p Spatio-temporal deposition profile of an experimentally produced turbidity current with a continuous suspension supply

A turbidity current is a turbulent, particle-laden gravity current that is driven by density differences resulting from the presence of suspended sediment particles. The current travels downslope, bearing a large amount of sediment over a great distance, and forms fluvial and submarine bedforms. Knowledge of the spatio-temporal deposition profile of turbidity-deposited sediment is important for a better understanding of sediment transport by turbidity currents. In the current study, the depositional process of experimentally produced quasi-steady turbidity currents in an inclined flume was investigated using an electrical-resistance-based depositmeter. It was found that the amount of sediment deposited along the flume bottom increases linearly with time at a specific rate, and decreases exponentially in the downstream direction. Inspired by this observation, the mass conservation law was modified for the suspension layer and a simple theoretical model was proposed involving two parameters, the initial sedimentation rate and the decay constant. The proposed model reproduces the observed data from the flume experiment with high accuracy, and is consistent with the sediment transport distance of a natural example, the Bengal Fan. It was concluded that the two parameters that govern the sedimentation process are the key to estimating the deposition profile of fluvial systems. (c) 2021 International Research and Training Centre on Erosion and Sedimentation/the World Association for Sedimentation and Erosion Research. Published by Elsevier B.V. All rights reserved.

Quasi-stationary flow structure in turbidity currents

Giovanni De Cesare, Shun Nomura

A turbidity current is a particle-laden current driven by density differences due to suspended sediment particles. Turbidity currents can transport large amounts of sediment down slopes over great distances, and play a significant role in fluvial, lake and submarine systems. To better understand the sediment transport process, the flow system of an experimentally produced turbidity current in an inclined flume was investigated using video processing. We observed that the current progresses with constant frontal velocity and maintains an unchanged global interface geometry. In addition, the spatio-temporal profiles of the inner mean and turbulence velocity obtained by ultrasound velocity profiler (UVP) showed that similar distributions were maintained, with low dissipation. The results indicate that the turbidity current progressed in a quasi-stationary state, which enabled long-distance sediment transport. To understand the mechanisms behind the quasi-stationary flow, we analyzed the forces acting on the turbidity current. We found that under particular densities of suspended particles, the gravitational force is balanced by the viscous forces along the slope direction. We conclude that this specific force balance induces the quasi-stationary flow structure, enabling the long-distance transport of a substantial amount of sediment downstream with low dissipation. (C) 2020 International Research and Training Centre on Erosion and Sedimentation/the World Association for Sedimentation and Erosion Research. Published by Elsevier B.V. All rights reserved.

IRTCES2020

Managing reservoir sedimentation by venting turbidity currents: A review

Sabine Chamoun, Giovanni De Cesare, Anton Schleiss

Reservoir sedimentation is an issue that dam operators are increasingly facing as dams are aging. Not only does it reduce a reservoir's capacity but it also affects its outlet structures such as bottom outlets and powerhouse intakes. Sedimentation may also impoverish downstream ecosystems. For these reasons, several strategies for sediment management are being investigated and applied worldwide. Among these methods, venting of turbidity currents reaching the dam can be very beneficial and economical. This measure helps in preserving a certain continuity of sediment transport in rivers obstructed by dams. However, several practical but also theoretical challenges hamper this technique, rendering its use less common and its aspects relatively unknown. The present paper aims to gather the actual state-of-the-art concerning turbidity currents venting and to present an outlook for future development and research in this field. (C) 2016 International Research and Training Centre on Erosion and Sedimentation/the World Association for Sedimentation and Erosion Research. Published by Elsevier B.V. All rights reserved.

Irtces2016

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

Influence of outlet discharge on the efficiency of turbidity current venting

Sabine Chamoun

EPFL2017