River Sihl-Lake Zurich-River Limmat Flood Protection, Relief Tunnel

Hochwasserschutz Sihl - Zürichsee - Limmat, Entlastungsstollen

Ideal locations were identified for the inlet structure in Langnau a. A. (Sihl) and for the outlet structure in Thalwil (Lake Zurich) and optimised in terms of hydraulic operation and costs. Flood peaks with a strong retention effect will be diverted via Lake Zurich into the River Limmat. This will provide sufficient protection for Zurich’s city centre and upstream, should the River Sihl burst its banks. It was possible to demonstrate the feasibility of a solid, lasting, yet simple flood protection concept, which was reaffirmed and fine-tuned with the aid of hydraulic bed-load model tests at the Swiss Federal Institute of Technology (ETH) Zurich.

Main Data

  • Inlet structure with a 130 m long longitudinal defence and reservoir
  • Free-flow tunnel with supercritical flow (length: 2.1 km, inner diameter: 6.6 m, gradient: 1.3 % - 3.3 %)
  • Outlet structure with stilling chamber and outlet (length: 90 m, 3 m below the surface of Lake Zurich)


    Zurich Canton, Office of Waste, Water, Energy and Air (AWEL)


    IUB Engineering Ltd.

    Construction period

    Design Phase: 2010 to date

    Construction costs

    Cost estimate: CHF 109 million

    Scope of services

    • Feasibility study with detailed study of options within the framework of a participatory process
    • Support in hydraulic bed-load model tests
    • In-depth feasibility study
    • In future: overall design for preliminary project, construction project and detailed planning for construction with optional services up to commissioning