Sub-Area Routing in SWMM

In SWMM sub-Area Routing has three options: Outlet, Impervious and Pervious. The Outlet option ((a) in the figure below) routes the runoff from both sub-areas directly to the subcatchment’s outlet. The Pervious option ((b) in the figure below) routes the runoff from the impervious sub-area across the pervious sub-area and then to the outlet, and the Impervious option ((c) in the figure below) routes the runoff from the pervious sub-area across the impervious sub-area and then to the outlet.

When the runoff from the impervious surface is routed across the pervious surface ((b) in the figure below), some of the runoff is lost to infiltration and depression storage in the pervious sub-area. The graph below shows the typical effect on runoff these three sub-area routing methods have when applied to a single subcatchment. Note that because the runoff produced by the pervious area in this case is negligible, the “100% routed to Outlet” and the “100% routed to Impervious” cases are practically the same.

The Sub-Area Routing – Pervious option can be used to model LIDs. This is done by representing the LID as the pervious sub-area, setting the subcatchment’s pervious values to those of the LID, routing the runoff from the impervious sub-area of the subcatchment to the pervious subarea, and defining the Percent Routed to represent the percentage of impervious surface connected to the LID.

Modeling LIDs with this method implies that the entire subcatchment’s pervious surface represented in this tutorial because the slope and width of the LID must be those of the subcatchment, which is not always the case.