In the framework of the construction of the new Nant de Drance Pumped Storage Scheme it was decided to increase the rated capacity of the power plant from 600 to 900 MW. This implied that the capacity of the existing upper reservoir had to be increased, which in turn implied that the dam would have to be heightened. 

The Vieux emosson dam

The 55-metre high Vieux emosson dam was initially completed in 1955, sitting at an altitude of 2'200 metres in the municipality of Finhaut in the canton de Valais, the site contains a second dam just downstream called the Barberine dam named after the torrent that feeds the reservoir which was submerged as a result of the construction of the  emosson dam further downstream in the 1970s. 

What did the dam heightening works entail?

The project converted the existing arch gravity dam into an arch dam by heightening it from 55 to 76.5 m (crest length 200 m). For geometrical connection reasons, the upper part of the existing dam has to be demolished (15’000 m³). A new 336 m³/s spillway equipped with a new outlet canal was also foreseen. The heightening enabled the storage volume to be increased from 11.2 to 24.6 cubic hectometres. The project necessitates the placement of 75'000 cubic metres of concrete. The construction was completed in July 2016 and the commissioning continues until August 2020.

The resulting Nant de Drance Pumped Storage project 

The Nant de Drance pumped storage project takes advantage of the difference in water levels between the existing reservoirs of Emosson (1930 masl) and Vieux Emosson (2205 masl). It will be equipped with 6 Francis pump-turbine of 150 MW each (900 MW, 2.5 billion kWh). The new scheme integrates two state-of-the-art technologies, a conventional pump-turbine and a variable-speed pump-turbine. The conventional unit pumps water into an upper reservoir during low energy demand and releases it to produce energy during peak times. The variable-speed unit regulates the level of the energy it consumes, contributing to better grid regulation. Conventional pump-turbines can only operate on a fixed quantity of energy, while variable-speed units can regulate the level of energy they consume. As a result the variable unit continues to function even at lower energy levels, assuring a steady refilling of the reservoir while contributing to stabilization of the network (more than 80% efficiency). The project includes 17 km of underground galleries and two vertical shafts of 425 metres high each. Excavated volume: 1.7 million cubic metres.

What services did Gruner provide? 

Gruner Stucky Ltd was mandated to do the design and works supervision of the dam heightening. This is invloved the

• the study of alternatives preliminary assessment;
• 3D numerical modelling of the existing and heightened dam
• Assessment of heightening repercussions on appurtenant structures;
• Bill of quantities, costs, schedules, construction permits;
• Elaboration of Tender documents and evaluation to the tender responses;
• Coordination with the Nant de Drance construction site;
• Elaboration of construction drawings

Work supervision

In addition to the dam heightening, Gruner Stucky Ltd was contracted in January 2014 by the Owner to assist the Consultant for the supervision of the construction works of the pumped storage's powerhouse and tunnels. 

Gruner's Dam heightening services page

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