On most mornings, the Schuylkill River Trail is where many Philadelphians step out for a breath of fresh air. Runners check their splits, cyclists zip below bridges, and dog walkers clutch leashes and coffee.

But after the remnants of Hurricane Ida and seven tornadoes struck Philadelphia in early September of 2021, those routines were put on pause as parts of the trail disappeared beneath a sea of sludgelike brown water. The Vine Street Expressway—running between the Schuylkill and Delaware rivers—was closed as water flooded the roadway, nearly reaching the overpasses.

“We learned that Ida was not an outlier so much as it was a stress test,” says co-author Leandro Pongeluppe, an economist. “The systems meant to hold the line are now being asked to perform under conditions they were never designed to handle.”

What makes this an unusual story, Pongeluppe notes, is that it is the poorest and wealthiest Philadelphians most at risk of exposure to flooding. For high-income residents, exposure is driven by extensive impervious surfaces and the ground sinking under the weight of dense infrastructure. For low-income communities, the risk is a product of high housing burden—financial strain of spending a disproportionate share of their income on housing—insufficient flood preparedness, and systemic neglect of environmental protections.

Analyzing ZIP code-level loan data, Pongeluppe also found that approved federal disaster loans did not come close to matching the scale of the damage.

Looking ahead, the team is securing funding for building an AI-powered “digital twin” of their Philadelphia model. Trained on their physical model, this twin would be able to process shifting storm forecasts and calculate street-level flood predictions in a matter of minutes rather than days.