Bissorte Dam

The Bissorte Dam (Barrage de Bissorte) is a gravity dam in the Maurienne Valley, in Savoie, France, about 7 km east of Valmeinier. It was built from 1930 to 1935 to supply a hydroelectric plant capable of generating 75 megawatts (MW) of power.

The complex was reconfigured from 1980 to 1986 in order to add a 750 MW pumped-storage power plant known as Super-Bissorte, the third most powerful in France after Grand'Maison and Montézic.

Location
The Bissorte Dam is perched on an ancient glacial basin, now alpine tundra, in the Massif des Cerces. It is close to the Valfréjus ski resort in the commune of Modane, near the Italian border.

History
During the 1920s, a period of intense stock market speculation on hydroelectricity in France, Bissorte's potential as a building site drew much attention. In 1930, work began on the construction of a 545 m gravity dam with a stone facing to protect its concrete structure. The power plant, known today as Bissorte 1, was put into operation in 1935 and can output 75 MW in a matter of minutes thanks to its three Pelton turbines.

In 1980, the electric utility company Électricité de France (EDF, Electricity of France) built a second dam, the Pont des Chèvres, on the Arc River below Bissorte and reorganized the power plant into a pumped-storage facility. Through the use of a new underground penstock, water that had been released through the turbines during times of peak energy demand would be pumped from the lower reservoir back into the upper one during times of lower consumption, replenishing the plant's supply of water and enabling a balanced production of power. Super-Bissorte, the new addition to the complex, was commissioned in 1986. It comprises two underground facilities, called Bissorte 2 and Bissorte 3, which can generate 750 MW through the use of four 150 MW reversible Francis pump-turbines and a 150 MW Pelton turbine.

The Bissorte concession agreement, which grants EDF the rights to operate the facility, expired at the end of 2014 and will be put up for bid, according to the Law of 16 Octobre 1919.

Design
The Bissorte Dam, which forms the upper reservoir (2050 m elevation), is a gravity dam on the Bissorte River (8.8 km long). It is 545 m long and 49 m wide at the base. A stone facing protects its concrete structure. The reservoir, Lake Bissorte (Lac de Bissorte), holds a water volume of 40,000,000 m3 that can now be brought to a maximum of 100,000,000 m3 after additional work was completed in 1957.

A second reservoir, situated on the Arc River downstream from the Bissorte Dam, is formed by the Pont des Chèvres Dam (925 m elevation), a gate-structure dam. It serves to stabilize the flow of tailwater from the Bissorte and Orelle plants, and functions as the lower reservoir in the pumped-storage system utilized by Super-Bissorte, one of France's major pumped-storage power stations, known as STEP (Stations de transfert d'énergie par pompage). The reservoir holds 1,500,000 m3 of water.

Penstocks
The original penstock, installed to pipe water into Bissorte 1, was a novelty at the time of its construction thanks to its stiffener rings, metal bands that provide reinforcement against the high tensile pressure of the water. A second penstock was installed underground with the addition of Super-Bissorte.

Hydroelectric plants
Bissorte 1 is equipped with three horizontal Pelton turbines that can output 25 MW each. It is 1144 m below the water level of Lake Bissorte.

Super-Bissorte is composed of two underground facilities: Bissorte 2, outfitted with four 150 MW reversible Francis pump-turbines, and Bissorte 3, furnished with a 150 MW vertical Pelton turbine.

Pumped-storage system
Super-Bissorte, like the other STEP facilities, is a strategic element of France's power grid. It provides a way to restore electricity in case of a power outage, like the one that occurred during the Europe-wide blackout on November 6, 2006. Due to an error of judgement on the part of a German electric company, French and Spanish power networks found themselves in abrupt isolation and unable to meet demand. However, the Réseau de Transport d'Électricité (RTE, Electricity Transmission Network) was able to offset the shortage by mobilizing nine hydroelectric dams: “At 22:15 hours, the hydroelectric plants of Bort, Montézic, Grand Maison, Villarodin, and Sarran-Bromat augmented production by 2,800 MW, followed by an increase of 1,140 MW at 22:20 hours from the Tignes, Super-Bissorte, La Bathie and Monteynard plants.”