الملخص الإنجليزي
Flash floods occur infrequently in Oman, resulting in losses of fresh water to the sea and deserts. In order to store and increase recharge to the aquifers, the Government of Oman has built 24 recharge dams, 52 storage dams and 6 protection dams across the Sultanate.
Due to high velocities of run-off water, large quantities of sediments are often transported, and deposited in the reservoir of dams. Sediments, which are of finer texture settle when stream velocity is reduced to zero. They lower the rate of recharge by acting as a restrictive layer. Sediment deposits are thick in Wadi beds and low in the flanks of the Wadi. Such differential settlement of sediments add to natural variation of infiltration rate of the native soil, and often reduce the rate of recharge.
In this study, the impact of sedimentation on artificial recharge to groundwater at Wadi Sahalnoot Recharge dam, Salalah, has been studied. Infiltration rates prior to the construction of the dam in 1993 were compared with infiltration rates measured in 2005 at the same locations. The comparison shows that the infiltration rate of the native soils have reduced considerably since the construction of the dam. This is possibly due to the migration of clay and loam particles through macro pores of the native soil profile, during percolation of recharging water. Analysis of the thickness of sediments after flood events in June 1993, May 2002 and September 2004 show that maximum depositions were at same locations after each event, coinciding with lowest positions in the Wadi.
The variation in sediment depth within the reservoir bed contribute to variation in infiltration rate. However, the decline in water level in the reservoir would represent an effective infiltration rate, reflecting the spatial variation in sediment deposition and infiltration rate of the native soil. Therefore, the effective hydraulic properties determining recharge were determined using a numerical model for unsaturated flow, HYDRUS2D. Changes to free water level following major floods were used to calibrate the model. The sensitivity of the model to calibrated parameters were determined, which was followed by scenario modelling.
