Global Mapping of Coastal Compound Flood Risk at 11 km Resolution

Compound floods, driven by the concurrence of river overflow and anomalous sea levels, are becoming more likely as climate extremes grow more unpredictable, yet many flood risk assessments fail to capture interactions between floods with different origins. Here, we quantify Coastal Compound Flood Risk (CCFR) at 0.1° resolution (∼11 km at the equator) by integrating flood hazard, population exposure, and empirical vulnerability, and further aggregate results at the sub-national level. We introduce a new hazard metric—Coastal Compound Flood Hazard (CCFH)—which captures combined riverine and oceanic flood volumes across multiple return periods. We further derive an effective vulnerability function based on the ratio between observed exposure and the fitted upper envelope of the GDP–CCFH relationship, reflecting the realized share of potential flood impact. We find that ignoring compound flood dynamics leads to substantial underestimation of exposure. Under the effective vulnerability scenario—compared to risk-averse and risk-taking assumptions—Asia accounts for 35.22% of global high-risk areas, followed by Africa (20.21%), Europe (17.02%), South America (9.89%), and North America (2.31%), with river deltas and low-lying coasts emerging as global hotspots. Our proposed CCFR captures systemic flood risk and reduces uncertainty by incorporating extreme flood events and data-driven vulnerability, which provides a stronger foundation for prioritizing investments in flood resilience.