Computational hydraulic modelling of fine sediment stirring and evacuation through the power waterways at the Trift reservoir

Taking advantage of the withdrawal of some glaciers, several new dams will eventually be constructed in Switzerland in the coming years as a part of the 2050 energy strategy. The Trift dam, located in Berner Oberlands in Switzerland, is one of these new projects. Reservoir sedimentation is however one of the main challenges for long-term sustainable operation of dam reservoirs that requires mitigation measures. Settling of suspended sediment may reduce reservoir live storage available for hydropower production and hamper operation of the bottom outlets. Jenzer-Althaus (2011) has experimentally tested a stirring device (hereafter called SEDMIX) as a mitigation measure to prevent reservoir sedimentation. The device induces an adequate level of upwind turbulence preventing sediment from settling near the dam, keeping them in suspension for progressive evacuation through the power intake during normal operation of the hydropower plant. This innovative system can be potentially installed in several reservoirs in Switzerland and abroad to avoid reservoir siltation due to fine sediments mainly transported by turbidity currents. The present study investigates the implementation of the SEDMIX device in the Trift reservoir by means of computational hydraulic modelling. The study proves the efficiency of the SEDMIX device in evacuating fine sediments during normal operation of the hydropower plant. The effect of other parameters such as the orientation and discharge of the SEDMIX jets, as well as the characteristics of the suspended sediments such as initial concentration and grain size, on the efficiency of the SEDMIX device will be evaluated in the future.