Climate Extremes at the City–River Interface: Insights from the Philadelphia-Schuylkill System

Hurricane Ida struck the U.S. East Coast in August 2021, pushing the Schuylkill River in Philadelphia to a record discharge nearly 100 times larger than its average flow. As one of the most severe disasters of the 21st century, Ida exemplifies the increasing frequency and intensity of extreme hydrometeorological events under climate change. Predicting urban flood pathways remains challenging due to the complex interplay of rainfall-runoff and river–tide–landscape interactions. To address this, we developed a high-resolution (street-resolved) flood model integrating LiDAR terrain data, bathymetric surveys, and land use-based surface friction across Philadelphia. We find that impervious surfaces and fragmented infrastructure exacerbate pluvial flooding and localized waterlogging, increasing exposure for both low- and high-income communities. Scenario-based simulations reveal a tipping point: a logarithmic increase in inundation areas for return periods above 100 years, and 2-7% additional flooding when peak discharge coincides with high tide—rising to up to 15% under projected sea level rise by 2100. Our findings underscore the compounding impacts of climate extremes in urban river systems and the need for integrated forecasting and adaptive planning—particularly in vulnerable, low-lying, and rapidly urbanizing regions worldwide.