Small Urban Watersheds
Small Urban Watersheds
(OP)
Does HydroCAD have any lower limtis on the watershed size? For urban watersheds less than 5 acres, I have been told the SCS TR-20 Method may not be reliable and a rational method should be used. I do not see any hints or errors when running small watersheds.





RE: Small Urban Watersheds
Some localities specify the Rational method for small waterersheds, while others specify the SCS/NRCS method regardless of size.
I generally recommend SCS over Rational whenever it is accepted by local regulations, partly because it provides a complete runoff hydrograph that is required for volume-sensitive calculations, such as detention ponds.
For details please see www.hydrocad.net/rational.htm
Peter Smart
HydroCAD Software
www.hydrocad.net