Possibilistic Flood Exposure: A “Worst Credible Case” Inundates Houston, TX

This study has shown how physically possible but statistically rare flood events—the “worst credible cases”—can expose deeper racial, infrastructural, and insurance disparities than conventional probabilistic planning often reveals. Current flood risk planning in the United States relies heavily on probabilistic modeling, which can underestimate the severe consequences of rare rainfall events that exceed statistical expectations. This constraint can obscure the potential for catastrophic inundation in densely populated areas and place extreme but plausible events outside the realm of responsible planning. Using Houston, Texas, as a case study, we examined how a “worst credible case” rainfall scenario would affect two urban watersheds within the same flood control district but with very different histories of development, drainage infrastructure, and flood protection. The study used an integrated possibilistic flood modeling framework that combined high-resolution hydrologic simulations, block group-level sociodemographic data, and National Flood Insurance Program coverage rates. Results showed that extreme but plausible rainfall could dramatically increase flood depths and amplify racial disparities compared with historical flood events. The analysis also revealed the systemic vulnerability of older conveyance-only drainage networks and identified a structural flood insurance gap shaped by watershed-level histories of infrastructural neglect and exclusionary development. These findings suggest that responsible flood planning should account not only for what is statistically likely, but also for what is physically possible and socially consequential.