Zurich – A new underground tunnel between Langnau am Albis and Thalwil will direct floodwater from the River Sihl into Lake Zurich. The groundbreaking ceremony for the project took place on 18 March. Construction work for the tunnel, which is around two kilometers long, is scheduled to be completed in 2026.

From 2026, extreme flood peaks in the Sihl are to be channeled into Lake Zurich via an underground tunnel, according to a statement from the canton of Zurich's building department. The tunnel, which will be two kilometers long with a diameter of 6.6 meters, will stretch between Langnau am Albis and Thalwil. On 18 March, the groundbreaking ceremony marked the start of construction.

Extreme flooding of the Sihl could cause severe flooding in the lower Sihl valley and in the city of Zurich, the statement said. The relief tunnel could avert possible damage for the city of Zurich alone of up to 6.7 billion Swiss francs.

The costs for the planning and construction of the tunnel, as well as for the associated ecological replacement measures on the Sihl and on Lake Zurich, are estimated at around 175 million Swiss francs. They will be largely borne by the canton of Zurich, with the federal government, the city of Zurich, the SBB and the Sihltal Zurich Uetliberg Bahn SZU AG sharing in the costs. Further information on the project will be provided on the internet by the canton of Zurich.