As a result of high water flows in August 2005, hydrological studies have been conducted that show that at Zurich, floods of the river Sihl with flood peaks of 310-600 m³s^-1 are possible. Based on these findings, the Cantonal Department of Waste, Water, Energy and Clean Air AWEL of Zurich decided to implement a regional flood warning system for the improvement of flood protection for the city of Zurich and the Sihl valley. The core of the flood forecasting system for the Sihl is the hydrological model PREVAH. Through construction at the main train station of Zurich (Löwenstrasse), flood risk is increased in the center of Zurich and at the construction site. In the case of critical predicted flood peaks of the Sihl, the following organizational measures are scheduled: (i) a preliminary reduction of the level of the lake Sihl to reduce peak flows in the area of Zurich main station and (ii) alarm organization to initiate a construction site clearance and a possible flooding of the construction site. Two different groups of experts decide about a possible preliminary reduction of the level of the lake Sihl or a construction site clearance and a possible flooding of the construction site. The system IFKIS-Hydro-Sihl was successfully used in the first phase of the construction project at the main station. This requires that the system is operated by well-trained staff which can deal in various situations with uncertainties of meteorological and hydrological model data and which is able to communicate these to the end users.

Project phases

• Phase immediate (2007-2008): In connection with the SBB train station construction site / Löwenstrasse, WSL examined how to deal with storms and heavy rainfall in the catchment area of the river Sihl.
• Phase medium term (2008-2009): WSL implemented a probabilistic flood forecasting system for the Sihl. Floods can already be detected early through a series of daily modeling, thus the safety of the city of Zurich in general and the situation during the construction activities at the train station Löwenstrasse (critical construction phase 2008 to 2011) in particular can be improved. Forecasts are presented in an easy to use visualization platform.

• Phase long term (2010-2011): The prediction system for the Sihl was one of the first regional modules implemented in the forecasting system FEWS (for "Flood Early Warning System") of the FOEN. The simulation results are also represented on the Common Information Platform for Natural Hazards in Switzerland (GIN) which was launched in March 2010. In addition, tools for the calculation of scenarios have been developed.

• Operational phase (2008-2015): Since 2008 (and until 2015) WSL ensures the operation of the forecast system. Every Friday, a report is written and sent to the end user.

• Regulation phase (2012-2015): Formulation of an optimized regulation of water release from the dam, with regard to the flood safety of the city Zurich, based on in-depth hydrological investigations.
• Phase "Limmat" (2012-2015): Assessment of the possible interplay between the Sihl and lake Zurich for increased protection against floods in the Sihl and Limmat valley.