Flash floods occur infrequently in the Sultanate of Oman, resulting in losses of fresh water to the sea and deserts. In order to store run-off water and increase recharge to aquifers, the Government has built recharge dams across the country. Due to high velocities of run-off water, large quantities of sediments are often transported and deposited in the reservoir of dams. Sediment deposits are thick in wadi beds and low in the flanks of the wadi. Such differential rates of sediment deposition add to natural variation of infiltration capacity of the native soil, and often reduce the rate of recharge. In this study, the impact of sedimentation on artificial recharge at Wadi Sahalanowt Recharge dam, Salalah, Oman, has been studied. Infiltration capacities prior to the construction of the dam in 1993 were compared with those measured in 2005 at same locations. The comparison shows that the infiltration capacity of the native soils has been reduced considerably since the construction of the dam. This is possibly due to the migration of fine clay and silt particles through macro pores of the native soil, during percolation of recharging water. Analysis of the thickness of sediments after flood events in 1996, 2002 and 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 contributes to variation in saturated hydraulic conductivity of the soil. However, the decline in water level in the reservoir would represent an effective infiltration capacity, reflecting the net effect of the sediments, native soil and migrated fine particles through the native soil. The effective hydraulic properties determining the recharge rate were determined using a numerical model for unsaturated flow, HYDRUS2D. Changes to free water level following May 2002 flood was used to calibrate the model. The sensitivity of the model to calibrated parameters was determined followed by a scenario modelling.

Flash floods occur infrequently in the Sultanate of Oman, resulting in losses of fresh water to the sea and deserts. In order to store run-off water and increase recharge to aquifers, the Government has built recharge dams across the country. Due to high velocities of run-off water, large quantiti...