Effect of reservoir management on the efficiency of a sediment bypass tunnel: Case study of Solis Reservoir, Switzerland

Sediment bypass tunnels (SBTs) divert sediment-laden flows from river systems around reservoirs to the tailwater reach and thus help prevent reservoir sedimentation. However, their bypassing efficiency largely depends on reservoir operation, particularly for type-B SBTs with an intake located within the reservoir. The present study aims to investigate the effect of reservoir operation conditions on the bypass efficiency of a type-B SBT at the case study Solis Reservoir in Switzerland. Four annual measurement campaigns were conducted in the reservoir between 2018 and 2021. Flow velocities were measured, and bathymetry was mapped using an acoustic Doppler current profiler at high spatial resolution along the elongated and narrow reservoir. In- and outflow sediment volumes were measured using turbidimeters and Swiss plate geophone systems and estimated by using state-of-the-art sediment transport equations, respectively. Two floods with one-year and five-year return periods, respectively, in 2019 and a one-year return period flood in 2020 were captured. The results show that the average sediment bypass efficiency, i.e. the ratio of outflowing to inflowing sediment volumes, increased from 17% to 88% by operating SBT. The results highlight that the SBT bypass efficiency is highly dependent on the reservoir water level. For high efficiencies above 170%, an optimal value of the reservoir drawdown level is around 813 m asl. Bypass efficiencies up to 250% indicate that the type-B SBT does not only stop sedimentation but can also help regain active storage volume of the reservoir if operated under optimal conditions in terms of reservoir water level. Without SBT operation, ca. 205,000 m³ of net sediment deposition volume would have resulted in an aggradation of 1 m on average from 2018 to 2021. The findings of this study contribute to improved SBT and reservoir operation regimes in terms of reducing the sedimentation rates and prolonging the reservoir lifetimes.