Barrage de Mauvoisin Projet de surélévation de la prise d'eau et vidange, impacts sur l'environnement

Mauvoisin dam - Service and bottom outlet raising project Environmental impacts

Biologiste diplomé ECOTEC Environnement SA, rue François Ruchon, 3. CH-1203 Genève

Auteur de correspondance : durand@ecotec.ch

Abstract

The Mauvoisin dam is located 1975 masl in the Alps of the Valais Canton in Switzerland. With a height of 250 m, the dam was built on a Rhone River tributary named the Dranse de Bagnes. The powerhouse is located on the river plain at 476.8 masl. The delivered power reaches 386 MW. During the last 30 years, sedimentation amounts to more than 10 million cubic meters of deposited material in the lake reservoir. Measurements indicate that the sediment layer can easily reach 20 to 40 cm in the vicinity of the service and bottom outlets. To preserve the storage capacity of the dam, it was decided to raise the service and bottom outlets. However, this would imply the flushing of more than 1 million of cubic meters of accumulated sediments down through the Rhone River into the Geneva Lake. An environmental impact assessment study was required to evaluate the transport conditions of such quantities of suspended material and to define measures for mitigating negatives effects. Even if the environmental study covered thoroughly all impacts and mitigating measures, public appraisal was negative and new solutions had to be considered. As for most dams, sedimentation is generally a strong threat for reservoir operation and perenniality. Unfortunately, hydro-systems are strongly affected by reservoir flushing. This opens the quest for alternative measures of sediment management.