Flood Risk Assessment Using Coupled 1D–2D Hydrodynamic Models: A Case Study of the Qujiang River Basin

Frequent flooding and potential levee breaches pose severe threats to life safety and economic development in the Qujiang River Basin, highlighting the need for integrated risk assessments to improve flood management strategies. This study developed a flood risk assessment framework that combines hydrological design, 1D/2D hydrodynamic models and flood impact analysis. Design flood hydrographs for 10-, 20-, 50-, and 100-year return periods were generated using the instantaneous unit hydrograph method, and breach scenarios were incorporated to evaluate extreme failure conditions. The results indicate that inundation extent, depth, and duration increase significantly with return period, with the 100-year flood producing a maximum depth of 10.04 m and an inundation duration of up to 70 h. Levee breach simulations reveal that the Lingshangang breach results in rapid but short inundation, whereas the Qujiang breach results in prolonged deep flooding depths, posing severe risks to critical infrastructure and densely populated areas. Socio-economic impact assessments demonstrate substantial losses under extreme flood scenarios. These findings provide valuable insights for targeted flood risk mitigation, emergency evacuation planning, and resilient land use management in vulnerable river basins.