Abstract
Flash floods occur periodically on Riyadh province, Saudi Arabia, due to various factors, including rugged topography and geological structures. Each year, it results tremendous loss of life and property damage across a wide area. The present study aims to identify potential suitable areas for stormwater management in Riyadh province-Saudi Arabia using a GIS-decision support system (DSS), in addition, to determine the runoff coefficient and the runoff depth for different land cover/use classes and different soil type. Moreover, it aims to study the effect of the Riyadh metro project in the generation of flash floods around the proposed metro lines. The results of the spatial distributions of modelled annual runoff depth varied from 9 to 180 mm/year, and annual runoff depth around the proposed metro lines ranged from 70 to 120 mm/year. The major cause of floods in Al-Riyadh province is the occurrence of extremely heavy rainfall over a short period and low water absorptive capacity of soil, leading to an increased overland flow. Therefore, despite the total rainfall amount being relatively small in Riyadh province, Saudi Arabia, the rainfall event can be very intense, hence causing problems of flooding. The high-potential risk of flash floods is within areas around line 1, 2, 4, and 6. The analysis indicates that construction of the Riyadh Metro will lead to an annual increase in the flash flood generation in the urban regions. The DSS was implemented to obtain suitability maps and to evaluate the existing SWH/Groundwater recharge (GWR) structures in the study area. The DSS inputs comprised maps of rainfall surplus, slope, runoff coefficient (RC), land cover/use, and soil type. Based on an analytical hierarchy process analysis taking into account five layers, the spatial extents of SWH suitability areas were identified by multi-criteria evaluation. The spatial distribution of the classes in the suitability map showed that the excellent and good areas are mainly located in the northeastern and northwestern parts of the study area. The southeastern and west southern parts almost have the same categories dominated by moderate and poor and unsuitable areas. On average, 22.17 % (84,356 Km(2)) and 31.56 % (120,085 Km(2)) of the study area are classified as excellent and good for SWH, respectively, while 23.98 % (91, 243 Km(2)) and 22.28%(84,775 Km(2)) of the area are classified as moderately suitable and poorly suited and unsuitable, respectively. Most of the areas with excellent to good suitability have slopes between 2 and 8 % and are intensively cultivated areas. Rainfall in these areas ranges from 120 to 230 mm. Most of the existing SWH/GWR structures that are categorized as successful were within the excellent (89.1 % of the structures) areas followed by good suitable (10.9 of the structures). Overall, results indicated that wadi Hanifah and Wadi Nisah have a moderate vulnerability to flooding, with high vulnerability in the northeast part of Al-Riyadh province. The use of a number of SWH sites in the excellent areas is recommended to ensure successful implementation of SWH systems.