Composite CN versus Percent Impervious - SCS Method HEC-HMS

[KatieTX]

Using the SCS CN Method in HEC-HMS:
Are both of these correct to model a developed area?
1 - compute the CN of the basin in the undeveloped state and input this into the 'CN' field. Compute the percent impervious in the developed state and enter this in the 'percent impervious' field. Run the model.

2 - compute the composite CN of the basin in the developed state by taking into account the percent of land that is undisturbed and the percent of land that is impervious due to development. Enter this CN in the CN field and 0 in the percent impervious field.

Example:
Original (undeveloped) basin - CN = 30
Developed basin has 14% impervious area. The 86% remaining is still in the undeveloped state (CN = 30).


Method 1 - CN = 30, percent impervious = 14%.
Method 2 - CN = (30*.86) + (98*.14) = 39.52; percent impervious = 0.

Which method is more correct for developed basins, or are both equally correct?

 

[Drew08]

Method 1 generally assumes the impervious area is directly connected to the outlet...there is no opportunity for infiltration of runoff from the impervious surface.

Method 2 generally assumes the impervious areas discharges to non-impervious areas such as retention areas or other BMPs where there is the opportunity for infiltration.

Either way, very few permitting agencies allow using a composite CN below 50. 30 is very extreme, and unless you’re in an extreme situation and can validate its use, you may want to consider using the fair or poor condition of the cover type, especially if it is intermixed with development.

 

[KatieTX]

Thanks Drew. I ran a simple HMS model using the two methods, and method 1 was less conservative. I am reviewing a drainage report where they used Method 1. From your explanation, it makes sense why it would be less conservative.

Also, the 30 was just an example. I thought of that when I wrote it but forged on ahead anyway ;-)