Analysis of the Shear Stress on Bed in Bottom Outlet of Dams During Sediment Transportation by Numerical Model

Although water and soil conservation activities reduce reservoir sedimentation, it is inevitable thatreservoirs fed by rivers transporting high amounts of sediment will experience sedimentation. TheGhezel-Ozan and Shahroud rivers, which flow into the Sefidroud reservoir dam, are both highlysediment-laden and transport significant amounts of sediment in both forms of bed and suspended loadto the reservoir. Hence, it seems that the only practical way to remove the sediment from the reservoiris to flush it out using the chasse method. But it consists of a highly-concentrated water flow of thesoil mass move through the bottom outlet that is called a density current. The flow causes significantdamages to the boundaries of the conduit by soil erosion and corrosion. In this paper, density currentin bottom outlets and its related problems are evaluated and the effects of concentration on wall shearstress are studied. To obtain the requirements for numerical modeling of three phase air-watersedimentflow pattern, the model was first calibrated with the experimental measurements of twophase air-water flow taken at Water Research Institute of Iran. Therefore, a numerical model based onmulti-phases systems and available theories of numerical models for sediment transport was developedwithout applying empirical coefficients. Two different concentrations of suspended sediment havebeen used and the results are compared. The results showed that wall shear stress increases bysediment concentration. For both cases, the shear stress increases up to a maxima after a short distancefrom the gate. The quantity of these maxims is higher than those measured by flow of pure water.Thus, it is concluded that the sediment flow causes intense shear stress, which is a major threat to thetunnel walls. These damages stimulate the pressure field, which also may trigger cavitations.