An integrated hydrological and hydraulic modelling approach for flash flood hazard assessment in eastern Makkah city, Saudi Arabia

Flash floods are among the most destructive natural disasters with associated adverse impacts on society and the environment. The present study assesses the adverse impacts of flash floods within the Wadi Mehassar basin on the holy sites (i.e., Al Mashaaer Al Muqaddassa) and urban areas, east of Makkah city, which are highly vulnerable to flash floods especially during short duration high intensity rainfall. The difficulty associated with assessing the flash flood hazards in the study area is attributed to the rapid alterations in topography with accelerated urban development and the lack of hydrometric stations to gauge the surface runoff. Thus, in order to achieve the goals of the study, The ArcGIS 10.4, ERDAS Imagine 2015, and the Watershed Modelling System WMS 11.0 were used to process geological and hydrological data and prepare the modelling inputs such as geological, metrological and hydrological groups of the drainage basins under examination. Moreover, a mathematical hydrologic model (HEC-1), was applied to produce the hydrograph curves of the Wadi Mehassar basin, based on the synthetic unit hydrograph of the Soil Conservation Service (SCS-UH). The two-dimensional hydraulic modelling (HEC-RAS 5.0.7) software was employed, to calculate and simulate the water surface elevations, flow velocities, flow depths and spread of the flash flood, in order to develop the hazard rating map of flash flood inundation. The model results identified 44% of total area as liable to flash floods, and out of which 5.4% indicated high risk, 13.3% showed medium risk and the remaining 81.3% area fell under low risk of flash flooding.