An extreme precipitation event with subsequent flooding of the Sihl river could cause damage in Zurich in excess of CHF 6.5 billion. The Office of Waste, Water, Energy and Air (AWEL) has therefore developed various measures in recent years to ensure protection against flooding of the Sihl. Following an assessment of various options and a broad-based consultation process with the stakeholders involved, the primary long-term measure chosen as the most suitable variant was a spillway tunnel from the Sihl into Lake Zurich at Langnau am Albis or Thalwil.

TBF supported AWEL technically and administratively in a client-support role from the preliminary study to the approval procedure.