Influence of outlet discharge on the efficiency of turbidity current venting

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.

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

Alluvionnement des retenues par courants de turbidité

Giovanni De Cesare

Reservoir sedimentation by turbidity currents All lakes created on natural rivers are subjected to reservoir sedimentation. The construction of a dam significantly modifies the flow conditions of natural streams inside and downstream of an artificial lake. Considering the sediment concentration, often high during the flood season, the entering flow shows a greater density than the ambient fluid. Suspended load can therefore be entrained along the reservoir bottom all the way down to the dam in the form of turbidity currents. Research was undertaken to better understand the physical phenomena that contribute to reservoir sedimentation. The study of turbidity currents at the bottom of a lake is based on a review of the relevant literature, on measurements of water and sediment motion on site, on physical modelling of turbidity currents and on numerical flow simulation. The two-year on site investigation in the test site of Luzzone in southern Switzerland showed the relationship between precipitation, water and sediment flow and turbidity current in the reservoir even for minor events. A simple relationship describing the inflow hydrograph and the sediment transport for small floods in the incoming river was established as a set of non-dimensional equations. These relations were implemented in the numerical flow code as an upstream boundary condition. No significant occurrence of high water was observed. It was therefore impossible to establish the necessary elements for extrapolation to important flood events with their implication for the reservoir sedimentation process. The laboratory investigation confirmed the results from numerical simulation of turbidity currents. The progressing flow proceeded as predicted by theory and numerical simulation. The numerical model developed is based on the general Navier-Stokes solver code CFX-F3D from Computational Fluid Dynamics Services. User defined sediment deposition and erosion modules were added in order to take into account sedimentation and the interactions between the turbidity current and the bottom of the reservoir. The flow in the physical model as well as in the real reservoir geometry was simulated with exactly the same boundary conditions. Comparison between computer simulation, physical model and on site measurement results showed a satisfactory agreement. The code was then used to simulate turbidity currents in two dimensions in a simplified reservoir geometry with all available flow - sediment interaction models for three different particle sizes. The results were compared to data from an existing 2Dflow simulation programme and they agreed well. The study of a thousand-year flood in the Luzzone reservoir using the developed computer model revealed the potential of such a tool. In particular, the impact on the sediment deposits was analysed. A Rapid evaluation of the incidence of such an extreme turbidity flow was made. Through this study, considerable knowledge has been acquired. It can be used to formulate proposals in order to allow limited transit of sediments through the lake during floods to evacuate them directly downstream of the dam. This operation has to take place during flood periods, when the upstream and downstream reaches carry enough water. The bottom outlet therefore participates partially in the sediment evacuation at the moment of their major occurrence during floods. Another advantage of this procedure would be the reduced impact on wildlife and vegetation in the downstream river.

EPFL1998

Sediment Evacuation from Reservoirs through Intakes by Jet Induced Flow

Jolanda Maria Isabella Jenzer Althaus

Reservoir sedimentation is worldwide a significant long term problem and requires in view of the current mitigation measures an alternative and more sustainable solution. This challenge motivated the present study with the purpose to develop an alternative efficient method to release sediment out of a reservoir. The concept is based on the release of sediment through the headrace tunnel and turbines whereby a special focus was set on the fine sediment in the area in front of the power intakes. Specific jet arrangements should provide the energy and generate the optimum circulation needed to maintain the sediment in suspension and enhance its entrainment into the power intakes during turbining sequences. This new idea was experimentally tested in a rectangular laboratory tank with the following dimensions: 2 m wide, 1.5 m high and 4 m long. Two jet configurations were systematically investigated: a configuration of four jets arranged in a circle on a horizontal plane and a linear jet configuration located parallel to the front wall. The influence of the jet characteristics (nozzle diameter dj, jet velocity vj, jet discharge Qj, and jet angle θ) and the geometrical configuration parameters on the sediment release was investigated. As initial condition an almost homogeneous sediment concentration distribution was induced by air bubbles. This condition simulated a muddy layer like in front of the dam by the fading of a turbidity current. The water level during all the experiments was held constant by releasing the same discharge through the water intake as was introduced by the jets (experiments with jets) or through the back wall (experiments without jets), respectively. Turbidity measurements combined with flow velocity measurements gave information about the sediment release efficiency. The sediment release (evacuated sediment ratio, ESR) is defined as the evacuated sediment weight Pout divided by the sediment weight initially supplied Pin and represents the normalized temporal integral of the released sediment amount: ESR = Pout/Pin. Analogously, the settled sediment ratio is the settled sediment divided by the sediment weight initially supplied Pin. Experiments without jets as reference configuration showed an almost linear relation between the sediment release and the discharge within the tested range: the higher the discharge, the higher the evacuated sediment ratio. For a constant discharge the ultimate sediment release as well as the settled sediment ratio was easily estimated by a simple physical approach taking into account the settling velocity and the flow field generated by the discharge through the water intake and the back wall. For the tested discharge range the sediment release was between 0.09 and 0.37 for reference configuration. Jets are effectively mixing: after roughly half an hour the standard deviation of the suspended sediment concentration was approximately 5 %, what in chemistry is considered as homogeneous. Consequently, less sediment was settled and, hence, the sediment release was higher than without jets and reached for the highest tested discharge (ΣQj = 4050 1/h) ESR = 0.73. Moreover, contrary to the experiments without jets, with jets resuspension of settled sediment was observed. Resuspension started once steady state conditions for the circulation were reached. It has been detected for discharges higher than an experimentally determined threshold. The observed evolution of the resuspension rate suggests that for a final stage all of the initially supplied sediment can be evacuated. The circular jet arrangement was identified as the most efficient configuration regarding sediment release. Additionally, the normalized optimal geometrical parameter combination was determined as follows: off-bottom clearance of the jet arrangement C/B = 0.175, water intake height hi/B = 0.25, distance of the jet arrangement to the front wall daxis/B = 0.525, distance between two neighbouring jets lj/B = 0.15, jet angle θ = 0° and water height in the tank h/B = 0.6. Under optimum conditions and with the highest tested jet discharge (ΣQj = 4050 1/h) after four hours a sediment release of ESR = 0.73 was achieved. Without jets and with the same discharge through the water intake the sediment release reached ESR = 0.37. The corresponding flow pattern in the transversal plane was similar to an axial mixer, which in the literature is reported as favourable for suspension. In the longitudinal flow patterns resulting from higher discharges, a single rotor was found between jets and water intake, whereas for smaller discharges the flow pattern was similar to a radial mixer. A variation of a single geometrical parameter within the tested range (i.e. 60 to 200 % of the optimum value) caused a sediment release reduction of up to 40 %, depending on the parameter and the duration. The linear jet arrangement was found to be much less favourable in view of sediment release. Its results were in the same magnitude as for the experiments without jets (ESR between 0.37 and 0.45). This is due to the direction of the induced rotation which is unfavourable regarding sediment suspension: the sediment is drawn to the bottom where it is settled and difficult to be put in suspension again. The efficiency of the jets was established by comparing the sediment release obtained under different conditions: once when jets were employed, once without jets. The predicted efficiency based on time and discharge independent empirical relationships is around 1.7 for the optimum jet configuration. Using the measured data the efficiency depends on discharge and increases with time. At the end of the transient phase and when resuspension started the efficiency was approximately 1.5. With the highest tested discharge the efficiency reached after four hours almost 2 (ΣQj = 4050 1/h). Due to the fine grain size used in the experiments (mean diameter of 60 µm) the application focuses on large reservoirs where the sediment is well sized along the thalweg and only fine particles are expected in front of the dam as it is the case for sediments transported by turbidity currents. In the case study of Mauvoisin with a 520 m long dam crest creating a large reservoir in Switzerland, a first attempt was made to up-scale the research results. Based on the available discharge and head of the existing water transfer tunnel a preliminary optimal circular jet arrangement was suggested. However, the width of the reservoir was estimated at approximately three times as large as optimal experimental conditions. Nevertheless, with a circular jet arrangement could definitely more sediment be evacuated than without jets. Moreover, the region near the outlet devices could be maintained free of sediment and their clogging could be avoided. An economic study revealed that a jet arrangement is a low cost installation which, based on the performed experiments, is essential when aiming for high sediment release and fighting against reservoir sedimentation.