Sihl Flood Relief Tunnel, Owner's engineering SIA phases 32-33

In view of the need to act regarding the flood protection of the city of Zurich in case of rare flood events, the canton of Zurich started a project to improve the long-term flood protection in the catchment area of the river Sihl, Lake Zurich and the river Limmat. Part of it is the approximately two-kilometer-long spillway tunnel Thalwil (Din = 6.6 m), which, by means of a partial diversion of the Sihl outflow, will lead the flood peaks of the Sihl into the Lake Zurich. The Lake Zurich acts as a retention basin and releases the diverted waters with delay to the Limmat, thus protecting the lower Sihl Valley and Zurich from extreme flooding. With the partial discharge of flood peaks, the outflow remaining in the river Sihl in the city of Zurich resp. at Zurich main station is limited to about 300 m³/s in case of a 500-year flood (HQ500 = 600 m³/s). Due to the inevitable environmental impacts on the Sihl and Lake Zürich caused by the construction of the spillway tunnel, extensive ecological compensating measures will be implemented on the Sihl in Langnau am Albis and on Lake Zurich in Richterswil
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WWEA: Office of Waste, Water, Energy and Air
Is Consortium
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Owner's support; Special studies