Physical and 3D numerical simulation of the flow in the tailrace of a hydroelectric power plant to design fishway entrances

This study is within the frame of the construction of the rail link between Geneva Cornavin station, Eaux-Vives station and Annemasse (CEVA). The project area is located at a cut-and-cover tunnel built by slurry walls located in the alluvium of the Foron River. The installation of the covered trench cuts the groundwater table flow, which causes a rise of the water table in spite of the partial evacuation by the Foron. It is therefore planned to add about thirty wells in order to transit the waters of the aquifer from one side of the tunnel to the other. A thorough reflection allowed to find an alternative solution to traditional pumping consisting of putting in place self-priming multiphase siphons which forms the basis of the following study. The operating principle of the system is based on the maintenance of a free-surface vortex in a vacuum in what will be called the sealed box which functions by keeping the siphon primed to allow water to be transferred from either side of the gallery. This study explores the performance of a physical model of the system with the following objectives: (1) To characterize the initiation and stability conditions of the system by maintaining the vacuum in the sealed box and (2) to characterize the limiting operating conditions of the system in relation to the ground water table levels which relates to insufficient air entrainment due to low outflow or submergence of the sealed box. The experimental installation is fabricated at a scale of 1:1 so as not to introduce scale effects by the presence of air, the water-air mixture and the free-surface vortex in a vacuum atmosphere