Flood hazard mapping in data scarce basins: A Rain-on-Grid approach in Kumasi, Ghana

Flood inundation mapping is essential for assessing and managing urban flood risks. However, in many low- and middle-income countries, hydrological data scarcity limits the ability to generate reliable hazard assessments. This study developed a transferable workflow for flood hazard mapping in ungauged urban basins, demonstrated in the Aboabo Basin of Kumasi, Ghana. Hydraulic simulations were performed using the Rain-on-Grid method within HEC-RAS 2D, which was calibrated using post-flood field surveys. Synthetic design storms corresponding to 5-, 50-, and 100-year return periods were used to generate the hazard maps. The results indicated maximum flood depths of 3.5, 3.9, and 5.3 m, with inundation extents of 12.7%, 16.9%, and 23.1% of the basin area, respectively. These findings show how rapid urbanization amplifies inundation risk and demonstrate the scalability of flood impacts with return periods. Beyond the case study, the workflow highlights the potential of participatory calibration and rainfall-driven hydrodynamic modeling to overcome data scarcity in ungauged basins with limited data. This approach offers practical guidance for strengthening urban flood risk management in rapidly developing regions worldwide.