and 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 a minimum 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 water added to produce a suitable consistency for the intended purpose. Urban Stormwater Management Short Course

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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 surrounding ground 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 shall be constructed in accordance with the Manufacturer’s 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 masonry cement 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 and litter 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 further deterioration 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 flows which 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 a recommended limit of 50% of the 1 month ARI. The grassed section shall be sized to provide additional flow capacity up to and including the design storm ARI. The composite drain components shall comply with the relevant design requirements specified for grassed swales and lined drains. 4.2 Geometry

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

C

Qminor 1

4 min

50 mm freeboard 1 4 min

Lined drain Design flow width + freeboard Figure 26.4 Recommended Composite Drain Cross-Section

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5.0 Grassed Swale 5.1 Location

A grassed swale, depression, or minor formalized overland flow path is generally located within parkland, open space areas, 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 and commercial 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 are involved. 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 or property boundary. In existing areas, this alignment may be varied depending on the alignment and depth of existing underground services within the road verge. The designer should consult the Local Authority for appropriate alignments in existing areas. Swales may also be located within road media strips, provided the median is of sufficient width to contain the swale plus 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 the side or rear of private properties, they shall be placed within a separate drainage reserve of minimum dimensions in accordance 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 due consideration 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 exceed 0.9 m. A ‘vee’ shaped section will generally be sufficient for most applications; however, a trapezoidal section may be used for additional capacity or to limit the depth of the swale. C

C Qminor 1

4 min

300mm freeboard 1 4 min

Design flow width + freeboard

(a) ' Vee' Shaped

300mm freeboard

Qminor 1 4 min 1

Batter

50

50

1

1 4 min

Base

Batter

Design flow width + freeboard (b) Trapezoidal Shaped

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 the design storm water level in the swale.

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5.5 Velocities and Grades

The average flow velocity in a grassed swale shall not exceed 2 m/s. If this is not practical, an underground pipeline, lined open drain, or grass reinforcement system should be provided. 5.6 Grassing

The grass species chosen for lining of grassed swales must be sturdy, drought resistant, easy to establish, and able to spread and develop a strong turf layer after establishment. A thick root structure is necessary to control weed growth and erosion. One or • • • • •

more of the following permanent grasses are recommended for permanent seed mixes: Axonophus compressus (Cow grass)

Vertiver grass Brachiaria sp. Cynodon dactylon ((Bermuda grass) Panicum virgatum (Switch grass)

The quality of the grass seed used is important. Grass seed shall be fresh, recleaned grass seed of the latest crop available. Grass seed may range from 20% to 100% purity. Compensation for purity and germination shall be by furnishing sufficient additional seed to equal the specified pure live seed product. 5.7 Dry Weather Flow Provision

For swales that will be subjected to dry weather flows, an underground pipe or surface invert should be provided in accordance with the requirements of Section 28.9.4. (MSMA, 2000) 5.8 Vehicular Crossings

As far as practical, the number of vehicular crossing points on swales should be kept to a minimum. Where crossing points are deemed necessary, they may be provided by any of the following methods: • at-grade crossing • box or pipe culvert • bridge structure At-grade crossings shall be constructed with a hard durable surface that will be stable under design flow conditions. The cross-section should be designed in accordance Standard Drawing SD F-42 to minimise the potential for ponding across the crossing caused by the buildup of the grassed surface over time on the low side of the crossing. Culvert and bridge crossings should be sized with sufficient waterway area to minimise changes to the flow regime on both sides of the crossing and to minimise the potential for blockages. Restrictions caused be these types of crossings will cause sediment to deposit on the upstream side of the crossing, which may become a maintenance problem. If entrance and exit velocities, particularly for culverts, are increased above the swale average velocity limit of 2 m/s, erosion protection measures will be required to prevent scouring of the swale (refer Chapter 29, MSMA). The level of culvert obverts and bridge soffits should be at least 50 mm above the design storm water level in the swale. 5.9 Maintenance

Periodical maintenance will be required to maintain the hydraulic capacity of a swale. Grass should be regularly mown and sediment, litter, and debris deposits removed, particularly at flow restrictions such as vehicular crossing points. Bare patches and scoured areas must be repaired by removing dead grass, filling scour holes, and reseeding with a recommended permanent grass seed mix.

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Runoff Conveyance

5.10 Design Charts

Surface Cover or Finish

Suggested n values Minimum

Maximum

Short grass cover

0.030

0.035

Tall grass cover

0.035

0.050

Trowelled finish

0.011

0.015

Off form finish

0.013

0.018

Grassed Swales

Lined Drains Concrete

Stone Pitching Dressed stone in mortar

0.015

0.017

Random stones in mortar or rubble masonry

0.020

0.035

Rock Riprap

0.025

0.030

Brickwork

0.012

0.018

Precast Masonry Blockwork

0.012

0.015

Design Chart 26.1

Suggested Values of Manning’s Roughness Coefficient, n

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Runoff Conveyance

3 Swale reserve width, R (m) ( including required freeboard )

2

1

y z

z

1

'Vee' shaped Section 1

0.5

Qn S01/2

Z=6 Z = 5.5

Value of

Z=5 Z = 4.5 Z=4 0.1

0.05

0.01

0.005 0.1

0.15

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Flow Depth, y (m)

Design Chart 26.2

Solution to Manning’s Equation for ‘Vee’ Shaped Grassed Swale of Various Side Slope

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10 Swale reserve width, R (m) ( including required freeboard )

9 0.9

8

1

y 1

50

50

4

1

1

Base width, B (m)

0 .8

7

4

6 0.7

5 Flow depth, y (m) 0.6

Base width, B (m)

0.5

3

5

Design Flow, QD (m3/s)

4

4

3 2

1

Use 'vee' shaped section

0.4

2

1.5

0.3

1 1

1.5

2

3

4

5

Longitudinal Grade, S0 (%)

Design Chart 26.3 Grassed Swale Base Width – Preliminary Estimate (Manning's n = 0.035, Average Velocity = 2 m/s)

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11

10

y

1

1

Z

Z Base width, B (m)

Qn S01/2 B 8/3

5

Z=3

Value of

Z = 2.5 Z=1

Z=2 Z = 1.5

Z = 0.5

1

Z=0

0.5

0.1 0.1

0.5 Value of

1.5

2

0.25

0.3

1

y B

0.1

Z=3 Z = 2.5 Z=2

0.05

Z = 1.5

Qn S01/2 B 8/3

Z=1 Z = 0.5

Value of

Z=0

0.01 0.06

0.1 Value of

y B

0.15

0.2

Design Chart 26.4 Solution to Manning’s Equation for Lined Drains of Various Side Slopes Urban Stormwater Management Short Course

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5.11 Worked Example: Design of Perimeter Swale (Case study: Application of Bio-Ecological Drainage System (BIOECODS) in Malaysia)

3.60m

2.40m

3.60m

5.11.1 Design Produce: Catchment area, A = 6,500m2, which landscape area = 4,600m2 and pavement =1,900m2. Reference 26.2.2

26.2.4 26.2.5 Figure 26.2

a)

Design Criteria In new development areas, the edge of a grassed swale should generally be located 0.5 m from the road reserve or property boundary. The depth of a grassed swale shall include a minimum freeboard of 50 mm above the design storm water level in the swale. The average flow velocity in a grassed swale shall not exceed 2 m/s. Recommended Grassed Swale Cross-Sections: Side slope = 1:4 min (batter); 1:50 (base)

Overland flow time: Overland sheet flow path length = 35m Slope of overland surface = (3.60-2.40)/35 = 3.5% Design Chart 14.1, overland flow time, to = 12 minute

b)

Flow time in Perimeter Swale: Reach length of perimeter swale = 130m The estimated average velocity = 0.25m/s Flow time in perimeter swale, td = (130/0.25)/60 = 8.7 minute

c)

Time of concentration Time of concentration, tc = to + td = 12 + 8.7 = 20.7 minute Assume: tc = 20 minute

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d)

Design Storm Table 13.A1 Lacation : Pulau Pinang and equation 13.2 for tc = 20 minute, Parameter a b c d FD (Table 13.3 - West Coast: 120mm) P30 (Equation 13.2) P60 (Equation 13.2) Pd (Equation n 13.3) Rainfall Intensity (mm/hr) (Equation 13.4)

Minor Storm 3.7277 1.4393 -0.4023 0.0241 0.47 68.32 92.83 56.80 170.41

Minor Storm: 10 year ARI: Where,

And,

Thus,

e)

10

I30 = 3.7277 + (1.4393) [In(30)] + (-0.4023) [In(30)]2 + (0.0241) [In(30)]3 I30 = 136.65 mm/hr P30 = 136.65/2 = 68.32mm

10

10

I60 = 3.7277 + (1.4393) [In(60)] + (-0.4023) [In(60)]2 + (0.0241) [In(60)]3 I60 = 92.83 mm/hr P60 = 92.83/1 = 92.83mm

10

P20 = 68.32 – (0.42) (92.83 - 68.32) = 56.80mm I20 = 56.80 (60) / 20 = 170.41 mm/hr

10

Runoff Coefficient

Design Chart 14.3, runoff coefficient, C for minor storm = 0.58 and major storm = 0.67 (Category 7: Landscape) and 0.91 for pavement (Category 1). f)

Average runoff coefficient Equation 14.8, Average runoff coefficient for minor storm, Cavg = [(0.58x4600) + (0.91x1900)] / 6500 = 0.68

g)

Peak flow Rational Formula (equation 14.7): peak flow for minor storm = 0.10m3/s Qminor /2* = C.I.A/ (3600,000) (2) = 0.68 (170.41) (6500) / (3600,000) (2) = 0.10m3/s * There are two perimeter swale in the catchment area to cater the peak flow.

h)

Perimeter Swale Sizing Longitudinal slope = 1:1000; Side slope 1:6 (batter) & 1:50 (base); Bottom width, B = 1.8m; Manning’s, n = 0.035, Depth, D = 175mm; Area, A = 0.50m2; Wetted perimeter, P = 3.93m; Hydraulic radius, R = A/P = 0.13m; Average velocity, V = 0.23m/s ( Q10) ... OK Thus, freeboard = 50mm, total depth = 225mm.

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6.0 Pipe Drains (Chapter 25, Volume 10, MSMA) 6.1 Locations and Alignments

Standardised locations for stormwater pipelines are provided to limit the negotiations needed when other services are involved and permit ready location by maintenance crews. 6.1.1 Roadway Reserves Stormwater pipelines should be located on the high side of road reserves to permit relatively short service tie connections to adjacent properties. Where there is significant advantage in placing a stormwater conveyance on an alignment reserved for another Authority, it may be so placed provided that both the Authority responsible for maintenance of the stormwater conveyance and the other Authority concerned agree in writing to release the reservation. UPVC and PE pipes shall not be placed in a reserve designated for another Authority or adjacent to an existing drainage or sewer flexible pipeline within a road reserve. Table 25.1 (MSMA, 2000) provides typical requirements for location of pipe drains and services within road reserves, however these may be varied by the local Authority. The relevant Authority should be consulted concerning their standard alignments for services. Table 25.1 Alignments within Roadway Reserves Pipe Diameter (mm) Alignment 375 to 675 under kerb line 750 to 1800 within median strip, or centreline of roadway 6.1.2 Privately Owned Properties Wherever stormwater pipelines are required along shared property boundaries, they should be located along the high side of the downhill property. Stormwater pipelines are often constructed in parallel to sewers and as the sewerage system is usually deeper, pipes connecting to stormwater ties have less problems in crossing over the sewer. Alignments shall be offset sufficient distance from building lines to allow working space for excavation equipment. Acceptable centreline offset alignments from property boundaries in residential, commercial, and industrial areas shall be in accordance with Table 25.2 (MSMA, 2000) Table 25.2 Alignments within Privately Owned Properties Pipe Diameter (mm) Rear Boundary Side Boundary 375 to 450 1.8 m 1.2 m (see Note) 525 to 675 1.8 m 1.5 m (see Note) Note: Where other hydraulic services or power poles are located on the same side of a property boundary, the centreline of the stormwater pipeline shall be located 1.8 m from the property boundary.

6.1.3 Public Open Space The location of stormwater pipelines within public land such as open space shall be brought to the attention of the operating Authority for consideration. As a guide, unless directed otherwise, stormwater pipelines shall be located not less than 3 m from the nearest property boundary. 6.1.4 Drainage Reserves A drainage reserve shall be wide enough to contain the service and provide working space on each side of the service for future maintenance activities. Minimum drainage reserve widths shall be in accordance with Table 25.3 (MSMA, 2000)

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Table 25.3 Minimum Drainage Reserve Widths Minimum Reserve Pipe Diameter, D (mm) Width (m)

Invert < 3.0 m deep 375 to 450 525 to 675 750 to 900 1050 to 1200 1350 to 1800

Invert 3.0 - 6.0 m deep 375 to 450 525 to 675 750 to 900 1050 to 1800

2.5 3.0 3.5 3.5 not less than 3 x D 3.5 4.0 4.5 not less than 4 x D

Note: Where other hydraulic services or electricity services are laid on the same side of the property boundary, the required reserve width shall be increased by 500 mm to provide horizontal clearance between services.

Pipelines up to and including 675 mm diameter may be located within privately owned properties if satisfactory arrangements are made for permanent access and maintenance. Larger diameter pipelines shall be located within public open space or outside privately owned properties in separate drainage reserves. Consideration should be given to the multi-purpose use of drainage reserves such as open space or pedestrian corridors. 6.1.5 Clearance from Other Services Where conflicts exist in the alignment and level of services, it will be necessary to ensure that adequate clearance is provided between the outer faces of each service. Minimum clearances have been established to reduce the likelihood of damage to stormwater pipelines or other services, and to protect personnel during construction or maintenance work. Under no circumstances shall stormwater pipelines be: • cranked to avoid other services or obstacles • located longitudinally directly above or below other underground services in the same trench Minimum clearances between stormwater pipelines and other services shall be in accordance with Table 25.4 (MSMA, 2000). The nominated clearance should make due allowance for pipe collars and fittings. Special protection may be provided to protect service crossings by concrete encasing the stormwater pipe for sufficient length to bridge the trench of the other service. Service

Table 25.4 Minimum Clearances Clearance (mm)

Horizontal

All services

600

Sewers Water mains Telephone High Pressure Gas Low Pressure Gas High Voltage Electricity Low Voltage Electricity

150 75 75 300 75 300 75

Vertical

Penetration by services through stormwater pipes should be avoided. Where it is necessary for a service to penetrate a stormwater pipe or manhole, allowance should be made for the hydraulic losses in the system resulting from the penetration. In addition, the service should be contained in a pipe or conduit of sufficient strength to resist the forces imposed on it by the flow, including debris, in the stormwater system. Unless agreed to the contrary by the relevant Authority, penetrations should be constructed using ductile iron pipe. To assist in the removal of debris collected on service pipes or conduits passing through a drainage system, it is recommended that a manhole be located at the pipe or conduit penetration. Where a stormwater pipeline crosses or is constructed adjacent to an existing service, the design shall be based on the work-as-executed location and level of that service. The design documents shall direct the contractor to verify the location and level of the existing service prior to constructing the stormwater pipeline in question.

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6.2 Design Criteria

6.2.1 Minimum Design Service Life Stormwater pipelines shall be designed for a minimum effective service life of 50 years. In composite PE pipe where steel ribs are used to structurally strengthen and stiffen the pipe, the ribs shall be ignored in determining the long-term vertical deflection, long-term external loading carrying capacity, and long-term buckling resistance of the installation. 6.2.2 Diameter Minimum diameters for stormwater pipelines shall be in accordance with Table 25.5 (MSMA, 2000). Table 25.5 Minimum Pipe Diameters Application Diameter (mm) Pipe draining a stormwater inlet and 300 crossing a footpath alignment * Any other pipe 375 For a non-self draining underpass, the 450 pipe shall be sized for 10 year ARI and shall not be less than Note: * 300 mm diameter pipes are permitted in this situation only, in order to provide more space in the footpath alignment for other utility services.

The maximum pipe diameter to be used depends on the availability of pipes from manufacturers. The use of large diameter pipes creates problems with clearance for other services. Box culverts or multiple pipes should be used if additional capacity is required. 6.2.3 Pipe Grades The longitudinal grade of a pipeline between drainage structures shall be calculated from centreline to centreline of such structures. 6.2.3.1 Maximum Grade Pipeline grades shall be chosen to limit the pipe full flow velocity to a value less than or equal to 6.0 m/s. In steep terrain it may be necessary to construct manholes with drops to dissipate some of the kinetic energy. 6.2.3.2 Minimum Grades Stormwater pipelines shall be designed and constructed to be self cleansing, e.g. free from accumulation of silt. The desirable minimum grade for pipelines shall be 1.0%. An absolute minimum grade of 0.5% may be acceptable where steeper grades are not practical. Such instances shall be brought to the attention of the relevant Authority for consideration before finalising designs. 6.2.4 Scour Stop Collars Pipelines laid on steep slopes shall be protected from failure due to wash-out of the pipe bedding. Where pipeline grades are greater than 7%, reinforced concrete scour stop collars shall be provided. 6.2.5 Vertical Angles Stormwater pipelines shall be constructed so that the bore of the pipe has no point where debris can lodge and cause reduction in capacity. The use of vertical angles is not permitted. 6.2.6 Curved Pipelines Curved stormwater pipelines are only permitted for diameters 1200 mm and above. Curves may be utilised wherever there are significant advantages in their use. Ad hoc curving of pipelines to avoid obstacles such as trees, power poles, gas mains etc. is not permitted. Curved pipelines should be positioned to follow easily identifiable surface features, e.g. parallel to a kerbline.

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6.2.7 Multiple Pipelines Where multiple pipelines are used, they should be spaced sufficiently to allow adequate compaction of the backfill between the pipes. The clearance between the outer face of the walls of multiple pipes should generally be in accordance with Table 25.6 (MSMA, 2000) although the relevant Authority may permit a lesser spacing in special circumstances to reduce structure costs, where reserve width is limited, or for relief drainage works. Table 25.6 Recommended Minimum Spacing of Multiple Pipelines Diameter of Pipes (mm) Minimum Clear Spacing (mm) Up to 600 300 675 to 1800 600 Notes: 1. The above minimum spacing may need to be modified to satisfy structural considerations, especially when laid at depth or under traffic loads 2. Where lean mix concrete vibrated in place or cement stabilised sand is used for backfill, the clear spacing may be reduced to 300 mm for all diameters, subject to structural considerations

6.2.8 Dead-end Pipelines Dead-end pipelines are those with no surface inlet or manhole on the upstream end. They are only used to provide a connection point for piped property drainage. Dead-end pipelines shall drain directly to a stormwater manhole or inlet. Connection of a dead-end pipeline to another stormwater pipeline by a branch connection or slope junction will not be permitted. A dead-end pipeline shall be constructed on a straight alignment and shall have a maximum length of 50 m. 6.3 Pipe Installation

Pipe class shall be selected to provide adequate strength to meet construction, overburden, and traffic loads. Pipe loadings shall be determined in accordance with the relevant Malaysian or British Standard or manufacturer’s recommendations for the selected pipe material. Designers must be aware of the effect of pipe installation conditions on pipe strength. This applies for all pipe materials, and particularly for flexible materials including PE and UPVC. In assessing pipe loadings, consideration shall be given to bedding support type (or embedment and site soil moduli), specified trench widths, method of installation, and live loads including construction loading.

Where load limits apply, the location and load limitation shall be clearly shown on the drawings. 6.3.1 Depth In general, stormwater pipelines shall be deep enough to serve the whole of the adjacent block(s) that are to drain to the pipeline (refer Section 25.7.1). 6.3.1.1 Minimum Depth Minimum cover over pipelines should normally be 0.6 m as measured from top of pipe to finished surface level. For pipelines under road pavements, the required cover shall be measured from top of pipe to pavement subgrade level. Minimum cover over FRC and SRC pipes may be less than 0.6 m. The pipe load class for any such design cover shall be in accordance with the relevant Malaysian or British Standard, or manufacturers’ recommendations. Minimum cover shall be increased to account for construction loading during pipe installation and traversing over the pipeline particularly when applied at subgrade level. The absolute minimum cover shall be 300 mm, unless the pipeline is protected from superimposed loads by a concrete slab. Minimum cover over UPVC and PE pipes shall be the greater of 0.6 m or as defined in the relevant Malaysian or British Standard, or manufacturers’ recommendations. For pipelines under road pavements, the required cover shall be at least 0.6 m from the top of the pipe to pavement subgrade level. 6.3.1.2 Maximum Depth The maximum depth of stormwater pipelines to invert level shall generally be 6 m. In special cases (e.g. for a short length of pipeline through a ridge), approval must be obtained from the Local Authority to exceed this limit.

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In this case, design checks shall be required to ensure that the pipeline has sufficient strength for the imposed load. 6.3.2 Pipe Trenching Trench excavation shall comply with the principles specified in the relevant standard or Manufacturer’s specifications for the pipe material used. The maximum trench width shall be the external pipe diameter plus 300 mm measured at the level of the crown of the pipe. The minimum trench width shall be 600 mm. In trenches where timbering is necessary, the trench width shall be increased sufficiently to maintain the minimum specified clearance between the pipes and the face of the timbering. The width of curved trenches shall be adequate to allow correct jointing of pipes. 6.3.3 Pipe Materials Stormwater pipelines shall be constructed from materials proven to be structurally sound and durable and have satisfactory jointing systems. The use of two or more types of pipe material on a single length of pipeline is not acceptable. Stormwater pipelines may be constructed with any of the following: • Fibre Reinforced Cement Pipes (FRC) • Steel Reinforced Concrete Pipes (SRC) • Unplasticised Polyvinyl Chloride Pipes (UPVC) • Composite Polyethylene Pipes (PE) All pipes shall comply with the relevant Malaysian Standards, where applicable, or British Standard. Alternative pipe materials may be acceptable. Proposals for the use of other materials shall be referred to the relevant Authority for consideration. 6.3.4 Pipe Bedding Adequate, properly placed and compacted pipe bedding material is essential to allow the pipe to develop its design strength to resist loads. Bedding material for pipes in trenches shall be a minimum 75 mm thick under the pipe barrel and a minimum of 25 mm under pipe sockets. The bedding shall be shaped and compacted to provide continuous support for pipes and precautions shall be taken to prevent disturbance or instability of the bedding due to groundwater. Bedding material shall consist of granular material of low plasticity such as quarry fines, or coarse river sand free from organic matter with a minimum 85% passing the 2.36 mm sieve and not more than 15% passing the 0.075 mm sieve. 6.3.5 Jointing Pipes need to be capable of resisting root intrusion, hydraulic pressure loadings, and preferably have some flexibility at joints. Unless otherwise approved by the local Authority, pipe jointing shall be as follows: • 375 mm diameter pipes shall be rubber ring jointed • 450 mm diameter and larger pipes shall be either rubber ring jointed or flush jointed. However, pipes designed to operate under hydraulic conditions that exceed 2.0 m head shall have rubber ring joints • 450 mm to 675 mm diameter pipes located under roadways shall have rubber ring joints For pipe diameters greater than 1000 mm, adhesive shall not be used to join flexible pipes within the road reserve. Locations of various joint types shall be shown on the design drawings. The maximum allowable head for all pipes shall be in accordance with the appropriate Malaysian or British standard. Where pipes are connected to rigid structures or are embedded in concrete, adequate flexibility shall be provided to minimise damage caused by differential settlement. Pipe connections to structures shall be constructed in accordance with Standard Drawing SD F-12.

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6.3.6 Branch Connections Pipeline junctions except for property ties shall generally occur within a stormwater inlet, manhole, or special structure. Stormwater inlets are described in Chapter 24, and manholes in Section 25.6. Pipe branches are acceptable for property ties. Branch connections may also be permitted in locations where a surface manhole is undesirable, provided that adequate structural strength can be achieved at the junction. Allowable sizes of branch connections into pipelines of 450 mm to 1800 mm diameter shall be in accordance with Standard Drawing SD F-11. Entry angles for branches shall be between 450 and 900 to the main pipeline. A manhole shall be constructed on the branch pipeline within 20 m of the branch connection. 6.4 Worked Example (Proposed Tuanku Heights Mixed Development of Daerah Seremban, Negeri Sembilan)

6.4.1

Main conveyance vehicle for stormwater runoff within every housing lot shall be in underground reinforced concrete pipes in place of the usual concrete line channels.

6.4.2

Pipe diameters were determined using the design Chart 25.B3 (k=0.30mm), given the runoff quantity and pipe gradient. A minimal of 1% gradient was chosen to encourage self-cleansing within the pipes.

6.4.3

Minimum 600mm bedding shall be provided as bedding from top of platform level to top of pipe.

Calculation

Reference

Output

Calculation for Underground Drain Pipes Sizing Location

:

Node 3

Subcatchment

:

1

Area

:

6770 m2

tc

=

40 min

Q10

=

144.3931 l/s

Design Criteria Table 25.5

i) φmin

=

375 mm

Sec. 25.3.3 (a) ii) Maximum Grade : Velocity < 6 m/s Sec. 25.3.3 (b) iii) Minimum grade = 1.0% Table 25.7

iv) k = 0.3 mm From Design Chart 25.B3 (k = 0.3 mm), With D = 375 mm Hydraulic gradient 1 % OK!

Q

=

230 l/s

>

Q10

V

=

2 m/s

<

6 m/s OK!

Therefore,

Pipe Size for node 3 is 375 mm with 1 % gradient.

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6.5 Design Chart

Table 25.7

Pipe Roughness Values (average condition)

n

k (mm)

Spun Precast Concrete

0.013

0.3

Fibre Reinforced Cement

0.013

0.15

UPVC

0.011

0.06

Pipe Material

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Design Chart 25.B1

Hydraulic Design of Pipes – Colebrook-White Formula – k = 0.06 mm

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Design Chart 25.B2

Hydraulic Design of Pipes – Colebrook-White Formula – k = 0.15 mm

Design Chart 25.B3

Hydraulic Design of Pipes – Colebrook-White Formula – k = 0.30 mm

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Design Chart 25.B4

Hydraulic Design of Pipes – Colebrook-White Formula – k = 0.60 mm

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7.0 Engineered Waterways (Chapter 28, Volume 11, MSMA) 7.1 Design Storm

Engineered waterways shall be designed to cater for flows up to and including the major system design ARI (refer Table 14.3, MSMA). Adjoining low-lying land may need to be acquired and/or reclaimed to ensure effective surface drainage and containment of the design ARI flow within an engineered waterway. 7.2 Location

Continuous designated overland flow paths shall be provided from the top of the catchment through the entire urban area. Engineered waterways may be located within designated drainage reserves, roadways, parkland and open space areas, and pedestrian ways. All engineered waterways shall be located wholly outside of privately owned lots. If circumstances arise where this arrangement cannot be provided, prior agreement to locate engineered waterways within privately owned areas must be obtained from the Local Authority and the private landowners affected. Piping of major system design flows may be considered as an alternative to an engineered waterway, but acceptable provision against the pipe being blocked or its capacity being exceeded will be required. Engineered waterways shall be provided along the alignment of existing watercourses and drainage depressions. Diversion of engineered waterways away from their natural paths will only be permitted in exceptional circumstances and only with the approval of the Local Authority. Wherever possible, landuse within engineered waterway corridors should be designated as public open space. Other types of landuse may be considered, but they must be fully compatible with the primary role of the waterway to convey flood flows up to and including the design storm. 7.3 Reserves

Reserves are required for all engineered waterways. These must be clearly defined on all development plans to ensure that future development does not encroach upon land inundated by flows up to and including the design storm. The prime function of reserves is to give ready access to personnel, plant and materials, which may, from time to time, be required for waterway and berm maintenance. No encroachment, especially earth fill that may inhibit such access or make such maintenance unduly difficult, shall be allowed on reserves. The minimum drainage reserve width shall be the top waterway width for the major storm ARI flow plus a 300 mm freeboard requirement. Maintenance width requirements may be incorporated within this reserve width by benching. If this cannot be achieved, the reserve width must be increased to include maintenance width requirements. Minimum widths to be provided for maintenance access shall be in accordance with Table 28.1 (MSMA, 2000) Table 28.1 Minimum Requirements for Maintenance Access Top Width of Waterway Minimum Requirements for Maintenance Access W ≤ 6m One side 3.7 m, other side 1.0 m W > 6m Both sides 3.7 m When planning development along a waterway for which a master plan is not yet available, a drainage reserve width shall be estimated based on the premise that the design storm flow will be catered for by a grassed floodway. This premise ensures that sufficient land will be available for the design of the waterway when carried out in conjunction with detailed landuse planning at a later date. 7.4 Freeboard

The freeboard above the design storm water level for all engineered waterways shall be a minimum of 300 mm. A higher freeboard should be considered at locations where superelevation or hydraulic jumps are anticipated.

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7.5 Grades

7.5.1 Minimum Grades Engineered waterways shall be constructed with sufficient longitudinal grade to ensure that ponding and/or the accumulation of silt does not occur, particularly in locations where silt removal would be difficult. The minimum longitudinal grade for engineered waterways shall be as follows: • •

0.5% 0.2%

Grassed floodways and natural channels Lined channels

Longitudinal grades shall not produce velocities less than 0.8 m/s if low flow inverts flowing full. 7.5.2 Maximum Grades Engineered waterways shall be designed with longitudinal grades that minimise the incidence of hydraulic jumps, dangerous conditions for the public, and erosion of surface linings and/or topsoil. Longitudinal grades shall be chosen such that the design storm average flow velocity will not exceed: • 4 m/s in lined channels and low flow inverts • 2 m/s in grassed floodways and natural waterways 7.5.3 Drop Structures Drop structures should be provided to reduce waterway longitudinal grades such that the design storm average flow velocities do not exceed the limits specified in the previous section. Drop structures shall be designed to ensure that the structures do not get 'drown out' due to high tailwater levels under the major system design flow plus freeboard. Design requirements for drop structures are provided in Chapter 29, Section 29.3 (MSMA). 7.6 Worked Example: Design of Ecological Swale (Case study: Application of Bio-Ecological Drainage System (BIOECODS) in Malaysia)

ECOLOGICAL SWALE

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Reference Table 28.1

Design Criteria Minimum requirements for maintenance access = 3.7m (One side) and 1.0m (Other Side) for top width of waterway ≤ 6m or Both sides = 3.7m for top width of waterway > 6m The freeboard above the design storm water level for all engineered waterways shall be a minimum of 300 mm. A higher freeboard should be considered at locations where superelevation or hydraulic jumps are anticipated. The minimum longitudinal grade for engineered waterways = 0.5% for grassed floodways and natural channels; Longitudinal grades shall not produce velocities less than 0.8 m/s if low flow inverts flowing full Longitudinal grades shall be chosen such that the design storm average flow velocity will not exceed 2 m/s in grassed floodways and natural waterways Side slopes = 1:6 min (batter); 1:50 (base) Side slopes = 1:4 may be provided in special circumstance Low flow inverts and pipes shall be sized for a minimum capacity of 50% of the 1 month ARI flow

28.6 28.7.1

28.7.2 28.10.2 28.10.4

7.6.1 Design Produce: A = 256,000m2 So = 0.1%. a)

Overland flow time: Overland sheet flow path length = 35m Slope of overland surface = (3.60-2.40)/35 = 3.5% Design Chart 14.1, overland flow time, to = 12 minute

b)

Flow time in channel: Reach length of ecological swale = 920m The estimated average velocity = 0.35m/s Flow time in ecological swale, td = (920/0.35)/60 = 43.8 minutes

c)

Time of concentration Time of concentration, tc = to + td = 12 + 43.8 = 55.8 minutes Assume: tc = 56 minit

d)

Design Storm Major Storm: 100 year ARI Table 13.A1 Lacation : Pulau Pinang and equation 13.2 for tc = 56 minute, Parameter a b c d Rainfall Intensity (mm/hr) (Equation 13.4)

Major Storm 2.7512 2.2417 -0.5610 0.0341 135.48

Major Storm: 100 year ARI: Where, 100I56 = 2.7512 + (2.2417) [In(56)] + (-0.5610) [In(56)]2 + (0.0341) [In(56)]3 Thus, 100I56 = 135.48 mm/hr e)

Runoff Coefficient Design Chart 14.3 (category 5), runoff coefficient, C for major storm = 0.70.

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f)

Peak flow By using Rational formula (equation 14.7), peak flow for minor storm = 4.21 m3/s and major storm = 6.75 m3/s. Qmajor = C.I.A/3600,000 = 0.70 (135.48) (256,000) / (3600,000) = 6.75m3/s

g)

Ecological Swale Sizing Longitudinal slope = 1:1000; Side slope 1:6 (batter), 1:50 (base); Bottom width, B = 2.5m; Manning’s, n = 0.035, Depth, D = 1200mm; Area, A = 11.64m2; Wetted Perimeter, P = 17.10m; Hydraulic radius, R = 0.68m; Average velocity, V = 0.70m/s ( Q100) ... OK Thus, Freeboard = 300mm, Total Depth = 1500mm

7.6.2 Low flow Design i) Design Rainfall Intensity for 1 month ARI Table 13.A1 Lacation: Pulau Pinang and equation 13.2 for tc = 56 minute, Parameter a b c d Rainfall Intensity for 2 Year ARI (mm/hr)

Minor Storm 4.5140 0.6729 -0.2311 0.0118 69.94

Where, 2I54 = 4.5140 + (0.6729) [In(54)] + (-0.2311) [In(54)]2 + (0.0118) [In(54)]3 Thus, 2I54 = 69.94 mm/hr Equation 13.5a, 1 month ARI rainfall intensity = 0.4x69.94 = 27.98 mm/hr ii)

Runoff Coefficient

Design Chart 14.3 (Category 5), runoff coefficient, C for 1 month ARI rainfall intensity = 0.30. iii)

Peak Flow

By using Rational formula (equation 14.7), peak flow for 1 month ARI rainfall intensity = 0.60 m3/s. Qlow flow = C.I.A/3600,000 = 0.30 (69.94) (256,000) / (3600,000) = 0.60m3/s Drainage capacity for low flow = 0.30 m3/s. Thus, no. of module needed = (0.60 - 0.30) / 0.038 = 8 Size Flow

Table: Details for Module 410 x 467 x 607 mm 2280 L/min @ 0.038 m3/s

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7.7 Design Chart

Surface Cover

Suggested n values Minimum

Maximum

0.030 0.035

0.035 0.050

0.050 0.100

0.070 0.160

0.040 0.100

0.050 0.120

0.025 0.035 0.050 0.030

0.033 0.045 0.080 0.070

0.025 0.035

0.060 0.100

0.025 0.030 0.040 0.070 0.110

0.035 0.050 0.080 0.160 0.200

0.011 0.013

0.015 0.018

0.016 0.018 0.020

0.023 0.025 0.025

0.015 0.020 0.025

0.017 0.035 0.030

0.011

0.015

0.012 0.015

0.014 0.017

0.017 0.020

0.019 0.024

Grassed Floodways Grass cover only Short grass Tall grass Shrub cover Scattered Medium to dense Tree cover Scattered Medium to dense

Natural Channels Small streams Straight, uniform and clean Clean, winding with some pools and shoals Sluggish weedy reaches with deep pools Steep mountain streams with gravel, cobbles, and boulders Large streams Regular cross-section with no boulders or brush Irregular and rough cross-section Overbank flow areas Short pasture grass, no brush Long pasture grass, no brush Light brush and trees Medium to dense brush Dense growth of trees

Lined Channels and Low Flow Inverts Concrete Trowelled finish Off form finish Shotcrete Trowelled, not wavy Trowelled, wavy Unfinished Stone Pitching Dressed stone in mortar Random stones in mortar or rubble masonry Rock Riprap

Roadways Kerb & Gutter Hotmix Pavement Smooth Rough Flush Seal Pavement 7 mm stone 14 mm stone Design Chart 28.1

Suggested Values of Manning’s Roughness Coefficient, n

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140 1.

3

120

Floodway reserve width, (m) ( including required freeboard )

2 1.

1 1. 1

6

100

1

50

Base width,

1

(m)

1. 0

90

6

1

50

0. 9

80

70

60

0. 7

Base width,

(m)

60

1.6

(m3/s)

50

55 50

1.5

40

45

0.6 40

1.4

35

30

30

Design Flow,

(m)

0. 8

Flow depth,

25

0.5 20 15

20 10 5

0.4 15

10 0.5

0.6

0.7

0.8

0.9

1.0

1.2

1.4

1.6

1.8

Longitudinal Grade,

2.0

2.5

3.0

3.5

4.0

4.5

5.0

(%)

Design Chart 28.2 Floodway Base Width – Preliminary Estimate (Manning's n = 0.050, Average Velocity = 2 m/s)

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2. 2

270

Floodway reserve width,

2. 0

250

2. 1

Runoff Conveyance

(m)

8

1. 9

( including required freeboard ) 1.

200

6

1

6

1

50

50

1

1. 6

1. 7

1

Base width,

1. 5

(m)

2 .7

1. 3

1. 4

150

1. 2

2.6 2 .5

1. 1

2.4

100

2.3

Flow depth, (m)

1. 0

90

0. 9

80

Base width,

0. 8

60

(m)

60 55 50

50

0. 7

Design Flow,

(m3/s)

70

45 40

35

40 30

25 20

0.6

30

15 5

0.5

10

20

15

10 0.5

0.6

0.7

0.8

0.9

1.0

1.2

1.4

1.6

1.8

Longitudinal Grade,

2.0

2.5

3.0

3.5

4.0

4.5

5.0

(%)

Design Chart 28.3 Floodway Base Width – Preliminary Estimate (Manning's n = 0.050, Average Velocity = 2 m/s)

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10

Type 1 Invert

5

1

Use Type 2 Invert Av era ge flo w

mm = 600

= 450

varies max 600 mm 1

1

10

10

1

1 1

= 1000 mm

ve loc ity

mm

>

4m /s -

red

uce lon git ud

1.0

ina lg

rad e

0.5

Design Flow,

(m3/s)

mm = 300

0.1

mm = 150 0.05

= 0 (Vee shaped invert) 0.01 0.5

1.0

2.0

Longitudinal Grade,

Design Chart 28.4

3.0

4.0

5.0

(%)

Low Flow Invert Size (Type 1 - Variable Depth, Manning’s n = 0.013)

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10

Use pipeline instead of invert

Type 2 Invert

9

00 m = 30 8

= 28

m

1

m 00 m

m 600 =2

(m3/s)

10

m 00 m

000 =2

Design Flow,

10

1 1

1

mm

00 = 24

= 22

1

varies 1000 to 3000 mm

m

7

6

600 mm

1

mm

= 18

5

m 00 m

mm 600 =1 mm 400 =1

4

200 =1

mm

3

Use Type 1 Invert

2 0.6

0.5

0.7

Longitudinal Grade,

Design Chart 28.5

0.8

0.9

1.0

1.1

(%)

Low Flow Invert Size (Type 2 - Variable Width, Manning’s n = 0.013)

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Runoff Conveyance

11

10

y

1

1

Z

Z Base width, B (m)

Qn S01/2 B 8/3

5

Z=3

Value of

Z = 2.5 Z=1

Z=2 Z = 1.5

Z = 0.5

1

Z=0

0.5

0.1 0.1

0.5 Value of

1

1.5

2

0.25

0.3

y B

0.1

Z=3 Z = 2.5 Z=2 Z = 1.5

0.05

Qn S01/2 B 8/3

Z=1 Z = 0.5

Value of

Z=0

0.01 0.06

0.1 Value of

Design Chart 28.6

y B

0.15

0.2

Solution to Manning’s Equation for Lined Channels of Various Side Slopes

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Runoff Conveyance

8.0 Culvert (Chapter 27, Volume 11, MSMA) 8.1 Freeboard

All culverts with a waterway area of 1.0 m2 or more should be designed with a minimum of 300 mm freeboard above the design water level. For large culverts the designer should consider increasing this freeboard to allow for the size of debris anticipated, up to a maximum of 1000 mm. 8.2 Design Precautions

Where debris accumulation is considered to be a problem, other design precautions should be taken, such as providing a smooth well designed inlet, avoiding multiple cells and increasing the size of culvert. If multiple cells are unavoidable, provision of a sloping cutwater on the upstream pier (wall) ends may help to align floating debris with the culvert entrance. 8.3 Relief Culvert

A relief culvert passing through the embankment at a higher level than the main culvert permits water to by-pass the latter, if it becomes blocked. The relief culvert could also be placed at a low level some distance away from the main culvert where it is not likely to be blocked. As this relief culvert is an additional requirement, the cost of both culverts should be compared with that of a larger culvert that will be less subject to blockage. 8.4 Debris Control Structures

These can be costly both to construct and maintain. Details of the various types of debris control structures may be found in Hydraulic Engineering Circular No 9, “Debris Control Structures” (US Federal Highway Administration, 1971). The choice of structure type depends upon size, quantity and type of debris, the cost involved and the maintenance proposed. However, for existing culverts, which are prone to debris clogging, it may be worthwhile to construct a debris control structure rather than replace or enlarge the culvert. 8.5 Flow Velocity

Culverts usually increase the flow velocity over that in the natural water course. Except when the culverts flow full, the highest velocity occurs near the outlet and this is the point where most erosion damage is likely to occurs. A check on outlet velocity, therefore, must be carried out as part of the culvert design if the outlet discharges to an unlined waterway. 8.6 Erosion of Conduit

Flow of the water subjects the conduit material to abrasion, and too fast a velocity for a given wall material will cause erosion to the conduit. Very fast flows can cause cavitation unless the conduit surface is very smooth, and this results in erosion taking place at a rapid rate. However, cavitation damage does not occur in full flowing pipes with velocity less than about 7.5 – 8 m/s and about 12 m/s in open conduits. The maximum velocity beyond which erosion will take place depends on factors like smoothness of conduit, quantity and nature of debris discharged and frequency of peak velocity. Commonly adopted maximum values based on experience are listed in Table 27.1 (MSMA, 2000) Table 27.1 Maximum Recommended Flow Velocities, (m/s) for various conduit materials Material Precast concrete pipes Precast box culverts In situ concrete and hard packed rock (300mm min) Beaching or boulders (250mm min) Stones (150 – 100mm) Grass covered surfaces Stiff, sandy clay Coarse gravel Coarse sand Fine sand

Maximum V (m/s) 8.0 8.0 6.0 5.0 3.0 1.8 1.3 1.3 0.5 0.2

– 2.5 – – – –

1.5 1.8 0.7 0.5

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8.7 Scour at Inlets

A culvert normally constricts the natural channel, forcing the flow through a reducing opening. As the flow contracts, vortices and areas of high velocity flow impinge against the upstream slopes of the embankment adjacent to the culvert. Scour can also occur upstream of the culvert, as a result of the acceleration of the flow, as it leaves the natural channel and enters the culvert. Upstream wing walls, aprons, cut-off walls and embankment paving assist protecting the embankment and stream bed at the upstream end of a culvert. 8.8 Scour at Outlets

If the flow emerging from a culvert has a sufficiently high velocity and the channel is erodible, the jet will scour a hole in the bed immediately downstream and back eddies will erode the stream banks to form a circular elongated scour hole. Coarse material scoured from the hole will be deposited immediately downstream, often forming a low bar across the stream, while finer material will be carried further downstream. Depending on the supply of sediment, the scour hole may gradually refill until after the next major flood occurs. The provision of wing walls, headwall, cut-off wall and apron is generally all the protection that is required at culvert outlets. The judgement of design engineers, working in a particular area is required to determine the need for any further protection. Investigation of scour and outlet protection at similar culverts in the vicinity of the culvert being designed may provide guidance on whether further protection is required. Periodic site visits and inspection after major flood events will also confirm whether the protection is adequate or further protection is required. In urban areas, the risk of outlet scour is generally unacceptable and therefore a choice must be made as to which type of scour protection is suitable for the site. The options available include the following: • Local protection of the stream bed material, in the case of unlined drains and waterways. • Flow expansion structure. • An energy dissipating structure Stream bed protection can be achieved with a concrete apron, rock riprap, or rock mattresses, or concrete filled mattresses. It is important that mattresses are anchored to the cut-off wall or apron at the culvert outlet, to stop them moving downstream. A geotextile filter is usually provided under the mattresses and may also be required under the rock riprap. Scour protection is discussed in detail in Chapter 29 (MSMA). An important parameter in the selection of an appropriate energy dissipater is the Froude Number, Fr of the outlet flow. Where an outlet has Fr < 1.7, a simple apron structure, riprap, or a flow expansion structure will suffice. Where 1.7 < Fr < 3 a riprap basin or horizontal roughness elements basin is appropriate. Where Fr > 3 a hydraulic jump basin will be required. Energy dissipaters are discussed in detail in Chapter 29 (MSMA). 8.9 Siltation

If the flow velocity becomes too low siltation occurs. Flow velocity below about 0.5 m/s will cause settlement of fine to medium sand particles. To be self-cleansing culverts must be graded to the average grade of the water course upstream and downstream of the culvert, and levels must represent the average stream levels before the culvert was built. Culvert location in both plan and profile is of particular importance to the maintenance of sediment-free culvert cells. Deposition can occur in culverts when the sediment transport capacity of flow within the culvert is less than in the stream. The following factors may cause deposition in culverts: • Culverts often provide a wider flow width at low flows than natural streams. This results in the flow depth and sediment transport capacity being reduced. • Point bars (deposition) form on the inside of stream bends and culvert inlet placed at bends in the stream will be subjected to deposition in the same manner. This effect is most pronounced in multiple-cell culverts with the cell on the inside of the curve often becoming almost totally plugged with sediment deposits. • Abrupt changes to a flatter grade in the culvert or in the channel upstream of the culvert will induce deposition. Gravel and sand deposits are common downstream from the break in grade because of the reduced transport capacity in the flatter section. Deposition usually occurs at flow rates smaller than the design flow rate. The deposits may be removed during larger floods, depending upon the relative transport capacity of flow in the stream and in the culvert, compaction and composition of the deposits, flow duration, ponding depth above the culvert and other factors. Siltation can also occur upstream of culverts if they are installed at incorrect levels, creating ponding areas. Such grading should generally be avoided. Urban Stormwater Management Short Course

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Case Study I

1.0

Introduction

Design criteria and design calculation for the “Membina dan Menyiapkan Wad Forensik Di Hospital Bahagia Ulu Kinta, Ipoh, Perak Darul Ridzuan” is presented for case study. 2.0

Proposed Drainage System

The Government of Malaysia via the Works Department is planning to construct a new building for the forensic wad of Tanjung Rambutan Hospital on the area of approximately 1.5 hectares in Ipoh, Perak Darul Ridzuan (Figure 1). In this project, the Government is planning to construct a drainage system that shall comply with the new guidelines which is published by Department of Irrigation and Drainage (DID) in the year 2000 and gazetted by the government in the following year, namely Urban Stormwater Management Manual for Malaysia or MSMA. This project consists of the construction of a single building, which includes administration unit, clinical unit, forensic block and wad. The project covers a catchment area of 1.51 hectares on medium soil type. More than 60 % of the total area has been developed into impervious area such as paved road and car park, sheltered walkway, and utilities other than the building. The origin of this area was cultivated field. The pre-development runoff was catered by roadside drain and other existing secondary drains before discharging to the nearest receiving water which is located at 200 meters at the downstream. Generally, the duration of this construction project is about 76 weeks where it started on 24 June 2002. For the drainage system, the construction started in early of January 2004 and ended on June 2004. The proposed drainage system which is known as Bio-ecological Drainage System (BIOECODS) for this project is consistent with objectives of new stormwater management approach which focus on the control of both the quantity and quality of urban runoff. This has been embodied in the concept of ecologically sustainable development which is aimed at ensuring that development can occur without long-term degradation of natural resources and the environment. The component of BIOECODS consists of grassed swale, i.e. perimeter swale and ecological swale, subsurface detention storage, i.e. Type A and Type B and also Dry Pond as the detention basin.

Ip

Perimeter Swale

Wad

Forensic Block

Administration & Clinical Unit

Ecological Swale

Detention Storage Type B Walkway

Car Park

Dry Pond

Sewerage Treatment Plant Detention Storage Type A

Figure 1 Project Layout Urban Stormwater Management Short Course

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Case Study I

3.0 On-site Detention 3.1 Design Storm The design storm for discharge from an OSD storage, termed the discharge design storm, shall be the minor system design ARI of the municipal drainage system to which the storage is connected (refer Table 4.1). The design storm for calculating the required storage volume, termed the storage design storm, shall be 10 year ARI. 3.2 Permissible Site Discharge (PSD) The PSD is the maximum allowable post-development discharge from a site for the selected discharge design storm and is estimated on the basis that flows within the downstream stormwater drainage system will not be increased. 3.3 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 facility does not overflow during the storage design storm ARI. 3.4 Site Coverage Where possible, the site drainage system and grading should be designed to direct runoff from the entire site to the OSD system. Sometimes this will not be feasible due to ground levels, the level of the receiving drainage system, or other circumstances. In these cases, as much runoff from impervious areas as possible should be drained to the OSD system. 3.5 Frequency Staged Storage Generally the most challenging task in designing OSD systems is locating and distributing the storage(s) in the face of 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

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. The depth of inundation and extent of area inundated in the above-ground storage is thus limited such that the greatest inconvenience to property owners or occupiers occurs very infrequently. The approach recognises that people are generally prepared to accept flooding which causes inconvenience, provided it does not cause any damage and does not happen too often. Conversely, the lesser the degree of personal inconvenience, the more frequently the inundation can be tolerated. Recommended storage proportions for designing a composite above and below-ground storage system using a frequency staged storage approach are provided in Table 19.1 (MSMA, 2000). A typical composite storage system is illustrated in Figure 19.1 (MSMA, 2000) Refer to Table 19.2 (MSMA, 2000) for recommended maximum ponding depths in the above-ground storage component. Table 19.1 Relative Proportions for Composite Storage Systems Storage Area Proportion of Total Storage (%) Below-Ground Above-Ground Storage Storage Component Component Pedestrian areas 60 40 Private Courtyards 60 40 Parking areas and driveways 50 50 Landscaped areas 25 75 Paved outdoor recreation areas 15 85

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Case Study I

3.6 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

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

Habitable building

Freeboard to building floor level

Maximum ponding level for storage design storm

Above-ground storage

'Beginning to pond' level for above-ground storage

Below-ground storage

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

Figure 19.1 Illustration of a Composite Storage System 3.7 Signs It is essential that current and future property owners are aware of the purpose of the OSD facilities provided. 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).

Colours: Triangle and “WARNING” Red Water Blue Figure and other lettering Black

WARNING ON-SITE DETENTION AREA STORMWATER LEVEL MAY RISE IN THIS AREA DURING HEAVY RAIN

Figure 19.5 Typical OSD Advisory Sign (UPRCT, 1999) 3.8 Above-Ground Storage There are few absolute requirements when designing an above-ground storage. The following guidelines allow the designer maximum flexibility when integrating the storage into the site layout. 3.8.1 Maximum Storage Depths Maximum storage depths in above-ground storages should not exceed the values provided in Table 19.2 (MSMA, 2000).

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Case Study I

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 3.8.2 Landscaped Areas Landscaped areas offer a wide range of possibilities for providing above-ground storage and can enhance the aesthetics of a site. The minimum design requirements for storage systems provided in landscaped areas are: • • • • • •

• •

maximum ponding depths shall not exceed the limits recommended in Table 15 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

3.8.3 Impervious Areas Car parks, driveways, paved storage yards, and other paved surfaces may be used for stormwater detention. Car park detention shares the same surface area with parked vehicles. If the detention is designed without regard for the primary use of the car park in mind, considerable inconvenience and damage to parked vehicles can occur when it rains. First and foremost, for the car park detention to be acceptable to its owners, it is necessary to ensure that the lot does not pond water frequently. Also, when the lot detains stormwater, it should be inundated for only a short period of time. Thus, it is important to recognise the limitations in ponding depths and the frequency of ponding. Failure to do so can lead to owners taking action to eliminate this nuisance after experiencing flooding on their property. The minimum 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

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Case Study I

4.0 Worked Example 4.1 Design Rainfall Design ARI = 10 YRS Table 13.A1: Location = Ipoh, Perak Darul Ridzuan A = 5.0707, B = 0.6515, C = -0.2522, D = 0.0138

4.2 Runoff estimation for storm durations less than 30 mins For a short duration, d, the rainfall depth

Pd = P30 − FD ( P60 − P30 )

(Eqn. 13.3)

P30, P60 = 30-minute and 60-minute duration rainfall depths from published design curves FD = Adjustment factor for storm duration, a function of 2P24hr (Table 13.3)

ln( R I t ) = a + b ln(t ) + c(ln(t )) 2 + d (ln(t )) 3 10

10

I30 = 135.92 mm/hr

(Eqn. 13.2)

I60 = 86.28 mm/hr

(Eqn. 13.3)

P30 = 135.92 x (30/60) = 67.96 mm

(Eqn. 13.4)

P60 = 86.28 x (60/60) = 86.28 mm

Say tc = 10 min, P10 = 67.96 – 1.28 (86.28-67.96) = 44.52 mm 10 I10 = 44.51 / (10/60) = 267.09 mm/hr 4.3 Summary of runoff estimation for storm durations less than 30 mins 10

Time (min)

FD (Table 13.3)

Pd (mm)

Id (mm/hr)

5.00

2.08

29.86

358.35

10.00

1.28

44.52

267.09

15.00

0.80

53.31

213.23

20.00

0.47

59.35

178.05

4.4 Calculation for Grassed Swale Design Criteria: 26.2.2 26.2.4 26.2.5 Figure 26.2

The edge of a grassed swale should generally be located 0.5m from road reserve or property boundary Freeboard = 50mm Average flow velocity shall not exceed 2 m/s TW Side Slope = 1:4 min (batter); 1:50 (base)

with Longitudinal slope, S = 1:500 and n = 0.035 Eqn. 14.4a

Manning’s Equation, V = Q = VA

1 2 / 3 1/ 2 R S n

1

D Z BW

Urban Stormwater Management Short Course

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Case Study I

SLOPE 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002

SLOPE 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002

BW (m) 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6

Summary of Perimeter Swale Capacity Side Slope, Z Depth, D TW A P (m) (m) (m) (sq.m) (m) 4 0.00 0.60 0.00 0.60 4 0.05 1.00 0.04 1.01 4 0.10 1.40 0.10 1.42 4 0.15 1.80 0.18 1.84 4 0.20 2.20 0.28 2.25 4 0.25 2.60 0.40 2.66 4 0.30 3.00 0.54 3.07 4 0.35 3.40 0.70 3.49 4 0.40 3.80 0.88 3.90 4 0.45 4.20 1.08 4.31 4 0.50 4.60 1.30 4.72

R (m) 0.00 0.04 0.07 0.10 0.12 0.15 0.18 0.20 0.23 0.25 0.28

V (m/s) 0.00 0.15 0.22 0.27 0.32 0.36 0.40 0.44 0.47 0.51 0.54

Q (cumec) 0.000 0.006 0.022 0.049 0.089 0.144 0.215 0.305 0.415 0.546 0.700

BW (m) 1 1 1 1 1 1 1 1 1 1 1

Summary of Ecological Swale Capacity Side Slope, Z Depth, D TW A P (m) (m) (m) (sq.m) (m) 3 0.00 1.00 0.00 1.00 3 0.05 1.30 0.06 1.32 3 0.10 1.60 0.13 1.63 3 0.15 1.90 0.22 1.95 3 0.20 2.20 0.32 2.26 3 0.25 2.50 0.44 2.58 3 0.30 2.80 0.57 2.90 3 0.35 3.10 0.72 3.21 3 0.40 3.40 0.88 3.53 3 0.45 3.70 1.06 3.85 3 0.50 4.00 1.25 4.16

R (m) 0.00 0.04 0.08 0.11 0.14 0.17 0.20 0.22 0.25 0.27 0.30

V (m/s) 0.00 0.16 0.23 0.29 0.34 0.39 0.43 0.47 0.50 0.54 0.57

Q (cumec) 0.000 0.009 0.030 0.064 0.110 0.170 0.245 0.336 0.443 0.569 0.713

4.5 Calculation for Open Concrete Drain Design Criteria: 26.3.3 26.2.5

Maximum depth = 0.5m (Without protective covering) ; 1.0 (With solid or grated cover) Maximum width = 1.0m, minimum width = 0.5m Freeboard = 50mm Maximum average flow velocity shall not exceed 4m/s

with Longitudinal slope, S = 1:375 and n = 0.013 Eqn. 14.4a

Manning’s Equation, V = Q = VA

SLOPE 0.00267 0.00267 0.00267 0.00267 0.00267 0.00267 0.00267

Width, W (m) 0.30 0.30 0.30 0.30 0.30 0.30 0.30

1 2 / 3 1/ 2 R S n

Summary of Depth, D (m) 0.00 0.05 0.10 0.15 0.20 0.25 0.30

300mm Concrete Drain A P R (sq.m) (m) (m) 0.000 0.30 0.00 0.015 0.40 0.04 0.030 0.50 0.06 0.045 0.60 0.08 0.060 0.70 0.09 0.075 0.80 0.09 0.090 0.90 0.10

V (m/s) 0.00 0.45 0.61 0.71 0.77 0.82 0.86

Q (cumec) 0.000 0.007 0.018 0.032 0.046 0.062 0.077

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Case Study I

SLOPE 0.00267 0.00267 0.00267 0.00267 0.00267 0.00267 0.00267

Summary of Depth, D (m) 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Width, W (m) 0.30 0.30 0.30 0.30 0.30 0.30 0.30

450mm Concrete Drain A P R (sq.m) (m) (m) 0.000 0.45 0.00 0.023 0.55 0.04 0.045 0.65 0.07 0.068 0.75 0.09 0.090 0.85 0.11 0.113 0.95 0.12 0.135 1.05 0.13

V (m/s) 0.00 0.47 0.67 0.80 0.89 0.96 1.01

Q (cumec) 0.000 0.011 0.030 0.054 0.080 0.108 0.137

W

D

4.6 On-Site Detention Design Catchment Area = 15100 m2 % Pervious Area = 20%, % Impervious Area = 80% tcs = 20 minutes, tc = 20 minutes Development Status Pre-development Post-development

Impervious Area C A (m2) 0 0.89

Pervious Area A (m2) C

0 12080

0.71 0.56

15100 3020

∑CA

I (mm/hr)

Q (l/s)

10721 12442.4

122.55 160.24

364.96 (Qp) 553.83 (Qa)

Using Equation 19.1 with Equations 19.1a and 19.1b for above-ground storage (MSMA):

Qp ⎞ ⎛ Q ⎞⎛ a = ⎜⎜ 4 a ⎟⎟ ⎜⎜ 0.333 t c + 0.75 t c + 0.25 t cs ⎟⎟ Qa ⎝ tc ⎠ ⎝ ⎠

b = 4 Qa Q p PSD =

a−

= 2701.43

= 808497.53

a 2 − 4b 2

= 342.78 l/s

Using Equation 19.2 with Equations 19.2a and 19.2b for above-ground storage (MSMA):

⎛ PSD ⎞ ⎟⎟ c = 0.875 PSD ⎜⎜ 1 − 0.459 Q d ⎠ ⎝

;

d = 0.214

PSD 2 Qd

SSR = 0.06 td (Qd – c – d)

Urban Stormwater Management Short Course

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Case Study I

td (mins)

I (mm/hr)

5 10 15 20 30 35 40

358.35 267.09 213.23 178.05 135.92 123.87 113.84

Impervious Area C A (m2) 0.89 12080 0.89 12080 0.89 12080 0.89 12080 0.89 12080 0.89 12080 0.89 12080

Pervious Area A (m2) C 0.84 3020 0.74 3020 0.65 3020 0.6 3020 0.51 3020 0.48 3020 0.46 3020

∑CA

Qd (l/s)

13288 12986 12714.2 12563.2 12291.4 12200.8 12140.4

1322.69 963.45 753.06 621.37 464.07 419.81 383.91

td (mins)

Qd (l/s)

PSD (l/s)

c

d

SSR (m3)

5 10 15 20 30 35 40

1322.69 963.45 753.06 621.37 464.07 419.81 383.91

342.78 342.78 342.78 342.78 342.78 342.78 342.78

264.26 250.95 237.27 223.99 198.24 187.52 177.01

19.01 26.10 33.39 40.47 54.18 59.90 65.50

311.83 411.84 434.16 428.30 380.95 362.02 339.36

Above-ground storage (drypond) provided = 160 m3 / 1.2 = 133 m3 Below-ground storage (module) required = 435 – 133 = 302 m3 Storage in Module: Length of Perimeter Swale = 780 m Volume of storage = 780*0.96*0.41*0.467 = 143.4 m3 Length of Ecological Swale = 320 m Volume of storage = 320*0.96*0.41*0.467*2 = 117.5 m3 Volume of Detention Storage A = 15*16*0.96*0.410*0.467*0.607 = 26 m3 Volume of Detention Storage B = 39*4*0.96*0.410*0.467*0.607 = 17 m3 Total volume of storage = 304 m3 Primary Outlet Sizing: The primary outlet orifice is sized to discharge the PSD assuming free outlet conditions when the storage is full. Equation 19.3 (MSMA):

Ao = Do =

PSD

342.78 x 10 −3

=

0.62

2g H o

Cd

4A o

π

=

2 x 9.81 x 0.60

4 x 0.1611

π

= 0.1611 m2

= 0.453 m = 453 mm

Proposed orifice 450mm dia. Secondary Outlet Sizing: Broad-crested weir is sized for the estimated major system ARI flow from the site for time tcs (20 minutes) for 50 year ARI. 50

I 20

= 211.86 mm/hr

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Case Study I

Using the Rational Method, the major system flow is calculated as follows: Impervious Area C A (m2) 0.90 12080

Pervious Area A (m2) 0.64 3020

∑CA

I (mm/hr)

Q (l/s)

12804.8

211.86

754

C

Provide 1.2m x 1.2m brickwall sump with 1 no. of 600mm (or 2 nos. of 475mm) diameter concrete pipe at slope 1:100 as secondary outlet structure. 600mm diameter concrete pipe capacity = 0.75 m3/s

Layout Plan

Outlet Structure Urban Stormwater Management Short Course

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Case Study II

1.0 Introduction Design criteria and design calculation for the “Cadangan Merekabentuk, Membina Dan Menyiapkan Sekolah Kebangsaan Sri Bandi 2 Di Atas Tanah Kerajaan, Daerah Kemaman, Terengganu Darul Iman” is presented for case study.

2.0 Proposed Drainage System The drainage plan proposed for the development area has been designed with the control at source concept and to satisfy the Stormwater Management Manual (MSMA). The engineered waterway which also functions as drypond facility to be blended with the landscape function as the stormwater quantity control. The internal drainage system consists of covered lined drain is used to cater for the post-development discharge generated from the development area. Swale is designed to cater for runoff from car park and road pavement for minor event. The engineered waterway which acts as drypond is then used to route the post-development discharge prior to the disposal of stormwater into the receiving waterbody. Three numbers of drypond (EW1, EW2 and EW3) have been used to manage the stormwater generated from subcatchment A, B, C and D. This has resulted in the attenuation of post-development discharge to the level of predevelopment rate (Table 1). This satisfies the requirement of zero peak flow contribution outlined in MSMA. Table 1 Pre-Development Post-Development Control Outflow from Discharge (m3/s) Discharge (m3/s) Drypond (m3/s) 5 yr 10 yr 5 yr 10 yr 5 yr 10 yr EW1 A 0.08 0.09 0.046 0.051 1.05* 1.19* EW2 B 0.06 0.07 0.028 0.031 EW3 C and D 0.89 1.27 0.60 0.76 * Note: Pre-development discharge generated from sub-catchment A, B, C, D and E.

Drypond

SubCatchment

3.0

Reservoir Routing

3.1

Level Pool Method

Level-pool routing is a procedure for calculating the outflow hydrograph from a pond reservoir, assuming a horizontal water surface, given its inflow hydrograph and storage-discharge characteristics. When a reservoir has a horizontal water surface, its storage is a function of its water-surface elevation, or depth in the pool. For hydrologic routing the inflow I (t), outflow O ( t ) , and storage S(t) are related by the continuity equation:

dS = I ( t ) − O( t ) dt

(14.14)

If an inflow I (t ) is known Equation 1 cannot be solved directly to obtain the outflow O ( t ) , because both O and S are unknown. A second relationship, the storage function, is required to relate I , S , and O. Coupling the continuity equation with the storage function provides a solvable combination of two equations and two unknowns. Integration of the continuity equation (Equation 14.14) over the discrete time intervals provides an expression for the change in storage over the j th time interval j Δt , S j+1 - S j , which can be rewritten as:

⎛ O + O j +1 ⎞ ⎛ I j + I j +1 ⎞ ⎟⎟ Δt ⎟⎟ Δt − ⎜⎜ j S j +1 − S j = ⎜⎜ 2 2 ⎠ ⎠ ⎝ ⎝

(14.15)

The inflow values at the beginning and end of the j th time interval are I j and I j +1 , respectively, and corresponding outflow values are O j and O j+1 . The values of I j and I j +1 , are known because they are pre-specified (i.e. the inflow hydrograph ordinates). The values O j and S j are known at the j th time interval. Hence Equation 14.15 contains two unknowns, O j +1 and S j +1 , which are isolated by multiplying Equation 14.15 by 2 / ∆t and rearranging the result to produce:

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⎞ ⎞ ⎛ 2S j ⎛ 2 S j +1 ⎜⎜ − O j ⎟⎟ + O j +1 ⎟⎟ = (I j + I j +1 ) + ⎜⎜ ⎠ ⎠ ⎝ Δt ⎝ Δt

(14.16)

In order to calculate the outflow O j +1 from Equation 14.16, a storage-discharge function relating 2S / Δt +O and O is needed. The method of developing this function using stage-storage and stage-discharge relationship is shown in Figure 14.10.

Inflow (I )

Ij+1

Volume = (Ij+Ij+1)Δt /2

Ij Δt

t

A

Pond Outflow (O )

Volume = (Oj+Oj+1)Δt /2

Oj+1

Inflow

A

Oj

Ij+1

Δt

Outflow

Storage ΔS

t

Ij t

Δt

ΔS = S (tj+1) - S (tj ) Inflow (I)

H (tj+1)

H (tj )

Outflow (O) SECTION A-A

Figure 14.10 Development of the Storage-Discharge Function for Hydrologic Pond Routing The relationship between water-surface elevation and reservoir storage can be obtained using topographic maps or from field surveys. The stage-discharge relationship is derived from hydraulic equations relating head and discharge for various types of spillway and outlet works. The value of Δt is the same as the time interval of the inflow hydrograph. For a given water-surface elevation, the values of storage S and discharge O are determined. Then, the value of 2S / ∆t +O is calculated and plotted against O. In routing the flow through the j th time interval, all terms in the righthand-side of Equation 14.16 are known, and so the value of 2S j +1 / ∆ t +O can be computed. The corresponding value of O j +1 can be determined from the storage-discharge function 2S /∆t +O versus O. To set up the data for the next time interval, the value 2S j+1 / ∆ t - O j +1 is calculated by:

⎛ 2 S j +1 ⎞ ⎛ 2 S j +1 ⎞ ⎜⎜ − O j +1 ⎟⎟ = ⎜⎜ + O j +1 ⎟⎟ − 2O j +1 ⎝ Δt ⎠ ⎝ Δt ⎠

(14.17)

The computation is repeated for subsequent routing periods. Input requirements for this routing method are: • • • • •

the storage-discharge relationship the storage-indication relationship the inflow hydrograph initial values of the outflow rate (O1) and storage (S1) the routing interval (Δt)

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An analysis procedure for hydrologic routing is shown in Figure 14.11 (MSMA, 2000).

Select hydrograph model or method

Select design ARI

Choose time step Δt

y select model or method appropriate for the purpose of the design e.g. storage routing, facility design, flood routing etc. y select design ARI for both minor and major drainage systems from Table 4.1

y Δ t should be the same as the time step of the inflow hydrograph

Calculate stage-discharge curve

Calculate stage-storage curve

Calculate next time step

y Equation 14.18

Calculate next value of outflow hydrograph

No

End of input hydrograph Yes

y a range of hydrographs with different durations will need to be developed to determine the hydrograph that produces the maximum storage

Select hydrograph that produces the maximum peak flow rate or storage volume

Figure 14.11 General Analysis Procedure for Hydrologic Routing

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4.0 Worked Example (Cadangan Merekabentuk, Membina Dan Menyiapkan Sekolah Kebangsaan Sri Bandi 2 Di Atas Tanah Kerajaan, Daerah Kemaman, Terengganu Darul Iman)

Reference

Calculation

Output

Design Rainfall ARI = 10 yrs (Major System) Table 13.A1

Location = Kuala Dungun, Terengganu a = 5.5077, b = -0.0310, c = -0.0899, d = 0.0050

Runoff estimation for storm durations less than 30 mins Eqn. 13.3

For a short duration, d, the rainfall depth Pd = P30 − FD (P60 − P30 )

P30, P60 = 30-minute and 60-minute duration rainfall depths from published design curves. FD = Adjustment factor for storm duration, a function of 2P24hr (Table 13.3) Eqn. 13.2

ln( R I t ) = a + b ln(t ) + c(ln(t )) 2 + d (ln(t )) 3 10

I30 = 106.10 mm/hr P30 = 106.10 x (30/60) = 53.05 mm P60 = 75.35 x (60/60) = 75.35 mm

Eqn. 13.3 Eqn. 13.4

10

I60 = 75.35 mm/hr

Say tc = 15 min, P10 = 53.05 – 0.74(75.35-53.05) = 36.55 mm 10

I10 = 36.55 / (15/60) = 146.18 mm/hr

Summary of runoff estimation for storm durations less than 30 mins (major system) Time (min)

FD (Table 13.3)

Pd (mm)

10

5

1.39

22.05

264.61

10

1.03

30.08

180.47

15

0.74

36.55

146.18

20

0.48

42.34

127.03

Id (mm/hr)

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Reference

Calculation

Output

Runoff Estimation for Subcatchment A Area = 1900 m2 Table 14.3 tc = 15 mins Chart 14.3 5I15 = 131.55 mm/hr; c = 0.79 (Group 4) 10 I15 = 146.18 mm/hr; c = 0.81 (Group 4) 50 I15 = 171.45 mm/hr; c = 0.84 (Group 4) Eqn. 14.7 Q5 = ciA = 0.05 m3/s ; Q10 = ciA = 0.06 m3/s ; Q50 = ciA = 0.08 m3/s

Q5 = 0.05 m3/s Q10 = 0.06 m3/s Q50 = 0.08 m3/s

For Water Quality Treatment: Area = 1900 m2 Table 14.3 tc = 15 mins Chart 14.3 2I15 = 116.96 mm/hr; c = 0.76 (Group 4) Eqn. 14.7 Q2 = ciA = 0.047 m3/s Q0.25 = 0.5*Q2 = 0.023 m3/s V0.25 = 0.5*2tc*Q0.25= 21.11 m3 Provide 0.45m x 0.45m subsurface storage along 110m length EW1 (22 m3)

Q2 = 0.047 m3/s Q0.25 = 0.023 m3/s V0.25 = 21.11 m3

Calculation for Grassed Swale:

26.1.1 26.2.4 26.2.5 26.2.3

Design Criteria: Contributing area = Subcatchment A1 Design Storm = minor (5 yr ARI), Q = 0.017 m3/s Freeboard = 50 mm Average flow velocity ≤ 2 m/s Side slopes not steeper than 4(H):1(V) Base side slopes shall be less than 50(H):1(V) Maximum depth = 0.9m Slope: 1 in 500 n = 0.035 Bottom Width (BW) = 0.6m Depth, D = 0.10 m Flow area, A = 0.10 m2 1

Flow velocity, V = n

R

2/3

R

2/3

S

1/2

S

1/2

= 0.22 m/s Drain capacity, Q = VA = 0.02 m3/s ( > Q5) Freeboard = 50 mm Depth, D = 0.15 m Flow area, A = 0.18m2 1

Flow velocity, V = n

= 0.27 m/s Drain capacity, Q = VA = 0.05 m3/s ( > Q10 = 0.019 m3/s) 26.1.2

Minimum requirement for maintenance access = 0.5 m both sides

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Reference

Calculation

Output

Calculation for Engineered Waterway:

28.3 28.6 28.7 28.10.2 28.10.6

28.10.7

Design Criteria: Type: Grassed Floodway Design Storm = 10 yr ARI Freeboard = 300 mm Minimum Longitudinal Grade = 0.2 % Maximum flow velocity ≤ 2 m/s Side slopes not steeper than 6(H):1(V) or 4(H):1(V) in special circumstances Floodway base side slopes shall be less than 50(H):1(V) Landscaping: - no trees than those with clean boles, strong crown structure, and no propensity for root suckering may be planted in the floodplain - minimum spacing of trees shall be 3 m - maintenance free ‘thicket’ zones used for hydraulic reasons shall have a minimum of 3m clearance from lot boundaries to provide access for mowing - no vegetation other than grass shall be planted within 3 m of a concrete invert in a floodway Advisory signs should be located at points of congregation and generally at about 500 m intervals along floodways within the 2 year ARI floodplain

EW1:

Contributing area = Subcatchment A Slope: 1 in 500 n = 0.035 Bottom Width (BW) = 0.60m Depth, D = 0.20 m Flow area, A = 0.28 m2 1

Flow velocity, V = n

R

2/3

R

2/3

S

1/2

S

1/2

= 0.32 m/s Drain capacity, Q = VA = 0.09 m3/s ( > Q10) Freeboard = 300 mm Depth, D = 0.50 m Flow area, A = 1.30 m2 1

Flow velocity, V = n

= 0.54 m/s Drain capacity, Q = VA = 0.70 m3/s ( > Q50)

Table 28.1

Top Width of Waterway = 4.60m < 6m Minimum requirement for maintenance access = 3.7 m and 1.0 at one side and other side respectively

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Case Study II COMPUTATION OF HYETOGRAPH (PRE-DEVELOPMENT-ARI 10)

Rainfall Duration

ARI 5

td (min) 30 60 120 180 360

ARI 10

Ln5It

ARI 50

5It (mm/hr) 95.49 67.82 46.87 37.43 25.28

Ln10It

Loss (mm) 21 25 31 35 44

Excess Rainfall (mm) 32.44 50.28 73.51 90.47 127.90

td =120min 10Ddt (mm) 2.21 8.75 22.79 15.29 6.62 8.75 6.91 2.21

td =180min 10Ddt (mm) 5.43 19.90 30.76 19.90 10.86 3.62

td =360min 10Ddt (mm) 40.93 52.44 14.07 10.23 6.39 3.84

73.51 15min Interval

90.47 30min Interval

127.90 60min Interval

4.56 4.22 3.85 3.62 3.23

I10t (mm/hr) 106.10 75.35 52.19 41.86 28.73

4.66 4.32 3.95 3.73 3.36

Ln50It

I50t (mm/hr) 127.72 92.25 65.14 52.93 37.27

4.85 4.52 4.18 3.97 3.62

TEMPORAL PATTERN (ARI 10) td (min) 30 60 120 180 360

10It (mm/hr) 106.10 75.35 52.19 41.86 28.73

Time Interval

td =30 min 10Ddt (mm) 5.19 8.11 10.70 2.92 3.57 1.95

1 2 3 4 5 6 7 8 9 10 11 12

32.44 5min Interval

Total Rainfall (mm) 53.05 75.35 104.38 125.59 172.37 td=60 min 10Ddt (mm) 1.96 3.52 8.45 6.03 11.66 5.08 4.47 2.87 2.41 1.56 1.41 0.85 50.28 5min Interval

INITIAL LOSS Percentage of pervious area is 0% Area Type Pervious

Percentage (%) 100

Initial loss (mm) 10

Proportional loss (mm) 20% of rainfall

Impervious

0

1.5

0

TOTAL LOSS FOR INITIAL LOSS OF IMPERVIOUS AREA 0.00 mm

INFLOW HYDROGRAPH FOR ARI 10 (RAINFALL DURATION 60min) Time (min)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105

D (m) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

0.000 0.002 0.004 0.008 0.006 0.012 0.005 0.004 0.003 0.002 0.002 0.001 0.001

A1(5min) 11950 (sq.m)

A2(10min) 11965

23.43 42.06 100.94 72.10 139.39 60.68 53.47 34.25 28.84 18.63 16.82 10.21

23.46 42.11 101.07 72.19 139.57 60.76 53.54 34.29 28.88 18.65 16.84 10.23

A3(15min) 9789

19.19 34.45 82.69 59.06 114.19 49.71 43.80 28.05 23.62 15.26 13.78 8.37

A4(20min) 9500

A5(25min) 4860

18.63 33.44 80.24 57.32 110.81 48.24 42.51 27.23 22.93 14.81 13.37 8.12

A6(30min) 836

9.53 17.10 41.05 29.32 56.69 24.68 21.75 13.93 11.73 7.57 6.84 4.15

1.64 2.94 7.06 5.04 9.75 4.25 3.74 2.40 2.02 1.30 1.18 0.71

A7(35min) 0

A8(40min) 0

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

A9(45min) 0

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

TOTAL DEPTH (cu.m) 23.43 65.52 162.25 226.25 337.23 358.30 329.73 284.70 216.91 152.50 112.32 82.91 52.94 31.33 16.26 5.33 0.71 0.00 0.00 0.00 0.00

Qinflow (cumec) 0.00 0.08 0.22 0.54 0.75 1.12 1.19 1.10 0.95 0.72 0.51 0.37 0.28 0.18 0.10 0.05 0.02 0.00 0.00 0.00 0.00 0.00

ARI 10 (D=60min) 1.40

Flow (cumec)

1.20 1.00 0.80 0.60 0.40 0.20 0.00 0

10

20

30

40

50

60

70

80

90

100

110

120

Time (min)

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Reference

Calculation

Output

Calculation for Drypond Sizing Table 4.1

Determine design storm criteria for the basin Design storm for primary outlet: 5 year ARI, 10 year ARI Design storm for secondary outlet spillway: 50 year ARI Determine the basin outlet limits The estimated pre-development flow hydrograph is tabulated in the following table. The basin outflow limits for 5 and 10 year ARI are 1.05 m3/s and 1.19 m3/s respectively.

Time Interval

Pre-Development Flow Hydrographs ARI (years) 5 10 Storm Duration (minutes) 30 60 120 30 60 120 30 Flow (m3/s) 0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1 0.18 0.07 0.07 0.21 0.08 0.08 0.26 2 0.46 0.19 0.32 0.52 0.22 0.36 0.67 3 0.80 0.48 0.89 0.92 0.54 1.00 1.16 4 0.85 0.66 0.85 0.97 0.75 0.96 1.23 5 0.82 0.99 0.45 0.95 1.12 0.51 1.20 6 0.70 1.05 0.39 0.80 1.19 0.44 1.01 7 0.42 0.97 0.36 0.48 1.10 0.41 0.61 8 0.22 0.84 0.18 0.25 0.95 0.20 0.32 9 0.11 0.64 0.03 0.13 0.72 0.04 0.16 10 0.04 0.45 0.00 0.04 0.51 0.00 0.05 11 0.00 0.33 0.01 0.37 0.01 12 0.24 0.00 0.28 0.00 13 0.16 0.18 14 0.09 0.10 15 0.05 0.05 16 0.02 0.02 17 0.00 0.00

50 60

120

0.00 0.10 0.28 0.69 0.96 1.43 1.52 1.39 1.20 0.92 0.65 0.48 0.35 0.22 0.13 0.07 0.02 0.00

0.00 0.11 0.47 1.28 1.23 0.65 0.56 0.52 0.26 0.05 0.00

Note: For storm duration 30 and 60 minutes, time interval = 5 minutes For storm duration 120 minutes, time interval = 15 minutes

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Case Study II COMPUTATION OF HYETOGRAPH - POST-DEVELOPMENT (SCA)

Rainfall Duration td (min) 15 30 60 120 180 360

ARI 5 Ln5It 4.88 4.64 4.33 3.96 3.73 3.34

ARI 10 5It (mm/hr) 131.30 103.73 75.65 52.41 41.76 28.27

Ln10It

Loss (mm) 5 5 7 8 9 11

Excess Rainfall (mm) 32.83 51.77 76.95 110.49 135.49 192.11

ARI 50 I10t (mm/hr) 150.10 114.51 83.49 59.21 48.16 33.91

5.01 4.74 4.42 4.08 3.87 3.52

Ln50It

I50t (mm/hr) 190.65 141.94 103.99 75.28 62.08 44.48

5.25 4.96 4.64 4.32 4.13 3.79

TEMPORAL PATTERN (ARI 10) td (min) 15 30 60 120 180 360 Time Interval

10It (mm/hr) 150.10 114.51 83.49 59.21 48.16 33.91 td = 15 min 10Ddt (mm) 1 10.50 2 16.41 3 5.91 4 5 6 7 8 9 10 11 12 32.83 5min Interval

Total Rainfall (mm) 37.53 57.26 83.49 118.42 144.47 203.45 td =30 min 10Ddt (mm) 8.28 12.94 17.08 4.66 5.69 3.11

51.77 5min Interval

td=60 min 10Ddt (mm) 3.00 5.39 12.93 9.23 17.85 7.77 6.85 4.39 3.69 2.39 2.15 1.31 76.95 5min Interval

INITIAL LOSS Percentage of pervious area is 20%

td =120min 10Ddt (mm) 3.31 13.15 34.25 22.98 9.94 13.15 10.39 3.31

td =180min 10Ddt (mm) 8.13 29.81 46.07 29.81 16.26 5.42

td =360min 10Ddt (mm) 61.48 78.77 21.13 15.37 9.61 5.76

110.49 15min Interval

135.49 30min Interval

192.11 60min Interval

Area Type Pervious

Percentage Initial loss Proportional loss (%) (mm) (mm) 20 10 20% of rainfall

Impervious

80

1.5

0

TOTAL LOSS FOR INITIAL LOSS OF IMPERVIOUS AREA mm 1.20

POST-DEVELOPMENT INFLOW HYDROGRAPH FOR ARI 10 - SCA (RAINFALL DURATION 60min) Time (min)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

D (m) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

A1(5min) 272 (sq.m)

0 0.003 0.005 0.013 0.009 0.018 0.008 0.007 0.004 0.004 0.002 0.002 0.001

A2(10min) 181

0.82 1.47 3.52 2.51 4.86 2.11 1.86 1.19 1.00 0.65 0.59 0.36

A3(15min) 1277

0.54 0.97 2.34 1.67 3.23 1.41 1.24 0.79 0.67 0.43 0.39 0.24

A4(20min) 0

3.83 6.88 16.51 11.79 22.80 9.92 8.75 5.60 4.72 3.05 2.75 1.67

TOTAL DEPTH (cu.m) 0.82 2.01 8.32 11.73 23.03 17.14 26.07 12.36 10.54 6.92 5.73 3.79 2.99 1.67 0.00 0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Qinflow (cumec) 0.000 0.003 0.007 0.028 0.039 0.077 0.057 0.087 0.041 0.035 0.023 0.019 0.013 0.010 0.006 0.000 0.000 0.000 0.000 0.000 0.000

ARI 10 (D=60min) 0.100

Flow (cumec)

0.090 0.080 0.070 0.060 0.050 0.040 0.030 0.020 0.010 0.000 0

10

20

30

40

50

60

70

80

90

100

110

Time (min)

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Reference

Calculation

Output

Calculation for Drypond Sizing- Drypond A (EW1) Compute the basin inflow hydrographs The critical storm duration for maximum basin storage has to be determined by routing post-development inflow hydrographs of different durations (longer than tc) through the basin as given in the following table.

Time Interval

Post-Development Flow Hydrographs From Sub-catchment A

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

5 30

60

0.00 0.01 0.02 0.05 0.06 0.07 0.02 0.02 0.01 0.00

0.00 0.00 0.01 0.03 0.04 0.07 0.05 0.08 0.04 0.03 0.02 0.02 0.01 0.01 0.01 0.00

ARI (years) 10 Storm Duration (minutes) 120 30 60 120 30 Flow (m3/s) 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.01 0.01 0.02 0.02 0.01 0.03 0.02 0.06 0.06 0.03 0.07 0.07 0.04 0.07 0.04 0.04 0.09 0.02 0.08 0.08 0.02 0.10 0.02 0.03 0.06 0.03 0.03 0.02 0.03 0.09 0.02 0.03 0.01 0.01 0.04 0.01 0.02 0.00 0.00 0.04 0.00 0.00 0.02 0.02 0.01 0.01 0.01 0.00

50 60

120

0.00 0.00 0.01 0.04 0.05 0.10 0.07 0.11 0.05 0.04 0.03 0.02 0.02 0.01 0.01 0.00

0.00 0.01 0.03 0.08 0.06 0.02 0.03 0.03 0.01 0.00

Note: For storm duration 15, 30 and 60 minutes, time interval = 5 minutes For storm duration 120 minutes, time interval = 15 minutes

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Reference

Calculation

Output

Calculation for Drypond Sizing- Drypond A (EW1) Compute the stage-storage relationship Basin Stage-Storage Relationship Elevation

Stage

Area

Δ Storage

Total Storage

(m)

(m)

(sq.m)

(m3)

(m3)

17.500

0.000

66

0.00

0.00

17.600

0.100

110

11.00

11.00

17.700

0.200

198

19.80

30.80

17.800

0.300

286

28.60

59.40

17.900

0.400

374

37.40

96.80

18.000

0.500

462

46.20

143.00

5 Year ARI Stage-Discharge Relationship Size the major primary outlet Minor primary outlet: 450 mm dia. concrete pipe Stage (m)

Discharge, Q (cumec)

Total Storage,S (cu.m)

(2S/delt)+Q (cumec)

0.000 0.100 0.200 0.300 0.400

0.000 0.007 0.027 0.059 0.102

0.00 11.00 30.80 59.40 96.80

0.00 0.08 0.23 0.45 0.75

0.600

Stage (m)

0.500 0.400 0.300 0.200 0.100 0.000 0.000

0.050

0.100 Q (cume c)

0.150

0.200

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Routing Results for 5 year ARI, 60 minute Basin Inflow Hydrograph Time (min)

Inflow (cumec)

Ij + Ij+1 (cumec)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160

0.00 0.00 0.01 0.03 0.04 0.07 0.05 0.08 0.04 0.03 0.02 0.02 0.01 0.01 0.01 0.00

0.00 0.00 0.01 0.03 0.06 0.10 0.12 0.13 0.12 0.07 0.05 0.04 0.03 0.02 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00

2SJ/ΔT-QJ

0.00 0.01 0.03 0.08 0.14 0.20 0.25 0.27 0.26 0.23 0.21 0.18 0.15 0.13 0.11 0.09 0.07 0.06 0.05 0.04 0.03 0.03 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.00

2Sj+1/Δt+ Outflow Stage (cumec) (m) Qj+1

0.00 0.00 0.01 0.04 0.09 0.18 0.26 0.33 0.36 0.34 0.31 0.27 0.23 0.20 0.17 0.14 0.11 0.09 0.07 0.06 0.05 0.04 0.03 0.03 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01

0.000 0.000 0.001 0.003 0.008 0.020 0.031 0.041 0.046 0.26 0.042 0.038 0.032 0.027 0.023 0.019 0.014 0.010 0.008 0.006 0.005 0.004 0.003 0.003 0.002 0.002 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.000

Stage (m)

0.000 0.100 0.200 0.300 0.400 0.500

Total Discharge, Storage,S Q (cumec) (cu.m)

0.00 0.01 0.03 0.06 0.10 0.15

0.00 11.00 30.80 59.40 96.80 143.00

(2S/Δt)+Q (cumec)

0.00 0.08 0.23 0.45 0.75 1.11

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Case Study II

Reference

Calculation

Output

Calculation for Drypond Sizing – Drypond A (EW1) 5 YEAR ARI (D=60min) 0.09 0.08 0.07

Inflow

Outflow

Q (cumec)

0.06 0.05 0.04 0.03 0.02 0.01 0.00 0

50

100

150

200

Time (min)

Size the major primary outlet 450 mm dia.concrete pipe 10 Year ARI Stage–Discharge Relationship Stage (m)

Discharge, Q (cumec)

Total Storage, S (cu.m)

(2S/delt)+Q (cumec)

0.000 0.100 0.200 0.300 0.400

0.000 0.007 0.027 0.059 0.102

0.00 11.00 30.80 59.40 96.80

0.00 0.08 0.23 0.45 0.75

0.600 0.500

Stage (m)

0.400 0.300 0.200 0.100 0.000 0.000

0.050

0.100 Q (cumec)

0.150

0.200

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Case Study II

Routing Results for 10 year ARI, 60 minute Basin Inflow Hydrograph Time (min)

Inflow (cumec)

Ij + Ij+1 (cumec)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155

0.00 0.00 0.01 0.03 0.04 0.08 0.06 0.09 0.04 0.04 0.02 0.02 0.01 0.01 0.01 0.00

0.00 0.00 0.01 0.03 0.07 0.12 0.13 0.14 0.13 0.08 0.06 0.04

2SJ/ΔT-QJ

0.00 0.01 0.04 0.08 0.15 0.22 0.27 0.30 0.28 0.25 0.22 0.17 0.13 0.11 0.09 0.07 0.06 0.05 0.04 0.03 0.03 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.00

2Sj+1/Δt+ Outflow Stage (cumec) (m) Qj+1

0.00 0.00 0.01 0.04 0.10 0.20 0.29 0.36 0.40 0.37 0.34 0.30 0.22 0.17 0.13 0.11 0.09 0.07 0.06 0.05 0.04 0.03 0.03 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01

0.000 0.000 0.001 0.004 0.010 0.023 0.035 0.046 0.051 0.27 0.047 0.042 0.036 0.026 0.019 0.014 0.010 0.007 0.006 0.005 0.004 0.003 0.003 0.002 0.002 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.000

Stage (m)

0.000 0.100 0.200 0.300 0.400 0.500

Total Discharge, Storage,S Q (cumec) (cu.m)

0.000 0.007 0.027 0.059 0.102 0.155

0.000 11.000 30.800 59.400 96.800 143.000

(2S/Δt)+Q (cumec)

0.000 0.080 0.232 0.455 0.747 1.108

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Case Study II

Reference

Calculation

Output

Calculation for Drypond Sizing- Drypond A (EW1)

10 YEAR ARI (D=60 min) 0.10 0.09 0.08

Inflow

Outflow

Q (cumec)

0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 0

50

100 Time (min)

150

200

Size the secondary outlet 450 mm dia. Concrete pipe 50 Year ARI Stage–Discharge Relationship Stage (m)

Discharge, Q (cumec)

Total Storage, S (cu.m)

(2S/delt)+Q (cumec)

0.000 0.100

0.000

0.00

0.00

0.007

11.00

0.08

0.200

0.027

30.80

0.23

0.300

0.059

59.40

0.45

0.400

0.102

96.80

0.75

0.600 0.500

Stage (m)

0.400 0.300 0.200 0.100 0.000 0.000

0.050

0.100 Q (cumec)

0.150

0.200

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Case Study II

Time (min)

Inflow (cumec)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165

0.00 0.00 0.01 0.03 0.05 0.10 0.07 0.11 0.05 0.04 0.03 0.02 0.02 0.01 0.01 0.00

Ij + Ij+1 (cumec)

0.00 0.01 0.04 0.08 0.15 0.17 0.18 0.16 0.10 0.07 0.05 0.04 0.03 0.02 0.01 0.00 0.00 0.00 0.00 0.00 0.00

2SJ/ΔT-QJ

0.00 0.00 0.00 0.01 0.05 0.10 0.19 0.27 0.33 0.36 0.34 0.31 0.27 0.23 0.20 0.17 0.14 0.11 0.09 0.07 0.06 0.05 0.04 0.03 0.03 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01

2Sj+1/Δt+ Outflow Stage (cumec) (m) Qj+1

Stage (m)

0.000 0.000 0.001 0.005 0.013 0.029 0.045 0.058 0.065 0.32 0.060 0.053 0.045 0.038 0.031 0.025 0.019 0.014 0.010 0.008 0.006 0.005 0.004 0.003 0.003 0.002 0.002 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.000

0.000 0.100 0.200 0.300 0.400 0.500

0.00 0.01 0.06 0.13 0.25 0.36 0.45 0.49 0.46 0.41 0.36 0.31 0.26 0.22 0.18 0.14 0.11 0.09 0.07 0.06 0.05 0.04 0.03 0.03 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01

Total Discharge, Storage,S Q (cumec) (cu.m)

0.000 0.007 0.027 0.059 0.102 0.155

0.000 11.000 30.800 59.400 96.800 143.000

(2S/Δt)+Q (cumec)

0.000 0.080 0.232 0.455 0.747 1.108

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Case Study II

Reference

Calculation

Output

Calculation for Drypond Sizing- Drypond A (EW1)

50 YEAR ARI (D=60 min) 0.12 0.10 Inflow

O utflow

Q (cumec)

0.08 0.06 0.04 0.02 0.00 0

50

100 Time (min)

150

200

Outlet Structure:

4.6 m

50 mm

450 mm dia. Concrete Pipe

1

450 mm

4 0.6 m

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