Curve number assignment

Curt Schoenfelder

In the SWMM 5 Curve Number infiltration method, how is the %impervious area of a subcatchment handled? Does SWMM assign a default Curve Number, of say 98 or 100 to the %impervious area? Thanks for any input.

Greg Kacvinsky

I believe that the % impervious variable is used to describe "directly connected impervious surfaces" that have no initial abstraction or infiltration losses. For instance, when using a CN of 80 to describe a residential development and using a 0% impervious area, the entire area has an initial abstraction, which means runoff does not occur anywhere until rainfall exceeds the initial abstraction. If you assign a % impervious variable to represent roadways that discharge directly to a sewer system, you're treating those areas as having no infiltration capacity, so you'll get an immediate runoff response from those areas (probably a more realistic way to build your model).

From our experience, using the % impervious variable has a much greater impact when analyzing the hydrologic response for smaller, more frequent storms (when the initial abstraction is a large percentage of the total rainfall).

Lew Rossman

I just wanted to clarify some things regarding this thread.

In SWMM, all impervious area is "directly connected" unless one uses the "Route To" option to have some fraction of its runoff be directed onto the pervious area of the subcatchment instead of flowing directly to the subcatchment outlet.

The Depression Storage parameter for the impervious area serves to model its initial abstraction, but a percentage of the area can assigned to have no initial abstraction by setting a value for the %Zero-Imperv parameter (e.g., if %Zero-Imperv is 20%, then 20% of the impervious area generates runoff immediately while the remaining 80% of the area begins to have runoff only after its depression storage depth gets filled).

The Curve Number assigned to a subcatchment is used only to compute infiltration from its pervious area. It plays no role in determining runoff from the impervious portion of the subcatchment.

Curt Schoenfelder

That is what I gathered after reading through the different segments of code looking for any related defaults, but I just wanted to make sure I wasn't missing something. I know in some modeling programs you can assign curve numbers to impervious areas, such as 98 to paved parking lots or roads. Thanks for the feedback.