Sedimentation | EPFL Graph Search

Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism. Settling is the falling of suspended particles through the liquid, whereas sedimentation is the final result of the settling process. In geology, sedimentation is the deposition of sediments which results in the formation of sedimentary rock. The term is broadly applied to the entire range of processes that result in the formation of sedimentary rock, from initial erosion through sediment transport and settling to the lithification of the sediments. However, the strict geological definition of sedimentation is the mechanical deposition of sediment particles from an initial suspension in air or water. Sedimentation may pertain to objects of v

Influence of outlet discharge on the efficiency of turbidity current venting

Sabine Chamoun

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.

EPFL2017

VENTING OF TURBIDITY CURRENTS: WHEN TO ACT?

Sabine Chamoun

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.

2017

Influence of outlet discharge on the efficiency of turbidity current venting

Sabine Chamoun

EPFL2017

VENTING OF TURBIDITY CURRENTS: WHEN TO ACT?

Sabine Chamoun

2017