Sediment evacuation from reservoirs through intakes by jet induced flow: Communication (Laboratoire de constructions hydrauliques, Ecole polytechnique fédérale de Lausanne)
Today’s worldwide yearly mean loss of storage capacity due to sedimentation is already higher than the increase of capacity by the construction of new reservoirs for irrigation, drinking water and hydropower. In Asia for example 80% of the useful storage capacity for hydropower production will be lost in 2035. In Alpine regions the loss rate in reservoir capacity is significantly below world average. The main process in narrow reservoirs is the formation of turbidity currents, which transport the fine sediments regularly near the dam, where they can increase sediment levels up to 1 m per year. The outlet devices such as intakes and bottom outlets are therefore in many reservoirs after 40 to 50 years of operation already affected. The effects of climate change will in future increase the sediment yield entering the reservoirs. Turbidity currents may be stopped and forced to settle down by obstacles situated in the upper part of the reservoir in order to keep the outlet structures free of sediments. Another new idea is to whirl up the fine sediments near the dam and intakes and keep them all the time in suspension, which allows a continuous release through the turbines. Mrs. Dr. Jolanda Jenzer Althaus studied this new idea for the first time with systematic hydraulic model tests combined with numerical simulations. Special water jet arrangements were developed which can be installed near the dam in front of the intake in order to generate an optimum circulation needed to maintain the fine sediments in suspension. In such a way a significant amount of sediment can be released continuously during powerhouse operation. In order to understand the involved physical processes in detail, systematic hydraulic model tests were carried out in a rectangular tank equipped at its front wall with an intake. First a configuration of four jets arranged in a circle on a horizontal plane in front of the intake was tested. For comparison in a second step also a linear jet configuration located parallel to the front wall with the intake was studied. Detailed measurements of flow velocity and sediment release helped to find the optimum combination of the parameters defining the circular jet arrangement. Finally, numerical simulations could reproduce the flow patterns.