Flood Risk Assessment Under Climate Change Scenarios in the Wadi Ibrahim Watershed

Flooding poses a significant hazard to urban areas, particularly under the pressures of climate change and rapid urbanization. This study evaluates the flood risk in the Wadi Ibrahim watershed, located in Makkah Al-Mukarramah City, Kingdom of Saudi Arabia (KSA), by analyzing the impacts of climate change on flood hazards. The analysis incorporates projections from the Coordinated Regional Climate Downscaling Experiment (CORDEX) regional climate model (RCM) for three climate scenarios: representative concentration pathway (RCP) 2.6, RCP 4.5 and RCP 8.5. A novel aspect of this study is the integration of 2D HEC-RAS rain-on-grid (RoG) hydrodynamic modeling with climate change projection analysis, which has not been previously applied in this watershed. Flood risk maps are generated for each scenario at three return periods: 50, 100, and 200 years. The results indicate an increasing flood volume and depth under future climate scenarios. The flood risk mapping shows an expansion of medium- and high-risk zones compared to current conditions. Under the current climate, the low-risk areas (0–0.5 m) slightly decrease from 13.9 km2 (50 years) to 13.8 km2 (200 years), while the medium- (0.5–2 m) and high-risk areas (>2 m) increase from 6.5 km2 to 7.0 km2 and from 7.2 km2 to 9.8 km2, respectively. Under RCP 2.6, the low-risk zones decline from 13.6 km2 to 13.0 km2, the medium-risk zones grow from 14.5 km2 to 16.2 km2, and the high-risk zones rise from 4.3 km2 to 6.5 km2. The higher emissions scenarios show greater risk increases, with the high-risk areas expanding from 5.3 km2 to 12.0 km2 under RCP 4.5, and from 9.5 km2 to 16.6 km2 under RCP 8.5. These findings underscore the escalating flood risks due to climate change and highlight the need for mitigation in the Wadi Ibrahim watershed.