Evaluating the Impact from Stormwater Runoff from Proposed Developments in Southern California using PCSWMM

Proposed developments have recently been required to address post-construction or operational phase stormwater runoff with the proposed implementation of design, sourcecontrol, and treatment best management practices (BMPs). Under these circumstances, evaluating runoff water quality is critical for discharges to Clean Water Act (CWA) Section 303(d) listed waters, environmental impact assessments (e.g. Environmental Impact Report), or choosing cost effective BMPs. Southern California like many other developing regions has developed BMP and water quality evaluation requirements under county and municipal jurisdictions in order to meet federal and local requirements. Simplified methods for predicting an event mean concentration (EMC) for a constituent of concern have been available, for example, calculations based on sources of available EMCs and land use data, such as the Nationwide Urban Runoff Program (NURP). Often this information is outdated and not specific for regional situations. Reports produced in 1999 and 2000 for Los Angeles County offer a robust water quality data source collected for multiple storm seasons (depending on available constituents), and the City of San Diego has likewise produced reports of stormwater runoff data from various land uses for recent years. These recent and regional data sources offer new insight into predicting EMCs for constituents of concern in Southern California stormwater runoff. This not only allows for more reliable regional EMC data, but more information such as peak pollutant concentrations, associated rainfall, and runoff volumes, to develop a more comprehensive approach to predicting potential stormwater behavior for future developments and BMP performance.Over the past three years, PCSWMM 2000 and PCSWMM 2002 (PCSWMM) have been used to predict pollutant concentrations for proposed developments. PCSWMM utilizes the EPA distributed Stormwater Management Model (SWMM) and can be used to predict pollutant behavior based upon the pollutant build-up and wash-off concept. Other models such as the Hydrologic Simulation Program-Fortran (HSPF) utilize the buildup/wash-off method, but are not specifically designed to address stormwater collection system modeling. Although much more data intensive, SWMM modeling offers expansive results compared to a simplified procedure of calculating land use weighted EMCs. Build-up and wash-off rates are based on numerous physical influences, and require estimations or empirical results based on literature sources or attempts at calibration. HSPFParm, an EPA distributed software package and database, provided for HSPF users to help determine appropriate HSPF coefficients, is one source for initially estimating common coefficients. Data from the Los Angeles (LA) and San Diego (SD) reports offer information including, EMCs, land use, drainage area, precipitation, and runoff volume that can assist in model calibration for refining build-up and wash-off rates and other model input.Development projects in the San Diego area have been evaluated utilizing PCSWMM to predict EMCs and other runoff values in support of environmental impact reports and required water quality assessments. The proposed development projects have included residential, hotels, parking lots, schools, churches, stores, and restaurants. The approach for determining PCSWMM coefficients such as build-up rates were initially determined from literature sources, including SWMM guidance documents and HSPFParm. Coefficients were also estimated from information in the LA and SD monitoring reports as input. From this point, modeling coefficients were updated from time to time as each development project came along. These refinements were based on new knowledge of local runoff data, new literature sources, and professional judgment. This paper offers a current and regional source of SWMM modeling coefficients and EMC values and describes efforts for modeling various development projects in the San Diego area.