Terrain-Based Flood Susceptibility and Exposure Mapping Using a HAND-GIS Framework: A Case Study from the Aseer Region, Saudi Arabia

Flooding poses a serious challenge in rapidly growing mountain cities, where steep relief, wadi networks, and expanding urban surfaces concentrate runoff along narrow drainage pathways. This study applies a terrain-based Height Above Nearest Drainage (HAND) workflow within a GIS environment to map flood susceptibility and infrastructure exposure across the Abha, Khamis Mushait, and Ahad Rafidah catchment in the Aseer Region of Saudi Arabia. A 30 m digital elevation model was processed in PCRaster to derive flow direction, flow accumulation, stream networks, subcatchments, and HAND surfaces under four contributing-area thresholds of 1, 5, 10, and 20 km2. The scenario design evaluates how drainage-representation uncertainty affects susceptibility and exposure estimates. Susceptibility was summarized for cumulative HAND classes of ≤5, ≤10, ≤20, and ≤30 m, then intersected with filtered building footprints and the road network to estimate infrastructure exposure. The analysis shows that mapped susceptibility varies with drainage representation, but the most critical building and road exposure remains concentrated within the same low-lying urban–wadi zone across all scenarios. The mapped extent of the HAND ≤ 5 m class declined from 367 km2 under the 1 km2 scenario to 99 km2 under the 20 km2 scenario. Buildings within HAND ≤ 5 m decreased from 26,449 to 5633, while road segments within the same class declined from 8758 to 1393. Even under more conservative stream thresholds, exposure remains focused within this same urbanized drainage belt, indicating persistent localized susceptibility. The findings show that HAND can be used as a practical first-pass screening tool for identifying flood-susceptible terrain and prioritizing exposed infrastructure in data-scarce environments, while the scenario-based threshold testing improves confidence in identifying robust hotspots for follow-up hydraulic modeling and urban risk management.