For the first time in its almost 40-year history, Lac d'Emosson in the canton of Valais will be drained dry between February and May 2012 in order to carry out maintenance work on the installations which are normally submerged. This is the first time the reservoir has been drained since being filled in 1973. A compatibility study has been conducted to devise measures to protect the environment and fish fauna.

Rarely is a reservoir completely drained, but Lac d'Emosson in the Trient Valley, canton Valais, is to be drained dry for the first time since it was filled with water in 1973. This exceptional measure will enable the normally submerged lake sections to be serviced.

The work will focus on the gate valves and bottom drainage. The gate valves at the head of the penstock will be renovated and upgraded. The owner, Swiss-French Électricité d'Emosson S.A., will also overhaul the dewatering conduit. Électricité d'Emosson S.A. is also taking this opportunity to carry out checks and maintenance work throughout the facility: The penstock from Ravoire and Corbes will be recoated in order to protect the steel lining against corrosion and to reduce friction loss. The tail underwater channel between La Bâtiaz power station in Martigny and the Rhone will be overhauled. Vallorcine power station, situated in France, will have the capacity of its turbine increased and be fitted with a new generator. As a result of the reservoir drainage, the plant will be out of operation for around 90 days between 14 February and 18 May 2012. The work, costing around CHF 20 million, will be carried out under difficult winter conditions. During the drainage operation, Lac d'Emosson will remain inaccessible and the access road will remain blocked until May. 

Environmental and fishery measures implemented
To identify the required environmental measures and monitor their effectiveness, Électricité d'Emosson S.A. conducted an environmental compatibility study in compliance with the applicable regulations in canton Valais and France. By measuring the underwater particle content, the trout stock in the affected streams can be better protected. The rinsing operation planned on completion of the work will restore the diversity of the natural eco-morphology which is necessary for the fauna. In conjunction with the Canton of Valais Office for Hunting, Fisheries and Wildlife, a protocol for monitoring the fish fauna and the fishery measures has been drawn up. Électricité d'Emosson S.A. has pledged a contribution of CHF 100,000 for restocking the lake with fish after drainage. The fish stock is expected to fully recover within three years. Fishermen need have no concerns about increased restrictions for the season beginning in 2012.