Since commissioning of the power stations Mauvoisin in 1956 (designed by Pöyry), relatively large sedimentation occurred in the Mauvoisin reservoir consisting of deposits of fine sediments, called 'Gletscherschliff' (approximately 330,000 m3/ yr). The sediment deposits have reached the intakes of the power and bottom outlet tunnels. If the water level is lowered under a critical level, this can lead to a large entry of suspended sediment load into the waterways and cause abrasion of the turbines, as well as blockages of valves.
In a planning study, various options were examined for the prevention or reduction of the sediment entry, with the result that new intake structures shall be provided for both the tunnels: For the power intake, around 38 metres, and for the bottom outlet, around 36 metres higher than the existing ones. The new intake structures must, via new tunnel sections, be connected to the existing tunnels. In addition new safety valves (butterfly valves and high pressure sliding gates) are being installed, in new underground chambers and adits.
As the Owner's engineer for renewal of intakes for power tunnel and bottom outlet, Pöyry provided feasibility study; final design, specifications and tender documents; tender evaluation; detailed design and civil works; construction drawings; review of suppliers' designs for electro-mechanical equipment; supervision of construction, factory tests, installation and commissioning of equipment.