Key Factors Influencing Flood Severity and Mapping of Flood Susceptibility in Northern Vietnam and Its Red River Delta From Landfalling Tropical Cyclones

Abstract

Flooding induced by landfalling tropical cyclones (TCs) poses significant hydrological and socio-economic challenges in Northern Vietnam, a region characterised by complex catchment topography, varying degrees of urbanisation, and frequent exposure to hydro-meteorological hazards. This region spans the Red River Delta (RRD), with its urban centres including Ha Noi, the capital of Vietnam. Using data covering 70 TC events from 1979 to 2023, we investigated interactions between TC characteristics, catchment properties, and observed flood responses, and further developed a systematic methodology for flood susceptibility mapping that incorporates aspects of flood severity covering affected area, duration, and magnitude. The severity metrics are shown to correlate with TC properties (particularly average daily precipitation and the proportion of area with total precipitation > 50 mm) and catchment-specific hydrological characteristics (particularly elevation, runoff coefficient, and antecedent soil saturation). These characteristics are merged via the analytic hierarchy process (AHP) combined with fuzzy comprehensive evaluation, to generate flood susceptibility maps that were benchmarked against provincial- and grid-level historical flood extent data. By identifying the underlying hydrological drivers that determine spatial variability in flood susceptibility, this study informs early warning system design and long-term flood management strategies for TC-affected catchments.