Problem

How does the "Subarea Routing" option work with Percent Impervious and Percent Routed options, with the EPA-SWMM runoff method in catchments?

Solution

When using the EPA-SWMM runoff method, you have the option of routing runoff in three ways, by way of the "Subarea Routing" option:

1) "Both to Outlet" - runoff on both the impervious and pervious portions of the catchment route directly to the outlet (catchment outflow node) The "Percent Routed" is ignored in this case.

2) "Impervious to Pervious" - a portion of the runoff from the impervious area routes to the pervious area, and then to the catchment outlet. The portion is controlled by the "Percent Routed" option and applies only to the impervious area in this case. 

3) "Pervious to Impervious" - a portion of the runoff from the pervious area routes to the impervious area, and then to the catchment outlet.T he portion is controlled by the "Percent Routed" option.

You also have the option to direct the outlet of one catchment to another catchment, by way of the catchment outflow node field.

The "Percent Routed" applies only to the impervious area (with the Subarea Routing" method set to "Impervious to Pervious" ) or only to the pervious area (with the Subarea Routing" method set to "Pervious to Impervious". (it does not apply to the entire catchment area).

Consider an example catchment that is 30% impervious and 70% pervious, and 75% of the impervious area connected directly to the outlet. In this case, the "percent routed" would be 25%. 75% of the runoff from the impervious area would route directly to the outlet, and 25% would route onto the pervious area, then the pervious area runoff would route to the outlet.

Below is a diagram to explain visually:

Note : Percent impervious field is applicable to EPA-SWMM Runoff Method only,  hiding of the percent impervious field was implemented in a patch for version 10.03.04.53 and later. If you still see this field for any other runoff method in your software, then make sure you have the latest version installed. Downloading OpenFlows / Water Infrastructure Software 

See Also

Percent Connected Impervious Areas and Percent Unconnected Impervious Areas 

Low Impact Development (LID)