Flood protection Sihl, Lake Zurich: spillway Thalwil

The project serves to protect the city of Zurich from floods from the Sihl. In the area of the concise left-hand bend of the Sihl below the Sihlwald and below the alluvial wood rake, the spillway takes the flood peaks with a regulated side outlet. From there, they are fed to Lake Zurich through a gravity gallery under the Zimmerberg.

For the spillway, ideal locations were found for the inlet structure in Langnau a. A. (Sihl) and for the outlet structure in Thalwil (Lake Zurich) and optimised in terms of their hydraulic function and costs. Flood peaks are diverted with a pronounced retention effect via Lake Zurich into the Limmat, so that sufficient protection against overflowing of the Sihl is achieved in the central area of the city of Zurich and above. The feasibility of a robust, durable and simple flood protection concept was demonstrated and confirmed and refined by means of hydraulic and shear engineering model tests at the ETH Zurich.

In order to verify the functionality of the structure, a physical model test was carried out for both the inlet structure and the outlet structure at the ETH Zurich's Laboratory of Hydraulics. The 3D models of the structures, which were further optimised during the tests, serve as the basis for the model construction.

• Intake structure with 130-metre-long longitudinal weir and collecting basin
• Free-flow adit with a shooting discharge (length: 2.1 km, internal diameter: 6.6 m, gradient: 1.3 % - 3.3 %)
• Outlet structure with stilling chamber and estuary structure (length: 90 m, three metres below the water surface of Lake Zurich)