How does the "Percent Connected Impervious Areas" and "Percent Unconnected Impervious Areas" work in catchments?
"Percent Connected Impervious Areas" and "Percent Unconnected Impervious Areas" are ways of representing impervious areas that are a part of another catchment. For example, there may be a grassy area and a driveway in the same catchment. One way to represent this would be to create a composite CN and area, where you represent the areas similar to the following screenshot:
This method will create a composite hydrograph. However, there may be cases where you want to determine exactly how the flow is directed between the impervious and other areas. That is where the Percent Connected Impervious Areas and Percent Unconnected Impervious Areas come in.
As seen in the screenshot above, you can enter the percent of the area for the catchment that is considered impervious. You have the option of considering this to be connected impervious areas or unconnected impervious areas (or both). Below is a description of the two options.
If you enter 0% for the these fields, there is no adjustment to the CN value. The sum of the Percent Connected Impervious Area and Percent Unconnected Impervious Area cannot exceed 30%. The reason for this is because, according to the U.S. Soil Conservation Service, the absorptive capacity of the remaining pervious areas is not significant enough to affect runoff. The flows from these areas would be more closely resemble impervious connected area flows.
Note that if you are using a CN value that already accounts for impervious adjustments (such as the CN values listed for commercial and residential districts in Table 2-2a, TR-55), then do NOT enter in % connected and/or % unconnected, otherwise you will be doubling the adjustment for impervious areas.
Mulitple CN numbers for a catchment and multiple catchments with a single outlet node