Local Damages Drive Network-Wide Costs in Compound Flood Prone Coastal Cities

Concurrency of extreme rainfall, river surges, and elevated tides can produce compound floods that single- or multi-hazard summation models frequently misrepresent—particularly in the Global South, where rapid urban expansion intensifies infrastructure stress. Here we show that, although summation-based approaches and a fully coupled hydrodynamic model yield comparable direct (infrastructure) damage estimates, indirect (user) costs can diverge by up to 40%. By integrating compound flood modeling with microscale traffic simulations in Kozhikode, India, we find that neighborhoods with minimal inundation can nonetheless face severe gridlock and up to tenfold increases in vehicular emissions, underscoring how local flooding reverberates throughout the city. This discrepancy in indirect costs risks substantial misallocation of limited resources and underscores the broader socio-economic burdens often overlooked by conventional frameworks. By capturing these correlated hazards and real-time road closures, we highlight the need for multihazard flood governance that effectively addresses both localized inundation and network-wide disruptions in rapidly urbanizing coastal cities.