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# tc and rainfall duration specified - pond size calculation

## tc and rainfall duration specified - pond size calculation

(OP)
Hi,

I'm a bit of a newbie on calculating detention ponds etc.
Our regulatory agency requires the following:
- Use of a one in 5-year design storm for the allowable pre-development run-off rate
- Use of a one in 25-year design storm for the post-development run-off rate
- Size of outlet control control that is equal or less than predevelopment velocities
- Use a min. 3hr rainfall duration event and a minimum 1hr time of concentration

The agency prefers use of the rational method. The questions is if we have a relatively small area to be developed into residential lots of 1 acre.
Pre-development tc = 40min / C = 0.15 and post development tc = 25min / C = 0.35.
How can I calculate the pond storage using a tc of 1hr and a 3hr rainfall event by using the rational method, which as far as I have read up assumes that the storm duration equals the time of concentration?
Any suggestions or comments would be appreciated.

### RE: tc and rainfall duration specified - pond size calculation

With modified rational method the rainfall duration is independent of the Tc. This allows you to develop a trapezoidal runoff hydrograph with a Tc of 1 hour and a rainfall duration of 3 hours. Frankly, I don't recommend the use of Rational method for pond sizing, but if you must that's how the hydrograph is developed.

Peter Smart

### RE: tc and rainfall duration specified - pond size calculation

(OP)
Thank you.
What method do you recommend for Pond Sizing?

### RE: tc and rainfall duration specified - pond size calculation

Any accepted hydrograph method, SCS/NRCS, Clark, & etc... I would say it depends on your regulating authority.

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