Simulating design storms in a changing climate: a physics-based approach to urban flood risk assessment

Design storms are widely employed to evaluate flood risks in urban areas. These synthetic storms are not direct representations of real extreme rainfall events but are simplified simulations designed to mimic such extremes. Typically derived from rainfall intensity-duration-frequency (IDF) curves, design storms represent scenarios under specific extreme rainfall conditions. To adapt these storms for future climates, IDF curves must first be recalculated to reflect projected climatic changes. We present a framework for updating sub-daily to daily IDF curves and corresponding design storms based on projected changes in rainfall temperatures and rainfall occurrence at the daily scale; information that is readily available from global and regional climate models without the need to bias-correct or further downscale the climatic data. Our approach utilizes the TENAX (TEmperature-dependent Non-Asymptotic statistical model for eXtreme return levels) model, a novel physics-based statistical tool that estimates future return levels of short-duration rainfall. This enables the development of future rainfall intensity profiles and corresponding design storms using reconstructed IDF curves for the projected climate. We demonstrate this method by re-parameterizing the Chicago Design Storm (CDS) to account for climate change impacts, using Zurich (Switzerland) as a case study. Specifically, we calculate changes in the IDF curve for durations ranging from 10 minutes to 3 hours by applying the TENAX model to estimate future 100-year return levels. The resulting synthetic 100-year return period design storms are constructed for both present and future climates, allowing us to produce flood inundation maps and evaluate shifts in flood risk for the city under changing climatic conditions.