Unconnected Impervious CN

StormShed3G™ applies an additional adjustment for unconnected impervious areas when computing a composite CN value. In the case where a CN of 98 or higher is included with the CN numbers for compositing, the program assumes that the areas with 98 or higher CN values are unconnected if the total impervious area is less than 30%. It then applies the following adjustment:

Where:
CNc = Composite runoff CN
CNp= Pervious runoff CN
Pimp= Percent imperviousness.

If the total imperviousness is greater than or equal to 30% the following equation is used: