Growing Pluvial Flood Hazard and Exposure From Heatwave‐Preconditioned Rainfall Extremes in Chennai Metropolitan Area, India

Abstract Sequential compound hazards—heatwaves followed by extreme rainfall—can cause greater impacts than their isolated occurrences, yet compounding effects of such hazards on urban flood risk remain unexplored. Using 40‐year (1982–2021) hourly rainfall observations in the Chennai Metropolitan Area, we show that the frequency of heatwave‐preconditioned rainfall extremes has increased from an average of 1.2 events/year in 1982–1991 to 3.2 events/year in 2012–2021. The tail of the probability distribution of heatwave‐preconditioned rainfall intensity decays slower than uncompounded events. Using a state‐of‐the‐art hydrodynamic model and joint exceedance probability of accumulated event rainfall and peak rainfall intensity, we find that the spatial extent of high‐hazard zones (1.4–3.5 m inundation depth) is up to ∼6 times larger compared to uncompounded events, while building and population exposure increase by 7 and 11 times, respectively. Flood risk reduction and climate adaptation should consider more frequent heatwave‐preconditioned rainfall extremes and their higher flood impacts.