Design Storm Duration
Design Storm Duration
(OP)
I have modelled a residential development for which we are required to attenuate the post development peak discharge to the level of the pre-development peak discharge for all events up to 1 in 25 year storm. The catchment is small (2 Ha) and the time of concentration 10 minutes. I have received back comment from the local authority who do not appear to be familiar with the SCS Methodology of using synthetic 24 hour storms with nested rainfall intensities. They comment ... it appears that the option selected in your programme only shows storage required after 11 hours of precipitation, and it is suggested that this antecedent condition is clearly intended for much larger catchments than your development. It is our experience that the critical storm is likely to be between 20 and 40 minutes duration, and storage volumes in the attenuation pond will be significantly more that Hydrocad calculates using the 24 hour storm.....
Anyone been faced with a similar response from the regulating authority and how did you convince them that the method used is correct ?
Anyone been faced with a similar response from the regulating authority and how did you convince them that the method used is correct ?





RE: Design Storm Duration
Whether either method is "correct" is very doubtful. No one knows the answer to that question with any certainty.
good luck
RE: Design Storm Duration
RE: Design Storm Duration
RE: Design Storm Duration
You may also want to use the free program NFF (National Flood Frequency) which uses regional regression equations to calculate flows and gives you a "generic hydrograph" for both urban and rural basins.
Whatever method you use, route several storms through your basin to see if the volume seems reasonable.
good luck
RE: Design Storm Duration
The request to determine the "critical storm" is probably based on experience with the Rational method. Proper use of the Rational method does require the determination of the critical duration (and the associated intensity) for any given point in the watershed.
However, the SCS method uses a "nested" synthetic rainfall distribution that effectively tests all durations in a single calculation, and does not require a specific determination of the critical duration.
The only advice I can offer is for the agency to consult some of the reference materials on the SCS method. (See my previous link.)