Design Storms Underestimate Flood Hazard and Risk Derived from Stochastic Storm Transposition

Flood hazard and risk data are essential to flood management and resilience efforts, includingdevelopment plans, construction requirements, and insurance mandates. However, many current hazardestimates are generated using design storms, which rely on simplified representations of extreme rainfalland assume a linear relationship between storm and flood frequency. We investigate whether thesesimplifications lead to underestimates of flood hazard and risk by comparing design storm-basedestimates against those derived from stochastic storm transposition (SST), which probabilisticallygenerates realistic rainfall fields. We model hazard and risk with each approach for a watershed in easternNorth Carolina and find that relative to SST, design storms underestimate flood frequency by an averageof 19 to 36 years and expected annual damage by 65 percent. Our findings demonstrate that probabilisticmethods and realistic rainfall fields are crucial to flood risk assessment and suggest that currentapproaches may not adequately prepare communities for flooding.