HydroCAD Subcatchments

[Gstein]

Hi All,

Can anyone explain the best method to model a subcatchment? For example, on a small lot, should I model three separate subcatchments for the Roof Area, Paving Area, and Landscape Area, or should I model it all as one subcatchment with each of the areas defined within? (all are being routed to the same link). To me, it seems as though it should yield the same results, but I do not understand why I am getting different results for both examples. Any ideas?

 

[psmart]

Generally, if the subareas are interspersed and drain to a common point, they would be modeled as a single subcatchment. By default, HydroCAD uses an area-weighted CN value to determine the runoff, which will generally yield a different result that adding the separate runoff values for each of the CN values. You can do the later by using a different subcat for each CN value (as you mentioned), or you can select the Weighted-Q option in HydroCAD. For a discussion of the CN weighting options please see

http://www.hydrocad.net/curvenumber.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net