LID Placement for Controls in InfoSWMM and SWMM5 w/ InfoSWMM Sustain

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

• place one or more controls in an existing Subcatchment that will displace an equal amount of non-LID area from the Subcatchment
• 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 sub catchment’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 sub catchment’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and Width properties may require adjustment to compensate for the amount of original Subcatchment area that has now been replaced by LIDs (see the figure below). For example, suppose that a Subcatchment which is 40% impervious has 75% of that area converted to a porous pavement LID. After the LID is added the sub catchment’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 %.

The second approach allows LID controls to be strung along in series and also allows runoff from several different upstream sub catchments to be routed onto the LID Subcatchment. If these single-LID sub catchments are carved out of existing sub catchments, then once again some adjustment 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 sub catchment’s standard surface properties (such as imperviousness, slope, roughness, etc.) are overridden by those that pertain to the LID unit.