A Synergistic Use of Remote Sensing and Hydrodynamic Techniques for Flash Flood Mitigation Toward Sustainable Urban Expansion in Najran Valley, Saudi Arabia

Flash floods are significant threats, particularly in semi-arid regions where prolonged rainfall and/or over-discharge can lead to catastrophic downstream flooding. While Saudi Arabia’s Najran region has been repeatedly pummeled by severe floods, the impact of these events on urban development in the Najran Valley, particularly in relation to dam discharge rates, has not been adequately assessed. This study seeks to assess mitigation strategies for simulated floodwater outflow (extent, depth, and velocity) from Najran and Al-Ain Dams into the Wadi Najran Valley. The study employed a synergistic approach integrating satellite optical Landsat-8/SRTM digital elevation model (DEM) imagery, ancillary (soil and geological) maps, and land-based rainfall measurements in the Geographic Information System (GIS), coupled with hydrologic/hydrodynamic modeling. Analyzing three basins—Wadi Najran Dam, Wadi Al-Ain Dam, and Wadi Najran Valley—using Watershed Modeling System (WMS) software revealed potential discharge rates of 347.82 m3/s and 16.59 m3/s for fully opened gates of Najran and Al-Ain Dams, respectively. The low-lying Wadi Najran Valley exhibited the highest flooding risk, with a runoff volume surpassing 428 million m3 and a peak discharge of 2481.71 m3/s. The 100-year flood simulations using the Hydrologic Engineering Center River Analysis System (HEC-RAS) demonstrated uneven floodwater drainage in the main canal, leading to bilateral overflow consistent with historical flood observations. Inundation risk is effectively mitigated by widening the existing channel into a trapezoidal open channel with a base of 300 m and a height of 3.76 m to handle a peak discharge of 3481.71 m3/s based on proposed hydraulic calculations. The adaptability of the implemented methodology to analogous urban climates globally justifies subsequent research to comprehensively assess its cost-effectiveness and feasibility.