230 - Statistical Analysis on the Footprint of Green Stormwater Infrastructure on Urban Hydrographs

Combined sewer overflows (CSOs) have been a growing challenge as urbanization and climate change have induced more frequent overflows, impairing stream health. One increasingly prolific solution for this challenge has been implementing green stormwater infrastructure (GSI), and Philadelphia has been a leader in widescale GSI implementation for CSO control for over a decade.  While many studies have shown the benefits of GSI in improving runoff retention, water quality, and CSO releases at the site and small subdivision scale, the influence of GSI on hydrographs in urban catchments has been less clear. This study evaluated the influence of GSI installations on a highly urban stream in Philadelphia by examining hydrological response to mild precipitation events (1.02 – 2.79 cm), which is a common design target of GSI. Peak flows recorded by USGS stream gages for two 5-year periods, before and after widespread adoption of GSI, were compared. While results indicated that the distribution of peak flow rates did not show statistically significant differences, the exceedance probability of flood discharge declined from 5.4% in the before period to 1.5% in the after period. The results demonstrate the complexity in assessing the impact of GSI on urban runoff and streams, and the stochastic nature of what GSI receives. Consequently, these findings indicate the hydrograph features that reflect the efficacy of GSI on controlling hydrologic response in urban settings, while emphasizing the importance of reflecting temporal dynamics in GSI analysis.