8 Mar 11 8:02
I appreciate the prompt response (and from you no doubt!)
I've used the SCS curve number method to tabulate initial abstraction (Ia), continual abstraction (Fa), and excess rainfall (Pe) from gross rainfall of a design storm (type III, 24hr). From this data I have added a portion of the abstraction back into the excess rainfall to account for subsurface collection via a interceptor drain at a specified depth. The determination of the portion of abstraction is based on the porosity, field capacity, initial moisture content, and depth of soil above the interceptor.
If we were not incorporating this model into a larger scheme of subcatchments and just wanted to find out the flow from the interceptor, we would calculate the water collected in the interceptor using an infiltration method (like Green-Amp or Horton's). In this case, however, we'd like to add what the interceptor collects to the excess rainfall on the surface. In this scenario they are going to the same place, so the outlet hydrograph will essentially be the addition of the hydrograph produced from the surface and the hydrograph produced by the interceptor. Since SCS CN method accounts for infiltration through abstraction, the value collected in the interceptor can be calculated. Since the surface runoff, and the intercepter runoff will both be collected and conveyed at the same point, we thought it would be appropriate to add them prior to applying the SCS unit hydrograph.
If for some reason this can't be done, I suppose an alternative would be to add the interceptor flow after the surface excess rainfall has been through the unit hydrograph. This might even be a better solution. I'd love to hear your thoughts.
Thank you for the quick reply. I'm available by phone if something isn't making sense.