Regional Differences in the Effectiveness of Low-Impact-Development Facilities
Understanding the mechanisms of LID
Low-impact-development (LID) facilities such as green roofs, bioretention swales, rainwater cisterns, rain gardens, impervious runoff dispersion, and permeable pavement can be used to reduce stormwater runoff.
LIDs use two mechanisms to reduce stormwater runoff: infiltration and evapotranspiration. LIDs slow the movement of runoff as it travels toward a stormwater conveyance system for collection and removal. This gives the native soil more opportunity to infiltrate some or all of the runoff. At the same time the opportunity for additional evaporation and transpiration (evapotranspiration) is increased.
Modeled LID Facilities
Six LIDs were modeled with WWHM4:
The WWHM4 model results are highly influenced by the assumed native soil characteristics. These characteristics (described above) control how much runoff is infiltrated into the native soil and removed from the stormwater conveyance system. The only LID where native soil characteristics are not important is the green roof, where infiltration into the native soil is not possible.
Permeable pavement offers the greatest annual runoff volume reductions of the six LIDs modeled because there is sufficient runoff storage in the gravel subgrade layer to allow for long-term slow infiltration into the underlying native soil. If infiltration into the native soil is not allowed, then the effectiveness of the permeable pavement is greatly reduced.
The WWHM4 model results are highly influenced by the assumed native soil characteristics. These characteristics control how much runoff is infiltrated into the native soil and removed from the stormwater conveyance system. The only LID where native soil characteristics are not important is the green roof, where infiltration into the native soil is not possible.
Impervious area disconnection appears to be the least effective LID of the six modeled LIDs in reducing annual runoff volume. The effectiveness of impervious area disconnection is directly dependent on the native soil.
Different LIDs perform differently in different climates. Green roofs, which rely on evapotranspiration, provide greater stormwater volume reduction benefits in climates where the potential evapotranspiration is relatively high compared to the precipitation.
Author's Bio: Douglas Beyerlein, P.E., is a co-founder and principal engineer with Clear Creek Solutions in Mill Creek, WA.