Storm water drain design Procedure

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The above link for simple excel software for designing the storm water drain cross section.
Input data to be entered as Catchment area, slope of the drain,area and perimeter of the assumed cross section etc.,
The following points to be consider for the Design.
1. Draw approximate LS.
2. Calculate catchment areas at 250 m intervals or at junctions of secondary drains.
3. Fix maximum water levels along the proposed drain such that they are below:
i) ground levels; and
ii) the top of secondary drains joining the proposed Primary/Outfall drain.
4. Calculate flows at 250 m intervals or at junctions of secondary drains assuming the rate of flows
as given below.
Use the Rational Method as given in CPHEEO Manual on Storm Water Drainage Systems 2019 Vol.I Part A (Engineering Design)
Q = 10 CIA
5. Assume section for each leg and calculate velocity adopting Manning’s formula
V = R 2/3 X S ½ / n
Where V = velocity in m/sec.
R = Hydraulic Radius
= Flow area (A) in sq.m. / Wetted perimeter (P) in m.
S = Drain slope.
N = Manning coefficient depending upon the nature of inside surface of drain (use n value of 0.015 for plastered surface).
6. Check for adequacy of the assumed drain section to discharge the calculated flow (Q) and that the velocity (m/sec) in the drain section in within 0.6 - 3.0 m/sec). If the capacity of the drain is insufficient, assume another section and repeat the procedure till the carrying capacity of drain section is approximately equal to the flow expected. Design the section of drain for each leg. Provide the necessary freeboard also.
7. Propose cover slabs depending upon the depth of drain and surrounding habitation.
8. Examine whether sufficient width of land is available, need for Land Acquisition and removal of encroachments.
9. As far as possible, try to align large drains away from habitation and commercial areas.
10. Calculate cost based on proposed length and cross section of different drain sections.

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