New Insights Into Global Flood Hazard and Risk Considering Levees Under a Changing Climate

Flooding is one of the most severe natural hazards worldwide, causing substantial economic losses and catastrophic impacts. Although levees are widely implemented for flood mitigation, few global flood models explicitly incorporate their influence on flood routing and risk assessment. In our previous study (Zhao et al., 2025, doi: https://doi.org/10.1029/2024WR039790), we developed a levee module for the CaMa-Flood model and generated levee parameters for global ungauged rivers. Furthermore, recent GPU-based computational optimizations in CaMa-Flood model (Kang et al., 2026, doi: 10.22541/essoar.176442648.85093032/v3) now enable simulations of global flood dynamics at high spatial resolution within a feasible timeframe.

These advancements allow us to analyze long-term changes in flood hazards and risks while explicitly accounting for levee effects. In this study, we forced the CaMa-Flood model with ensemble CMIP6 runoff data to simulate historical and future river hydrodynamic changes at the daily scale, comparing scenarios with and without levee considerations. Our results reveal that levees significantly mitigate global flood hazard. Driven by this protection, urbanization rates within levee-protected areas have substantially outpaced those in unprotected regions over the past four decades. Conversely, flood hazard in unprotected river reaches increases due to the hydrodynamic effects induced by levee construction. Regarding flood exposure, multi-model estimates show that while levees have historically played a crucial role in shielding a substantial fraction of the floodplain population and slowing the growth rate of flood exposure, rapid population growth has largely offset these protective benefits. Consequently, the absolute flood-affected population under levee protection in the present day remains at a historically high level, comparable to that of previous decades without levees. Furthermore, projections indicate that flood exposure will continue to increase through the mid-21st century under changing climate conditions. Overall, this work provides new insights into global flood modeling and risk assessment and supports improved flood management and decision-making in levee-protected regions.