Zurich builds CHF175 million flood tunnel as drought worsens
Tuesday 11th August 2026 on 21:30 in
Switzerland
Zurich is completing a CHF175 million flood-relief tunnel as drought leaves rivers with low water levels and fish under heat stress, SRF reported.
The project’s nearly 90-metre outlet channel is being built at Thalwil on Lake Zurich. Almost 13 million litres of water, equivalent to about five Olympic-sized swimming pools, were pumped from a sealed construction pit up to 12 metres deep.
The structure is scheduled to enter service in early 2027. During extreme flooding, it will divert some of the Sihl river’s water directly into Lake Zurich through the tunnel, which is more than two kilometres long.
The tunnel is intended to reduce the risk of flooding large parts of central Zurich and the main railway station. The canton estimates the potential damage from flooding at almost CHF7 billion.
Martin Neukom, Zurich’s cantonal director of public works, said the canton had identified further flood risks, including the Eulach in Winterthur and the Altbach in Kloten. Protective projects are being used to address those risks, while a preliminary project is currently under way for the Trepsenbach in Dietikon.
Neukom said the contrast between current drought conditions and the flood-protection project was not a contradiction. Climate change was making both dry periods and heavy rainfall more frequent and more intense, he said. The tunnel had therefore been designed for future weather extremes and very severe flooding, although no structure could provide absolute protection.
At present, however, some waterways in the canton are carrying little water, temperatures are high and fish are under stress. If the drought continues, it could also affect construction of the tunnel.
Project manager Adrian Stucki said work might not be carried out as planned because the waterways were too warm and carried too little water. The project team did not want to place additional stress on fish populations and risk causing them to die.
“Too much water is difficult, but too little water is too,” Neukom said, adding that the options for dealing with drought were much more limited than those for flood protection.