Synergies entre la production hydroélectrique et la protection contre les crues : cas d'étude de la Sihl en Suisse

Located at the confluence of two rivers, the Limmat and the Sihl, the city of Zurich has always faced the risks posed by flood events. Following the 2005 flood event, a tunnel project was launched to partly divert flood water from the Sihl to Lake Zurich upstream of the city centre. As an alternative to such a diversion tunnel, the diversion of excess flood water through the 80-year-old existing Etzelwerk pumped-storage scheme would present the advantage of a synergy between two objectives: enhanced flood protection on the one hand and the rehabilitation of renewable energy production infrastructure with an increase in installed capacity on the other. The concept consists in managing the water levels of the upstream reservoir on the Sihl upstream of Zurich, as function of meteorological and river flow forecasts. Flood routing and flood peak attenuation are obtained through the preventive drawdown of the upper reservoir, whilst producing hydropower. Reservoir drawdown must however respect stringent rules of water level variation, given its location in a densely populated catchment. The performance of the concept is highly dependent on the reliability of the weather and river flow forecasts. The main consequence is an increase in the rated discharge and installed capacity at the Etzelwerk powerplant, justified by its contribution to flood protection. The paper presents the hydrological and hydraulic flood routing studies carried out to assess the performance of different powerhouse discharges in protecting Zurich from floods with return periods between 300 and 500 years. A Routing System model of the watershed was prepared and calibrated over a period of 32 years including major flood events of recent years. Using rainfall load cases of 100, 300 and 1000 years return period, different flood events were produced and routed through the system considering different powerhouse equipment discharge. As a result, a discharge of 147 m(3)/s (four times more than the present discharge), was identified as the minimum discharge fulfilling the required reduction of the flood risk level in Zurich, leading to an installed capacity for the Etzelwerk scheme of 600 MW.