Improved flood hazard mapping through depth-velocity analysis: evidence from the Roon and Ly Hoa river basins, Vietnam

Accurate flood hazard mapping is essential for risk-informed planning, especially in regions prone to deep inundation and high-velocity flows. In Vietnam, most hazard maps rely on flood depth, potentially underestimating risks in areas influenced by rapid hydrodynamic conditions. This study proposes an integrated flood hazard assessment using flood depth and flow velocity, applied to the two river basins in Quang Tri province during the extreme flood event of October 2020. Hydrological and hydraulic simulations were conducted using the MIKE+ modelling suite, with model calibration and validation supported by flood marks, gauged water levels, and Sentinel-1 satellite imagery. The combined outputs were classified into six hazard levels (H1–H6) following the Australian hazard rating curve. Results show that velocity-inclusive hazard maps capture a significantly larger and more severe risk area than depth-only maps, particularly along river channels and urbanised floodplains. Approximately 60% of the Roon basin and 80% of the Ly Hoa basin fall within H3–H4 zones, unsafe for most individuals and vehicles. Moreover, the analysis identifies buildings in H5 zones, requiring structural reinforcement or relocation. This integrated approach enhances the realism and utility of flood hazard maps, especially in data-scarce contexts like central Vietnam.