Manhattan Pluvial Flood Simulations Under Two Extreme Rainfall Scenarios Show Similar Severity Patterns but Contrasting Peak Timing Across Districts

Urban pluvial flooding is often summarized by peak depth, peak extent, or cumulative burden, but these metrics do not distinguish when flooding first becomes hazardous from when the hazardous area becomes largest. This study introduces an activation–occupancy framework to quantify these two times and applies it to building‐resolving shallow‐water simulations of Manhattan ( at m resolution) under an Ida‐derived hyetograph and New York City's 1‐hr design storm. Under the Ida‐derived rainfall at the 0.3‐m threshold, district activation peaks occur within a 35‐min span, whereas the times of maximum hazard extent are much more dispersed. Under the shorter design storm, peak timing compresses sharply. Rainfall is spatially uniform in both simulations, so the 35‐min activation spread reflects routing alone and provides a lower bound on the variability of activation timing; spatial rainfall heterogeneity lies outside the modeled scenarios. Across the two modeled rainfall scenarios, severe‐flooding geography remains broadly consistent: the same seven districts exceed the borough reference at 0.2, 0.3, and 0.4 m, with the strongest rank preservation for eventual extent and hazardous area–time burden. At 0.5 m, that set contracts to six because Lower East Side/Chinatown falls just below the borough reference in both scenarios. East Harlem remains the principal exception in design‐storm timing at the deeper thresholds. These diagnostics distinguish severity patterns relevant to adaptation from scenario‐specific timing information for warning and response.