Flood safety in the Sarneraatal - Optimization of the outlet structure of a relief tunnel using 3D CFD modelling

A flood relief tunnel from Lake Sarnen is planned to ensure flood safety in the Sarneraatal. The tunnel flows into the Sarneraa below the Wichelsee. An outlet structure is planned at the end of the flood relief tunnel. Here, the relief water volumes are controlled by roller gates. Before the water flows into the Sarneraa a stilling basin is provided at the end of the flood relief tunnel to reduce the water energy. Between the water level of the Sarnersee and the outlet structure there is a water level difference of approx. 20 m. Using 3D-CFD simulation the functionality of the outlet structure is examined and optimized. The following questions were examined:

• Position of the alternating jump and energy dissipation in the stilling basin
• Flow conditions (velocities, water levels) in the stilling basin and in the Sarneraa with different combinations of tunnel discharge, Sarneraa discharge and gate positions