ABSTRACT
Hydropower is the major source of electricity in Switzerland contributing about 57% (36 TWh/yr) of the total annual generation. Therefore, water storage in hydropower reservoirs is crucial to balance the electricity demand over variable river flow. With the increase in storage demand and climate-related stress it becomes important to sustain the existing reservoir storage capacities. Sedimentation impairs the sustainable operation of reservoirs by reducing the storage volume and may also cause dam safety related issues by the interference of sediment deposits with dam outlets.
Sediment Bypass Tunnels (SBTs) are an effective countermeasure to reduce or even stop sedimentation and contribute to a sustainable use of reservoir storage capacity. This study investigates the performance of an SBT constructed at Solis reservoir in the Swiss Alps, operated by ewz. The SBT was commissioned in 2012 to mitigate continuous propagation of sediment aggradation towards the dam since its construction in 1986. As the inlet of the SBT is located within the reservoir and therefore typically submerged, optimized reservoir operation is required during the intended period of sediment bypassing.
Annual field measurements were conducted to measure the reservoir bathymetry, sediment concentrations, transport rate and sediment particle sizes on the bed to derive the reservoir’s sediment balance. The measurements between October 2018 and August 2019 are analyzed to investigate bypass efficiencies of the SBT. The results indicate that the efficiency of the SBT was 80%, and thus considerably higher than the previous efficiency rate of 17%, due to adaptation of the reservoir operation to a lower water level during SBT operation. This implies that with proper synchronization of SBT and reservoir operation, this type of SBT can be highly efficient.
Furthermore, a 1D numerical model is applied to investigate the processes of sedimentation and sediment management for the Solis reservoir. The data from the field measurements is used to set-up, calibrate and validate the model aiming at investigating the performance of the SBT. The model can reproduce the sedimentation as well as SBT operation in terms of longitudinal bed profile evolution and deposition volume. Moreover, the model also allows for simulating additional scenarios, including e.g. no SBT operation, to compare the effects of different operation modes.
