LID Placement

There are two different approaches for placing LID controls within a subcatchment:

1.     Place one or more controls in an existing subcatchment that will displace an equal amount of non-LID area from the subcatchment

2.     Create a new subcatchment devoted entirely to just a single LID practice

 The first approach allows a mix of LIDs to be placed into a subcatchment, each treating a different portion of the runoff generated from the non-LID fraction of the subcatchment. Note that under this option the subcatchment's LIDs act in parallel -- it is not possible to make them act in series (i.e., have the outflow from one LID control become the inflow to another LID). Also, after LID placement the subcatchment's Percent Impervious and Width properties may require adjustment to compensate for the amount of original subcatchment area that has now been replaced by LIDs (Fig 53). For example, suppose that a subcatchment that is 40% impervious has 75% of that area converted to a porous pavement LID. After the LID is added the subcatchment's percent imperviousness should be changed to the percent of impervious area remaining divided by the percent of non-LID area remaining. This works out to (1 - 0.75)*40 / (100 - 0.75*40) or 14.3 %.

Fig. 53: Adjustment of Subcatchment parameters after LID replacement (Source: User Manual for EPA-SWMM 5.1)

Under this first approach, the runoff available for capture by the subcatchment’s LIDs is the runoff generated from its impervious area. If the option to re-route some fraction of this runoff to the pervious area is exercised, then only the remaining impervious runoff (if any) will be available for LID treatment. Also, note that Green Roofs and Roof Disconnection only treat the precipitation that fall directly on them and does not capture runoff from other impervious areas in their subcatchment.

The second approach allows LID controls to be strung along in series and also allows runoff from several different upstream subcatchments to be routed onto the LID subcatchment. If these sungle-LID subcatchments are carved out of existing subcatchments, then once again some agjustment of the Percent Impervious, Width, and also the Area properties of the latter may be necessary. In addition, whenever an LID occupies the entire subcatchment the values assigned to the subcatchment’s standard surface properties (such as imperviousness, slope, roughness, etc.) are overridden by those that pertain to the LID unit.

Normally noth surface and drain outflows from LID units are routed to the same outlet location assigned to the parent subcatchment. However, one can choose to return all LID outflow to the pervious area of the parent subcatchment and/or route the drain outflow to a separate designated outlet. (When both of these options are chosen, only the surface outflow is returned to the pervious sub-area.)