Simplified OSD Design Procedure

The main objective in the design of OSD facilities is to determine the required storage (SSR) to reduce the post development discharge to that of the pre-development level (PSD) . In order to achieve this the outlet structure needs to be sized appropriately. The OSD Design Manual simplifies most of the processes involved in the design of OSD facilities required for a proposed site, taking into consideration the design storm for short duration rainfall less than 1 hour, the critical time of concentration for various slopes and catchment areas, the PSD and the SSR for the site. The determination of each parameter was based on the assumptions that Peninsular Malaysia can be divided into five design regions having different uniform design storms, as shown in Figure 5.A1.

(a) Permissible Site Discharge (PSD)

The maximum allowable discharge leaving the site is given in litres/second/hectare (l/s/ha), or in litres/second (l/s) when applied to a specific site. The maximum PSD is about 65 l/s/ha. This needs to be adjusted in accordance with the design regions as shown in Figure 5.A1 and the results were summarised in

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• approve preliminary drainage plan;

• development/subdivision consent conditions; and

• landscape and architectural/building plans.

• select discharge control device for each storage;

• establish level of outlet(s) and ensure free outfall if possible; and

• finalise required volume(s).

• distribute final storage volume(s) to minimize;

nuisance ponding conditions to property owners;

• check underground storages for access and ease of maintenance; and

• ensure sufficient weir capacity for storage overflows.

• ensure storage design ARI flows are conveyed to storage for all areas designed to drain to storages; and

• 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 an detail to allow builders/plumbers to construct system; and

• specify construction materials.

• prepare calculation sheets for each storage system;

and

• prepare maintenance schedule outlining necessary maintenance practices.

• review other plans prepared for the development for any anomalies or conflicts with detailed drainage plan; and

• check all stormwater-related development consent conditions have been satisfied.

Figure 5.13: Detailed Design Procedure for OSD Storage Systems (DID, 2000) Obtain copies of approved plans and

conditions

Select discharge control device and finalise storage volumes

Design storage systems

Design drainage conveyance system

Prepare detailed design drawings

Prepare calculation sheets and maintenance schedule

Review design

Submit detailed drainage plan with building plan application

MSMA 2^nd Edition. 2012 | DID Malaysia

5-16 On-site Detention

Tabel 5.A1. Table 5.A3 shows the inlet sizes which represent the PSD capacity. The examples of how to use these tables and figures are given in Appendix 5.B.

(b) Site Storage Requirement (SSR)

The minimum volume (in m³/hectare or in m³ when applied to a specific site) is required for storage to ensure that spillage will not occur when the outflow is restricted to the PSD. The minimum SSR for an OSD storage that has a Discharge Control Pit (DCP) that achieves the full PSD early in the storm event is presented in Table 5.A1 which shows the values in accordance with the design region, in m³/ha. Information about SSR for major towns in Peninsular Malaysia is provided in Table 5.A2.

Table 5.A3 can be used to determine the size of the OSD storage, and its inlet and outlet structures for different types of development projects in Malaysia when the project area, terrain slope and percentage of impervious area are known. Examples of how to use these tables are presented in Appendix 5.B.

(c) Minimum Outlet Size

To reduce the likelihood of the DCP outlet being blocked by debris, the outlet opening shall have a minimum internal diameter or width of at least 30 mm and shall be protected by an approved screen. Stainless steel well screen fabric is more preferable instead of mesh sreen in controlling trash entering the orifice due to the nature of easier cleaning after accumulating trash. Table 5.A3 provides the pipe diameter for the outlet in respect to the PSD requirements in different regions in Peninsular Malaysia.

(d) Overflow Pipe/Spillway

An overflow system must be provided to allow the storage compartment to surcharge in a controlled manner if the capacity of the system is exceeded due to a blockage of the outlet pipe or a storm larger than the design ARI. An overflow can be provided by installing a pipe/weir and the size of pipe is given in Table 5.A3. The weir size can be calculated based on pipe area, which can be determined for a given pipe diameter. Figure 5.A2 can also be used to determine the pipe sizes.

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REFERENCES

1. Creager W.P. and Justin J.D. (1950). Hydroelectric Handbook, 2nd Edition, John Wiley and Sons Inc., New York.

2. Department of Irrigation and Drainage – DID (2000). Urban Stormwater Management Manual for Malaysia, Volume 7, Government of Malaysia.

3. Li W.H. and Patterson C. (1956). Free Outlets and Self-Priming Action of Culverts, Journal of Hydraulics Division, ASCE, HY 3.

4. Stahre P. and Urbonas B. (1990). Stormwater Detention for Drainage, Water Quality and CSO Management, Prentice Hall, New Jersey, USA.

5. Upper Parramatta River Catchment Trust, UPRCT (1999). On-site Stormwater Detention Handbook, 3rd Edition, Australia.

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REGION 4