Assessment of future flood inundation under the impacts of climate change and human activities in Central Vietnam

This study evaluates the combined impacts of climate change and reservoir operations on flood dynamics in the Vu Gia Thu Bon River Basin, one of Vietnam’s most flood-prone regions. The analysis employs an integrated modeling chain, coupling a hydrological model and a hydrodynamic model to simulate streamflow and flood inundation across various scenarios. The hydrological model is forced by downscaled climate projections from 19 CMIP6 GCMs under SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios. The analysis focuses specifically on the evolution of flood events associated with a 20-year return period at different Global Warming Levels (GWLs). Climate projections show a significant increase in extreme rainfall by the end of the century, albeit with a large inter-model spread. Hydrological simulations indicate that the current cascade of reservoirs provides a substantial buffering capacity for flood events with a 20-year return period, still reducing peak discharge and flood extent even under 3°C GWL compared to the historical pre-dam period. However, at higher GWL, intensified hydrological extremes and increased downstream rainfall may eventually overwhelm current water management infrastructures. The results underscore the necessity of moving beyond "stationary" flood design assumptions, as the probability of extreme rainfall continues to rise.