Abstract
Urban stormwater management traditionally utilizes pipes, gutters, and other concrete features to collect and manage runoff. Increased imperviousness resulting from urbanization can cause undesirable effects such as higher peak flows during storm events, reduced groundwater recharge, and increased pollutant discharge to receiving waters. Newer stormwater management approaches, however, focus on distributed facilities to retain and infiltrate stormwater throughout urban catchments. Low-Impact Development (LID) aims to mimic natural systems and minimize negative ecosystem impacts while still providing adequate flood protection. For this project, a hydrologic model was developed using EPA’s Stormwater Management Model (SWMM5) for the Davis Manor neighborhood located in Davis, California. The goal of this research is to develop a planning model that quantifies runoff volume reduction and stormwater treatment from various LID retrofit project alternatives. The scenarios modeled in this project include varying densities (high, medium, and low) of street-based rain gardens, which are uniformly distributed throughout the neighborhood. In addition, the effects of capturing various percentages of runoff in infiltration-type Best Management Practices (BMPs) located in the front yards of private residences were analyzed. Combination scenarios using selective placement of street-based rain gardens in conjunction with rooftop disconnects were also modeled. Depending on the density of rain garden deployment, runoff volume reductions for LID installation in streets alone ranged from fifteen percent (15%) to forty-two percent (42%) of baseline values, while reductions for rooftop disconnects ranged from eight percent (8%) to twenty-one percent (21%). The highest values listed for these scenarios may represent practical upper limits to what can be achieved with these strategies. Higher density placement of street-based rain gardens would require significant removal of parking. Capturing all flows associated with rooftop runoff is unrealistic because of limitations associated with adequate spacing from building foundations and streets in front yards. The combination option which included the lowest density of street BMPs in select subcatchments and eighty-seven and a half percent (87.5%) capture of rooftop runoff resulted in a runoff volume reduction of thirty-eight percent (38%). Because historical runoff data for the Davis Manor community are not available, this model could not be calibrated against observed runoff values. While the runoff volume reductions presented in this case study are not verified, they can be used to compare the relative effects of potential project scenarios.