Coastal Flood Risk Assessment for Global Coastal Cities: Integrating Land Subsidence, Climate Change and Urban Expansions

In the context of climate change, the combined effects of coastal land subsidence and sea level rise exacerbate coastal flood risks by altering relative sea levels. This study leverages high-resolution land subsidence rate data obtained from Interferometric Synthetic Aperture Radar (InSAR) and employs the LISFLOOD-FP two-dimensional hydrodynamic model to simulate coastal flooding for 43 coastal mega-cities globally. Our findings indicate that, when considering subsidence, over 76% of these cities experience an expansion in inundation areas under both Baseline and SSP5-8.5 scenarios. Furthermore, we conduct a quantitative assessment of the relative contributions of land subsidence and climate change to coastal flood inundation, identifying 19 cities where land subsidence plays a dominant role. Moreover, the impact of urban expansion on coastal flood risk cannot be underestimated, particularly in coastal cities that experience rapid urbanization and extensive coastal reclamation activities. By incorporating annual data on the expansion of settlements, reclaimed coastal areas, and urban built-up areas, we evaluate the dynamic changes in coastal flood exposure and uncover a long-term trend of increasing potential impacts of coastal flooding in mainland China's coastal regions, which is at a continental scale. Specifically, the area of settlements located in coastal flood hazard zones has grown to 6.5 times its original size, while the area of reclaimed land within these zones has expanded to 26.3 times its original extent. The insights from this study provide a valuable reference for sustainable development strategies and measures to address the escalating coastal flood hazards in coastal cities worldwide.