Grande Dixence Dam

01 In brief

At the head of the Val d'Hérémence, high above Sion in the Swiss canton of Valais, stands the tallest gravity dam ever built. The Grande Dixence is 285 metres high and holds back the Lac des Dix, a reservoir of 400 million cubic metres. Completed in 1961 after a decade of construction, its 5.96 million cubic metres of concrete weigh about 15 million tonnes — more than the Great Pyramid of Giza.

The dam is only the visible part of Switzerland's most powerful hydroelectric complex. Meltwater from 35 glaciers is gathered by about 100 kilometres of galleries and fed into the reservoir, then dropped through power stations at Fionnay, Nendaz and Bieudron with a combined output of around 2,000 megawatts. In effect, the Grande Dixence is a battery: it charges up with summer snowmelt and discharges into the winter peak.

02 Why it's interesting

• 01 At 285 metres it is the tallest gravity dam in the world and the tallest dam of any type in Europe.
• 02 Its 15 million tonnes of concrete make it heavier than the Great Pyramid of Giza.
• 03 The reservoir is refilled by meltwater from 35 glaciers, routed through about 100 kilometres of galleries from catchments between the Mischabel, Matterhorn and Mont Gelé massifs.
• 04 The Bieudron powerhouse, completed in 1998, drives three Pelton turbines of 423 megawatts each under a 1,883-metre head — world records at commissioning.

03 How it works

A gravity dam resists the pressure of the water behind it in the most direct way available: sheer weight.

The Grande Dixence is 700 metres long along its crest, 15 metres wide at the top and 200 metres thick at the base. The concrete was poured as blocks some 16 metres thick, with special joints designed for cohesion, resistance and impermeability, and a grout curtain sealed the foundations down to a depth of 200 metres. Inside the wall, 32 kilometres of tunnels and shafts let engineers observe the structure's behaviour; dam keepers walk those galleries regularly and monitor the surrounding cliffs for any change.

What makes the scheme exceptional is the water supply. The natural catchment of the Dixence is modest, so the engineers borrowed water from across the region: 75 intakes and a chain of pumping plants gather meltwater from 35 glaciers between the Mischabel, Matterhorn and Mont Gelé mountains and carry it through about 100 kilometres of galleries — the main collector runs 24 kilometres inside the mountain at 2,400 metres — into the Lac des Dix, on average around half a billion cubic metres of water a year. The reservoir therefore works as seasonal storage, filling with summer melt and draining through the winter half-year, when Swiss electricity demand peaks.

Generation happens in three stages. Water dropping from the reservoir drives the Fionnay and Nendaz power stations — together about 680 megawatts — and, since 1998, the Bieudron station near the banks of the Rhône, which alone produces 1,269 megawatts from three Pelton turbines of 423 megawatts each under a head of 1,883 metres. After the turbines, the water is returned to the Rhône. In a typical year the complex generates on the order of 2,000 gigawatt-hours.

Dam, reservoir and collection-network figures from Grande Dixence SA's technical pages; generation, household and storage-energy equivalents from the operator and Alpiq's asset page, with trade-press corroboration.

04 The organisation behind it

The dam belongs to Grande Dixence SA, a company headquartered in Sion whose largest shareholder is the energy group Alpiq with a 60 per cent stake. Construction ran from 1951 to 1961 and commissioning began in 1961; the Cleuson–Dixence extension that added the Bieudron powerhouse was built between 1993 and 1998, more than doubling the complex's output.

05 In the real world

The Grande Dixence matters because of when its water arrives. Swiss hydropower output broadly follows the melt season, while demand follows the heating season, and storage dams are how the gap is bridged — the complex's roughly 2,000 gigawatt-hours a year, the equivalent of 500,000 households and about a fifth of Switzerland's storage energy, are generated disproportionately in winter. The dam has also become a fixture of Valais hiking country, from easy walks along the reservoir to the Ibex Trail above it.

06 Can you visit?

07 Sources & further reading

1. Grande Dixence (dam page) — Grande Dixence SAgrande-dixence.ch
2. Production plants — Grande Dixence SAgrande-dixence.ch
3. Collecting works — Grande Dixence SAgrande-dixence.ch
4. Guided tours inside the dam — Visit Grande Dixencevisit-grande-dixence.ch
5. Grande Dixence (asset page) — Alpiqalpiq.com
6. Grande Dixence — an engineering achievement — Water Power Magazinewaterpowermagazine.com