Flood protection for the River Sihl, Lake Zurich, River Limmat: selection of the Thalwil flood relief tunnel option

After comparing the various options to protect the lower Sihltal valley and the city of Zurich from extreme flooding of the River Sihl, the government council has selected the option for the further design of the relief tunnel between Langnau am Albis and Thalwil. The Thalwil relief tunnel, with a length of 2 km, diverts the flood peaks of the River Sihl into Lake Zurich. The result is a small increase in the level of Lake Zurich by 5 cm, which will be compensated by increasing the relief capacity of the River Limmat at the Münsterbrücke and Rathausbrücke bridges.

The main reasons the relief tunnel option was chosen was reliability (irrespective of weather forecasts), the very favourable cost-benefit ratio and the lower environmental impact.

The relief tunnel will cost CHF 130 million, construction will take 2.5 years and will be completed in 2023 at the earliest.

IUB Engineering has been involved in the development of this exciting project right from the very first considerations in the feasibility study to preparation of the preliminary design study for the Office of Waste, Water, Energy and Air of Zurich Canton (AWEL). IUB Engineering is currently developing the construction project.

Further information:


Hochwasserschutz Sihl - Zürichsee - Limmat, Entlastungsstollen
Hochwasserschutz Sihl - Zürichsee - Limmat, Entlastungsstollen
Hochwasserschutz Sihl - Zürichsee - Limmat, Entlastungsstollen