Assessing the influence of green stormwater infrastructure implemented for combined sewer overflow control on urban streamflows

Combined sewer overflows (CSOs) have been an increasing challenge as urbanization and climate change have induced more frequent overflows, impairing stream health. In response to this challenge, many cities, including Philadelphia, have implemented green stormwater infrastructure (GSI) to manage stormwater and reduce CSOs. While many studies have shown the benefits of GSI to improve runoff retention, water quality, and CSO releases, the influence of GSI on urban hydrographs in larger watersheds (e.g., in large cities such as Philadelphia) has been less clear. This study evaluated the influence of GSI installations on a highly urban stream in Philadelphia by examining its hydrological response to mild precipitation events (1.02–2.79 cm), the target of GSI design in Pennsylvania. Using two USGS stream gages on a stream in the Tookany/Tacony-Frankford Watershed (80 km2) in Philadelphia, peak flows for two 5-year periods before and after implementation of a GSI-focused program were identified and compared. The complex and highly dynamic nature of urban watersheds paired with the design constraints of GSI make defining their impact on hydrographs challenging. Therefore, delineating the goal of reducing CSOs through GSI using streamflow may not provide a comprehensive picture. Aggregating peaks indicated that the distributions of peak flow rates were not statistically significantly different between the two periods, but the exceedance probability of flood discharge decreased from 4.6 % in the before period to 1.7 % in the after period. The results demonstrate the complexity in assessing urban streams and their interactions with urban runoff and GSI, and the stochastic nature of what GSI receives (i.e. precipitation, meteorological conditions, prior GSI conditions).