A modeling framework for assessing compound impacts of storm surges and riverine floods on inundation in ungauged coastal catchments of Central Vietnam

The simultaneous occurrence of river flooding and storm surges can potentially exacerbate flood risks compared to individual occurrences. This study investigates compound flooding from severe typhoons in a data-scarce coastal river catchment using an integrated numerical modeling framework (rainfall-runoff, 1D/2D hydrodynamic, and storm surge model) combined with satellite precipitation and imagery. Our results reveal that compounding impacts increase inundation in low-lying areas, with a 4.67% rise in flooded extent and an 11.63% increase in maximum water depth. Potential hazard maps under a hypothetical super typhoon scenario indicate even greater risks, with up to 30% more inundated area and 50% higher maximum water depth than the most extreme historical event. The paper highlights two key points: (i) riverine flooding-storm surge interactions must be considered to avoid underestimating risks; and (ii) the developed framework offers a pratical approach for other river catchments facing similar challenges due to limited hydrological records.