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

Document Type : Technical Note

Authors

Abstract

Although water and soil conservation activities reduce reservoir sedimentation, it is inevitable that
reservoirs fed by rivers transporting high amounts of sediment will experience sedimentation. The
Ghezel-Ozan and Shahroud rivers, which flow into the Sefidroud reservoir dam, are both highly
sediment-laden and transport significant amounts of sediment in both forms of bed and suspended load
to the reservoir. Hence, it seems that the only practical way to remove the sediment from the reservoir
is to flush it out using the chasse method. But it consists of a highly-concentrated water flow of the
soil mass move through the bottom outlet that is called a density current. The flow causes significant
damages to the boundaries of the conduit by soil erosion and corrosion. In this paper, density current
in bottom outlets and its related problems are evaluated and the effects of concentration on wall shear
stress are studied. To obtain the requirements for numerical modeling of three phase air-watersediment
flow pattern, the model was first calibrated with the experimental measurements of two
phase air-water flow taken at Water Research Institute of Iran. Therefore, a numerical model based on
multi-phases systems and available theories of numerical models for sediment transport was developed
without applying empirical coefficients. Two different concentrations of suspended sediment have
been used and the results are compared. The results showed that wall shear stress increases by
sediment concentration. For both cases, the shear stress increases up to a maxima after a short distance
from 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 the
tunnel walls. These damages stimulate the pressure field, which also may trigger cavitations.