Hi,
We usually use the SCS method to estimate run off volume for sizing retention elements. On many projects, the result coming from CS using the SWMM explicit engine and SCS hydrograph method (with 24hrs storm event rainfall hyetograph of 25 years return period) are matching that calculated from a a quick hand calculation of the SCS formula ( basically calculating the potential maximum retention depth S in mm using the composite area CN and applying a formula to calculate the actual direct run off depth Q in mm and then multiply this depth by area of the catchment to get the volume of run off). For hand calculation we use total depth from a 24hrs storm event of the 25 years return period.
One engineer recently suggested consideration of the rainfall intensity for 25 years return period and at the 24th hour from an IDF curve (to represent the average intensity over that 24 hour storm event duration) and then applying the rational formula (with the weighted run off coefficient and project area) to calculate the discharge rate and then from that through multiplication with time, the volume for 24 hours event. Of course the result is almost twice that calculated from SCS.
I'd like to hear feedback on validity or limitations (catchments / small Tcs)
Thx