Code of practice on surface water drainage
Area Served by Drainage System
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- Maximum Allowable Peak Runoff
| Area Served by Drainage System Return Period (Tb Catchment of less than 100 ha 10 years Catchment of 100 to 1000 ha 25 years Catchment of more than 1000 ha 50 years Airport runway or any area as specified by the Board 100 years 7.1.4 Time of Concentration The peak runoff (Qr) occurs at the point of design when all parts of the catchment receiving a steady, uniform rainfall intensity are contributing to the outflow at this point. This condition is met when the duration of rainfall equals the time of concentration (tc). The time of concentration (tc) consists of the overland flow time (to) plus the drain flow time from the most remote drainage inlet to the point of design (td), viz. tc = to + td. The overland flow time (to) varies from 5 minutes to 15 minutes, depending on the overland travel distance, land topography and characteristics. The drain flow time (td) shall be estimated from the hydraulic properties of the drainage channel. 29 7.1.5 Maximum Allowable Peak Runoff The following types of developments are required to control the peak runoff discharged from the affected development sites a) New erection and reconstruction works to commercial, industrial, institutional and residential developments greater than or equal to 0.2 hectares in size and b) Additions & Alterations (AA) works to existing commercial, industrial, institutional and residential developments where affected area is greater than or equal to 0.2 hectares in size such as i) Addition of new buildings or ii) Extension to existing buildings or iii) Partial reconstruction of existing buildings iv) Any combination of the above The maximum allowable peak runoff to be discharged to the drains will be calculated based on a runoff coefficient of 0.55, and for design storms with a return period of 10 years and for various storm durations of up to 4 hours (inclusive. Peak runoff reduction can be achieved through the implementation of ABC Waters design features and/or structural detention and retention features, such as i) Detention tanks ii) Retention/sedimentation ponds iii) Wetlands iv) Bioretention swales; v) Bioretention basins or rain gardens vi) Porous pavements, etc. Details (calculations and/or hydraulic model results) showing how the proposed system meets the required peak runoff rates shall be submitted and endorsed by PE (Civil. For systems that include ABC Waters design features to meet the required peak runoff rates, the details must be endorsed by an ABC Waters Professional, who is also a PE (Civil. For design guidance on detention tank systems, QPs can refer to the Technical Guide for Onsite Stormwater Detention Tank Systems, available on the PUB website. For design guidance on the ABC Waters design features, QPs can refer to the ABC Waters Guidelines and relevant chapters in the Engineering Procedures, available on the PUB website. Due consideration shall be given to meeting ABC Waters storm water quality objectives, which will often require treatment of storm water runoff using ABC Waters design features. Download 3.37 Mb. Share with your friends:
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