Code of practice on surface water drainage
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- Maximum Velocity
- Sub-critical Flow
- Freeboard
| Top Width of the drain (W) Bottom Width of dry weather flow (D.W.F) Depth of D.W.F W ≤ m Cm < W ≤ 4.5m 0.6m 0.15m 4.5m < W ≤ 6.0m 0.9m 0.15m 6.0m < W ≤ 9.0m 1.2m 0.20m 9.0m < W ≤ 12.0m 1.8m 0.25m W > mm m 7.3.2 Maximum Velocity The velocity of flow in a drain shall not be too great to cause excessive scouring or hydraulic jumps. Hence the velocity of flow in a concrete-lined drain shall be limited to a maximum of 3.0 ms or below the critical velocity, whichever is lower. For an earth stream, the maximum velocity shall be limited toms. Further limitation of the maximum velocity shall be complied with when specified by the Board. 7.3.3 Sub-critical Flow Drains are designed to carry sub-critical flows. Critical state of flow exists when the Froude Number is equal to one. An open channel flow at or near the critical state shall be avoided asunder such a condition the water surface is unstable and wavy. In order to secure greater flow efficiency, channel flow shall be designed so that the Froude Number shall fall within the range from 0.8 decreasing to such minimum value as to achieve a practical flow depth and permissible flow velocity. 7.3.4 Freeboard Freeboard refers to the depth from the top of the drain (cope/bank) to the top of the water surface in the drain at design flow condition. Sufficient freeboard shall be provided to prevent waves or fluctuation of the water surface from overflowing the cope/bank. Generally, a depth of freeboard equivalent to 15% of the depth of the drain is required. 32 8 INTEGRATION OF ADJACENT WATERCOURSES WITH DEVELOPMENTS AND ABC WATERS DESIGN FEATURES WITHIN DEVELOPMENTS 8.1 Integration of Adjacent Watercourses with Developments 8.1.1 As part of the Active, Beautiful, Clean Waters (ABC Waters) Programme, integration of adjacent watercourses with proposed developments is encouraged, whilst satisfying the engineering requirements for drainage functions without posing any public safety, maintenance or environmental problems. These include, but are not limited to a) Enhancing the accessibility or connectivity to waterbodies/waterways, eg. creating community spaces for adjacent to watercourses and providing boardwalks to bring people closer to water, etc b) Providing aesthetic treatment to waterways, eg. use of a more natural finish such as block pitching or imprints on canal walls, planting along the canal walls, etc c) Installing ABC Waters Design features to detain and treat storm water runoff closer to the source (see Clause 8.2 for further details. 8.1.2 The designs of the aesthetic waterbodies, waterways and safety barriers need not be constrained by the standard features as shown in this Code of Practice as long as these facilities fulfill the drainage requirements and do not give rise to any public safety, maintenance or environmental problems. 8.1.3 The approval of the competent planning authorities shall also besought for all such proposals. Where appropriate, the aesthetic waterbodies and watercourses shall be integrated into the development proposals and be maintained by the developers/owners of these developments. Download 3.37 Mb. Share with your friends:
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