Sea level rise amplifies fluvial flooding in coastal megacities: a multi-scenario analysis

Climate change will contribute to sea level rise (SLR), impacting coastal land use, groundwater salinity, and coastal flooding. Previous studies have analyzed the direct impact of SLR on flooding by considering coastal subsidence and the enhancement of high tide events; however, it also impacts river hydrodynamics, further worsening fluvial flooding. Here, we analyzed how SLR could enhance fluvial flooding via backwater effects and impact coastal megacities under various climate and SLR scenarios using a global hydrodynamic model. We found that the future mean inundation area for these cities is projected to increase by up to 11.2 ± 9.0% under the warmest scenario. Similarly, the projected increase in mean flood exposure is by up to 11.1 ± 9.1% at the end of the century. Cities with high increases in flood exposure are located less than 100 km from the coast, while some cities located far inland could be impacted. Climate tipping scenarios show an even greater impact. More than three-fourths of examined cities were affected due to the backwater effect of SLR in the warmest scenario, with a minimum of 141% larger inundated area compared to simple coastal subsidence, suggesting consideration of SLR’s effect on fluvial flooding is indispensable for future flood risk assessment.