Code of practice on surface water drainage

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7.2
Computation of Discharge Capacity
7.2.1
Steady Uniform Flow Condition Drains are designed for steady uniform flow conditions and one-dimensional method of analysis is used.
7.2.2
Manning's Formula Drains shall be designed to have discharge capacities (Qc) adequate to cope with the estimated peak runoffs (Qr. The size, geometry and the bed gradient of a drain determine its discharge capacity (Qc). With the required discharge capacity (Qc) determined which must be equal to or larger than the peak runoff (Qr, the size of the drain is computed from the Manning's Formula where Q
c
= discharge capacity of drain (ms) n = roughness coefficient A = flow area (m) P = wetted perimeter (m) RAP hydraulic radius (m) S = bed gradient
7.2.3
Roughness Coefficient The value of the roughness coefficient (n) depends on the drain's flow surface and is given below
Boundary Condition
Roughness Coefficient (n)
Unplasticised Polyvinyl Chloride(UPVC)
0.0125 Concrete
0.0150 Brick
0.0170 Earth
0.0270 Earth with stones and weed
0.0350 Gravel
0.0300 Note Where there are different flow surfaces within a drain section, equivalent roughness coefficient maybe used.
2 1
3 2
S
AR
n
1
Q
c
=

31
7.3
Design Considerations
7.3.1
Minimum Velocity and Dry Weather Flow The velocity of flow in a drain shall not be lower than 1.0 ms for self-cleansing action to take place. However, the flow rate during dry weather may fall to a low level where this minimum velocity cannot be achieved. The problem can be solved by introducing a small channel in the drain to confine the dry weather flow to a smaller flow section. The dimensions of such a dry weather flow channel is given below or unless otherwise stated by the Board. The design of Type C channel is as shown in Drawing No. 1.

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