Improving regional flood risk assessment using flood frequency and dendrogeomorphic analyses in mountain catchments impacted by tropical cyclones

Abstract River floods frequently occur when tropical cyclones hit land. Nonetheless, systematic, long-term discharge data remain rather scarce in many tropical countries, which prevent proper analysis of peak discharges occurring during floods. The Terraba catchment is the biggest and most dynamic catchment in Costa Rica. In this study, we developed regional flood-frequency analyses combining tree-ring based estimation and measurement of peak discharge at monitoring stations during tropical cyclones to derive flood quartiles. Flood quartiles were combined with the Topographic Wetness Index (TWI) to determine regional flood hazards along floodplains. The flood risk assessment was based on a high-resolution mapping of infrastructure, population density (as a measure of exposure), and a social development index (to represent vulnerability). We show that peak discharge of cyclone-induced floods can be assessed accurately with flood-frequency analyses including dendrogeomorphic reconstructions and systematic discharge measurements. We also show that regional flood risk assessments can be performed in large-scale catchments if both coarse and detailed inputs are used. The results of this study will be useful for the development of flood risk schemes promoting resilience of local populations.