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.
EPFL2011
