Water resources of wadi systems of Southern Saudi Arabia

This study is a water resources assessment of two wadi systems in the southern Arabian Shield using seismic refraction, electrical resistivity (VES) and borehole geophysical techniques, pumping tests and ground water quality data and an examination of artificial recharge. Wadi Baysh (flowing southwest to the Red Sea) and Wadi Habawnah (flowing east to the Rub Al Khali) have similar catchment areas (approximately 10000 km2) and are underlain by igneous and metamorphic rocks but contrast in their hydrological cycles. The mean annual rainfall of Wadi Baysh is 350 mm while Wadi Habawnah receives 150 mm. The mean annual water discharges of Wadi Baysh and Wadi Habawnah are 75 and 10 mcm respectively. Hydrochemical classification, evolution of groundwater and its suitability for agricultural and domestic usage were studied in both wadis. In the upper and middle parts of both wadis the solute concentrations reach 1200 mg/l whereas in the lower parts solute concentrations range between 2000 and 3500 mg/l. Both wadis show high average values of transmissivity and storativity determined from pumping tests and grain size analysis. The geophysical surveys confirmed that the unconfined aquifer thickness is less than 42 m in both wadis. The seismic velocities of the aquifers range between 536 and 1817 ms-1 while the resistivities range between 23 and 125 ohm-m in both wadis. The igneous and metamorphic bedrock resistivity range between 3400 and 10260 ohm-m. The sediment has a high potential for water supply in the middle and lower parts of both wadis. The transverse resistance of the saturated part of the aquifers is used for correlation with the hydraulic transmissivity in which a computed empirical function may be applicable to similar aquifers in other Arabian Shield wadis. This study introduces a plan for the surface and subsurface storage that should help to manage the perennial yield and minimise the mining yield. The analysis reveals that building small reservoirs in both wadis is economically justifiable

The use of watershed geomorphic data in flash flood susceptibility zoning: a case study of the Karnaphuli and Sangu river basins of Bangladesh

The occurrence of heavy rainfall in the south-eastern hilly region of Bangladesh makes this area highly susceptible to recurrent flash flooding. As the region is the commercial capital of Bangladesh, these flash floods pose a significant threat to the national economy. Predicting this type of flooding is a complex task which requires a detailed understanding of the river basin characteristics. This study evaluated the susceptibility of the region to flash floods emanating from within the Karnaphuli and Sangu river basins. Twenty-two morphometric parameters were used. The occurrence and impact of flash floods within these basins are mainly associated with the volume of runoff, runoff velocity, and the surface infiltration capacity of the various watersheds. Analysis showed that major parts of the basin were susceptible to flash flooding events of a ‘moderate’-to-‘very high’ level of severity. The degree of susceptibility of ten of the watersheds was rated as ‘high’, and one was ‘very high’. The flash flood susceptibility map drawn from the analysis was used at the sub-district level to identify populated areas at risk. More than 80% of the total area of the 16 sub-districts were determined to have a ‘high’-to-‘very-high’-level flood susceptibility. The analysis noted that around 3.4 million people reside in flash flood-prone areas, therefore indicating the potential for loss of life and property. The study identified significant flash flood potential zones within a region of national importance, and exposure of the population to these events. Detailed analysis and display of flash flood susceptibility data at the sub-district level can enable the relevant organizations to improve watershed management practices and, as a consequence, alleviate future flood risk