Influence of outlet discharge on the efficiency of turbidity current venting

Abstract

Sediment yield into reservoirs was underestimated during the design of dams in the past. As a result, today, reservoir sedimentation is endangering the sustainability of dams. Moreover, the obstruction of rivers hinders their capacity to transport sediments and causes the alteration of their morphology and ecosystems. The rate of sedimentation is expected to increase in the future due to climate change. Turbidity currents are one of the main processes transporting sediments into long and deep reservoirs during floods. They are capable of transporting suspended sediments from the plunge point at the delta to the dam. Hence, venting of turbidity currents through bottom outlets is an appealing solution to reduce reservoir sedimentation. Using a flume of 8.55 m length and 0.27 m width, venting was investigated experimentally and numerically. Governing parameters such as the bottom outlet's discharge, the timing of venting relatively to the arrival of the turbidity current at the dam, the duration of venting, the reservoir's bed slope as well as the outlet's dimensions and level were studied. The efficiency of venting corresponding to the amount of sediments evacuated by the water used from the reservoir could be highlighted by systematic tests. The efficiency of venting increased with steeper bed slopes since the turbidity current was less reflected at the dam. Therefore, venting should be applied from the very beginning of dam impoundment to keep the cone upstream of the outlets free of sediments and the steepest bed slope possible close to the dam. The effect of the venting degree -defined as the ratio between outflow and turbidity current discharges- on the efficiency of venting was systematically studied. For turbidity currents reaching the outlet on a horizontal bed in the experimental configuration, a venting degree of about 100% resulted in the highest venting efficiency. For steeper reservoir bed slopes (i.e., 2.4% and 5.0%), the optimum efficiency can be obtained with a venting degree of about 135%. Venting was the most efficient when synchronized with the arrival of the turbidity current at the dam. Therefore, a gauging station should be placed around 300 m upstream of the low-level outlet to measure parameters such as velocity, indicating the arrival of the turbidity current. Furthermore, venting should last as long as there is inflow and should be maintained after the end of the flood for a duration that depends on the outflowing sediment concentration. In practice, venting can be stopped when the muddy lake has been evacuated and the vented water becomes clear again. This also allows cleaning the downstream river from fine sediment deposits after the venting operation. To optimize venting efficiency and minimize the dead storage, the outlet should be positioned at the lowest level possible. In addition, the height and width of the bottom outlet's entrance should be chosen in a way to create an aspiration cone that corresponds approximately to the dimensions of the body of the turbidity current. In order to keep the size of the low-level outlet reasonable, multiple outlets can be used to create the required aspiration cone.