The pervious slope, SLPP, is the average slope of the pervious areas. This is not the catchment slope from highest point to lowest point, but an average when considering only the pervious are-as. For example, if the catchment consists of a residential subdivision, this value would represent the average slope of the pervious lot surface. In this example the slope would not be less than 2% or whatever the municipal minimum is.
The overland flow length, LGP, should be set to the representative value for the pervious areas. It is not the length of the catchment from high point to low point. This value represents the average length over which flows from pervious areas would travel before being intercepted by channels, sewers, or roads. For example, in a residential subdivision this value might be the representative lot length which is typically 40 m.
The Manning’s roughness coefficient for pervious surfaces, MNP, should be selected based on sheet flow and not channel flow. This is a common mistake for modellers. Most listed values of Manning’s values are for channel flow, whereas the pervious runoff simulated is sheet flow. Therefore, if we assumed a grassed surface then the sheet flow Manning’s roughness coefficient would be approximately 0.25, whereas the channel roughness coefficient for the same material might be 0.025.
For an ungauged urban catchment, the pervious storage coefficient, SCP, should be set to 0, which will let the program determine the storage coefficient.