Bypassing efficiency evaluation and optimization of sediment bypass tunnel operation in a narrow reservoir using 1D numerical modelling

Sediment Bypass Tunnels (SBTs) are hydraulic structures designed to mitigate reservoir sedimentation, mainly used in Switzerland, Japan, and Taiwan. The SBT efficiency depends on multiple factors including the incoming sediment yield, SBT inlet location, reservoir water level, and the timing of SBT operation relative to a flood event. To evaluate bypassing efficiency and optimize SBT operation, a methodology is developed to integrate sediment bypassing processes in a 1D numerical model for simulating the sedimentation dynamics under varying hydraulic conditions. The well-monitored case of the Solis reservoir in Switzerland with SBT operation serves as a good basis for this study. After calibrating and validating the model with the field data, three different categories of SBT operation scenarios are studied for a 5-year flood event: i) no SBT operation, ii) the effect of reservoir water level during SBT operation, and iii) the optimum duration of SBT operation. The simulations reveal that SBT operation is highly effective in reducing the amount of sedimentation by 89% compared to no SBT operation. This equals to the prevention of a 9% loss in active storage volume. However, the SBT efficiency is highly sensitive to reservoir operation. A maximum efficiency is achieved at a lower reservoir water level below the minimum operating level for energy generation, while it also releases higher sediment concentrations to the downstream reach. Furthermore, a longer duration of SBT operation increases the bypassing efficiency and minimizes the loss of active storage volume but goes along with a reduction in hydropower generation. Thus, a comparison of net benefits for different scenarios is suggested to derive an optimal SBT operation mode for similar situations. Overall, the applied methodology serves as a useful basis for evaluating and optimising the sediment management efficiency of SBTs and can thus contribute to improving the sustainable operation of reservoirs.