Grande Dixence Dam

The Grande Dixence Dam is a concrete gravity dam on the Dixence at the head of the Val d'Hérémence in the canton of Valais in Switzerland. At high, it is the tallest gravity dam in the world, fifth tallest dam overall, and the tallest dam in Europe. It is part of the Cleuson-Dixence Complex. With the primary purpose of hydroelectric power generation, the dam fuels four power stations, totaling the installed capacity to , generating approximately annually, enough to power 400,000 Swiss households.
The dam withholds Lac des Dix (Lake Dix), its reservoir. With a surface area of 4 km², it is the second largest lake in Valais and the largest lake above 2,000 m in the Alps. The reservoir receives its water from four different pumping stations; the Z’Mutt, Stafel, Ferpècle and Arolla. At peak capacity, it contains approximately of water, with depths reaching up to . Construction on the dam began in 1950 and was completed in 1961, before officially commissioning in 1965.
History.
In 1922, Energie Ouest Suisse (EOS) became established with a few small power stations. To generate substantial amounts of electricity, EOS looked to the Valais canton which contains 56% of Switzerland's glaciers and stores the largest amount of water in Europe. In 1927, EOS acquired the license for the upper Dixence basin. In 1929, 1,200 workers constructed the first Dixence dam which would be complete in 1935. The first dam would supply water to the Chandoline Power Station which has a capacity of 120 MW.
After the Second World War, growing industries needed electricity and construction on the Cleuson Dam began in 1947 and was completed in 1951. The original Dixence dam was submerged by the filling of Lac des Dix beginning in 1957, it can still be seen when the reservoir level is low.
Plans for the Super Dixence Dam were now being finalized by the recently founded company, Grande Dixence SA. Construction on the Super Dixence Dam soon began later in 1950. By 1961, 3,000 workers had finished pouring of concrete, completing the dam. 
At 285 m, it was the world's tallest dam at the time, but it was surpassed by the Nurek Dam of Tajikistan in 1972 (300 m). It remains the world's tallest gravity dam.
In the 1980s, Grande Dixence SA and EOS began the Cleuson-Dixence project which improved the quality of electricity produced by building new tunnels along with the Bieudron Power Station. By the time the Cleuson-Dixence Complex was complete, the power generated had more than doubled.
The construction of the dam was documented in the short film Opération béton, the first film directed by Jean-Luc Godard.
Characteristics.
The Grande Dixence Dam is a high, long concrete gravity dam. The dam is wide at its base and wide at its crest. The dam's crest reaches an altitude of . The dam structure contains approximately of concrete. To secure the dam to the surrounding foundation, a grout curtain surrounds the dam, reaching a depth of and extending on each side of the valley.
Although the dam is situated on the relatively small Dixence, water supplied from other rivers and streams is pumped by the Z’Mutt, Stafel, Ferpècle and Arolla pumping stations. The pumping stations transport the water through of tunnels into its reservoir, Lac des Dix. Water from the high Cleuson Dam, located to the northwest, is also transported from its reservoir, the Lac de Cleuson. Three penstocks transport water from Lac des Dix to the Chandoline, Fionnay, Nendaz and Bieudron power stations, before being discharged into the Rhône below. All the pumping stations, power stations and dams form the Cleuson-Dixence Complex. Although the complex operates with water being pumped from one reservoir to another, it does not technically qualify as a pumped-storage scheme.
Most of the water comes from glaciers when they melt during the summer. The lake is usually at full capacity by late September, and empties during the winter, eventually reaching its lowest point around April.
Power stations.
Chandoline Power Station.
The Chandoline Power Station was the power station for the original Dixence Dam. The Grande Dixence Dam submerged the original dam but the power station still operates with water received from the reservoir of the Grande Dixence Dam, Lac des Dix. The power station is the smallest of the four, producing from five Pelton turbines with a gross head of .
Fionnay Power Station.
The Fionnay Power Station receives water from the Grande Dixence Dam by a long tunnel with an average gradient of 10%. Once the tunnel reaches a surge chamber at Louvie in Bagnes, it turns into a penstock which descends at a gradient of 73% for until it reaches the power station. The water, now flowing at a maximum rate of spins six Pelton turbines, generating a combined maximum capacity of .
Nendaz Power Station.
After arriving at the Fionnay Power Station from the Grande Dixence Dam, water then travels through a pressure tunnel which eventually leads into the Péroua surge chamber, above the Nendaz Power Station. The water, which remains at a maximum rate of spins six Pelton turbines, generating a combined maximum capacity of .
The Nendaz power station is located within mountains between Aproz and Riddes and is the second-largest hydroelectric power station in Switzerland after the Bieudron Power Station.
Bieudron Power Station.
The water travels down a long penstock from the Grande Dixence Dam before reaching the Bieudron Power Station down. The water spins three pelton turbines, generating a combined capacity of . The power station was constructed after the Nendaz and Fionnay power stations. The power station was built by both Grande Dixence SA and Energie Ouest Suisse between 1993 and 1998 at a cost of US$1.2 billion.
The Bieudron Power Station alone holds three world records, for the height of its head (), the output of each Pelton turbine and the output per pole of the generators . It was taken out of service in December 2000 after the rupture of a penstock. The power station became partially operational in December 2009 and fully operational in 2010.