VENTING OF TURBIDITY CURRENTS: WHEN TO ACT?

During floods, sediments are transported from watersheds into reservoirs, slowly decreasing water volumes and this leading to economic losses. Thus, in the long term, sedimentation endangers reservoirs’ sustainability. Sediments can also block low-level hydraulic structures such as bottom outlets and powerhouse intakes and cause abrasion of gates and turbines. Additionally, the trapped sediments induce downstream starvation and thus the impoverishment of the river’s morphology and ecosystem. Many measures are taken to deal with the sedimentation of reservoirs. Among the most common methods is venting turbidity currents approaching the dam. In fact, these sediment-laden currents carry the major part of sediments found near the dam and thus their evacuation before they settle can be a very effective method to reduce sedimentation. However, dam operators lack information and guidelines to perform efficient venting operations. The present research experimentally and numerically investigates the venting of turbidity currents applied with different timings of outlet opening: (1) before the current reaches the outlet, (2) after the current has reached the outlet and climbed up to the top of the dam, and (3) after the upstream reflection of the muddy lake has begun. The high data acquisition frequency offers the possibility to examine temporal variations of inflow and outflow concentrations and discharges and thus variations of the efficiency of venting in time during the tests. In addition, the experimental results are extended numerically for a better understanding of the effect of opening timing on venting. Results show that opening the outlet before the current reaches the wall can be more efficient than opening after the current has reached the wall. Outputs of this study lead to crucial information for dam operators dealing with reservoirs facing high sedimentation rates due to the formation of turbidity currents.