Conclustions: % Impervious vs. Composite CN

[tsoukup]

After a discussion with StormNET tech support and research from various SWMM expert users, the following information is a compilation of information gained. It may or may not be accurate so use at your discretion and please comment on anything that is inaccurate. This post applies to the EPA SWMM hydrology method and SCS Curve Number infiltration within StormNET.

In summary: Use the %impervious input on the physical properties tab. On the CN tab, select a CN for the pervious areas only. If the pervious areas within the subbasin exhibit different parameters (ie.. diff soil types or diff veg) then a composite CN for the different pervious conditions can be calculated. Impervious areas should be accounted for on the physical properties tab not in the CN tab.

Summary based on other user forum responses:

"Based on my re-read of the StormNET & SWMM5 manuals (no luck) and the source code, the answer I have now is the CN should be for the pervious land only. Below is a section of SWMM source code from "subcatch.c," and within the function of "getSubareaRunoff()."

// --- compute infiltration loss rate for pervious subarea // (inflow consists of runon + precip - evap) infil = 0.0;
if ( i == PERV )
{
if ( Subcatch[j].infil == j )
{
infil = infil_getInfil(j, InfilModel, tStep, (subarea->inflow - surfEvap), subarea->depth); }
if ( infil > availMoisture - surfEvap )
{
infil = MAX(0.0, availMoisture - surfEvap); }

So as the code shows, the infiltration calculation is carried out to pervious area only (the CN is used later in the function of "infil_getInfil()"). Thus, the CN should be for the pervious land only instead of a composite CN for the whole subcatchment.

Unfortunately this is not highlighted either in the manual or the annotation of the CN input window."

"By putting a composite CN and listing a %imperviousness it would double count the effects of imperviousness."

"You will miss the power of SWMM if you use a composite CN and no % impervious by not getting a much more realistic dynamic response of the runoff process that occurs in urban areas, where impervious areas govern the peaks and, often, volumes."