Sediment Evacuation from Reservoirs through Intakes by Jet Induced Flow

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,