Philadelphia and the Schuylkill under extreme hydrometeorological events

The Schuylkill River, a lifeline for Philadelphia, faces intensifying stress from urbanization and increasingly severe extreme hydrometeorological events (EHMEs) driven by climate change. Understanding how urban estuarine rivers respond to EHMEs remains challenging due to limited high-resolution data and the complexness of human-modified landscapes. Here, we combine long-term hydrological records, a 1-m resolution urban landscape model, remote sensing, citizen-generated data, and advanced hydrodynamic simulations to examine the Schuylkill River's response to EHMEs, focusing on Hurricane Ida's unprecedented flood on September 1, 2021. Ida triggered the river's highest-ever recorded flow discharge of 3,367.7 m³/s—nearly 100 times its average flow. This unique dataset enabled us to build a comprehensive flood model, capturing the dynamics of urban flooding and its impacts on Philadelphia's population at the street level. Our analysis reveals that past hydrological conditions and high-resolution urban terrain models are essential for accurately resolving water pathways and identifying the most vulnerable populations during EHMEs. Furthermore, we discovered that extreme discharges in the Schuylkill have intensified over the past century, underscoring escalating flood risks for Philadelphia's residents. Our numerical experiments reveal that extremer flow discharges added to high tide conditions, working as a "downstream water gate", can create significant expansions of the flooded area, penetrating through Philadelphia's most densely populated neighborhood. These findings highlight the urgent need for integrated research on EHMEs in urban estuaries worldwide to enhance flood preparedness and resilience.