Curve Numbers vs % Impervious

Umair Numan

When using Curve number infiltration method, should % impervious be set to 0 and a composite CN value be used? Furthermore, if modelling a subcatchment consisting entirely of asphalt, would in be appropriate to set the % imp to 0 and set the N perv to 0.011 (for asphalt) and the CN to 98?


Robert Dickinson

CN infiltration is only used for pervious surfaces - it does not apply to impervious surfaces. I would check out the swMM5 reference manual for Hydrology - there are some excellent tips on how to use the CN for Infiltration.

This makes sense to me assuming your asphalt has some infiltration in the cracks- Furthermore, if modelling a subcatchment consisting entirely of asphalt, would in be appropriate to set the % imp to 0 and set the N perv to 0.011 (for asphalt) and the CN to 98?