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In arid regions, flash floods often occur as a consequence of excessive rainfall. Occasionally causing major loss of property and life, floods are large events of relatively short duration. Western Saudi Arabia is an extremely arid region where rainfall is characterized by extremely high spatial and temporal variability. This area is subject to environmental hazards such as flood risks because most of the infrastructure is located on wadis and flood plains. In this paper, an integrated approach was adopted in which geographic information system and rainfall–frequency analysis using annual maximum 24-h rainfall were utilized to identify, assess and mitigate hydrological and environmental flood hazards in the study area. The study shows that the prediction of annual maximum 24-h duration rainfall along with prediction of 25-, 50-, 100- and 200-year return periods for the best fit of Gumbel pdfs. In addition, probable maximum precipitation was also estimated for 50- and 100-year return periods, which will aid decision-making for future project designs and flood protection studies. [ABSTRACT FROM AUTHOR]

Accelerated economic development in arid areas results in harmful stress on limited surface and groundwater resources. Flash flood and rainfall harvesting is a promising renewable resource. Using a theoretical water budget approach, this paper investigates groundwater recharge after a flash flood in three small dam reservoirs and one moderate-capacity dam reservoir in the Al-Alb basin, central Saudi Arabia. This approach is validated against water level measurements at five recharge wells. Watershed modeling of this basin was conducted using an analysis of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) digital elevation models, in which the morphometric variables were calculated. Landsat-8 and Spot-5 images from April 2015 were classified to identify different land-use and land-cover categories. The soil conservation service curve number (SCS CN) method was used to estimate the effective rainfall, where the estimated CN values of different land uses and the spatially distributed time-area zones of the catchment were identified. The rainfall-runoff relationship was assessed using the Hydrologic Engineering Center–Hydrologic Modeling System (HEC-HMS) model, and the results were validated using the actual reservoir storages of these dams. The lag time, time of concentration, time to peak, and maximum of discharge storage reservoir sizes and volumes were estimated for the main and subbasins of the Al-Alb basin. This research shows that considerable amounts of water can be harvested using these small dams (0.21×106 m3 to 6.95 ×106 m3). The hydrographs show that the construction of smaller dams upstream of the main dam at the Al-Alb location decreases the main peak magnitude by 14%, and the main runoff volume by 3%. Because of the high evaporation rate and low natural groundwater recharge as a result of siltation process in the dam lakes, recharge wells were drilled in the dam reservoir, increasing groundwater recharge by 44% and decreasing evaporation loss by 86%. With time, well recharge efficiency decreases, owing to clogging by the siltation process. Consequently, it is recommended to periodically clean and maintain the recharge wells. [ABSTRACT FROM AUTHOR]