Using spatially-identified effective impervious area to target green infrastructure retrofits: A modeling study in Knoxville, TN

There is a need for enhanced guidance in siting distributed, infiltrative green infrastructure (GI) practices, especially in densely developed urban watersheds where retrofits come at a high cost. To maximize the hydrologic benefit of GI practices on urban streams, the disconnection of effective impervious areas (EIA), or those impervious areas hydraulically connected to the stormwater network, has been identified as a strategic management approach that is expected to have the greatest impact. The overall effect of full disconnection of spatially-identified EIA on watershed hydrology is uncertain because this type of full disconnection is rarely brought to full-scale implementation. In this study, spatial EIA identification is used to parametrize an urban runoff model using the United States Environmental Protection Agency's Storm Water Management Model (SWMM). The calibrated model is used to assess runoff reductions resulting from GI practices distributed through the watershed via different placement strategies, both spatially-informed and not. Full treatment of the spatially identified EIA using bioretention cells was compared to two scenarios treating the same area of impervious surfaces, but with random placement either among all impervious areas or placement focused in areas of higher imperviousness. Model results indicate that substantially higher runoff reduction could be realized by targeting EIA, with a median runoff reduction of nearly 30% more than other treatment scenarios across storm events ranging from 1.27 to 20.7 mm using this strategic siting. Further improvements in optimizing distributed infiltrative GI practice placement are needed and targeting of spatially-identified EIA appears to be a viable method for increasing the hydrologic improvements realized through watershed scale implementations.