Assessing the effect of Best Management Practices on water quality and flow regime in an urban watershed under climate change disturbance

Urban streams and water bodies have become increasingly polluted due to stormwater runoff from increased urbanization. Improved water quality and reduced flood peaks are the ultimate goals of stormwater management to achieve safe and healthy urban water bodies, with additional benefits of increased green space and increased domestic water supply through potential recycling and groundwater recharge. In this research, Low Impact Development (LID) and Best Management Practices (BMPs) are assessed as natural methods to manage stormwater by applying the EPA System for Urban Stormwater Treatment and Analysis INtegration (SUSTAIN) model. Ballona Creek watershed in the Los Angeles basin (128 square miles with 61% impervious land cover) was chosen as a case study area to more specifically investigate the mechanisms through which different BMP types achieve compliance with water quality regulations, reduce peak flows, and encourage recharge through infiltration. This research illustrates how the characteristics of distinctive BMP types influence compliance and flow regimes. Model results show that infiltration-dominated BMPs reduced the total pollutant load at the outlet, but residual pollutants were more concentrated resulting in worse compliance with water quality standards. However, out of 86,000 acre-feet per year (AFY) of runoff from the whole watershed during the modeled period of 2004-2008, these BMP types infiltrated 66,000 AFY of water (76% of the total) for potential reuse and groundwater recharge, and reduced peak flows of larger storm events up to 60%. Treat and release-dominated BMPs resulted in lower pollutant concentrations and better compliance at the outlet, but higher pollutant loads were observed and only 34,000 AFY was infiltrated (40% of the total), with minimal peak flow reduction. Assessing future changes in precipitation and temperature due to climate variability further illustrated the beneficial and limiting characteristics of the five BMP types. Due to their poor peak flow reduction and infiltration capacity, treat and release BMPs would not provide as much benefit for future climate scenarios in which more intense precipitation events might occur. Stormwater modeling at the watershed scale can ultimately inform strategic BMP selection based on current and future hydrologic characteristics and desired outcomes.