Mont St Michel returns to the sea

December 08, 2015

Mont St Michel Barrage

A twenty year project to save the UNESCO world heritage site Mont St-Michel from becoming landlocked was inaugurated by France’s President Hollande in October 2015. Nickel-containing stainless steel plays an important role in this feat of engineering.

The Mont St-Michel is a granite rock situated about 600 meters off the northwest coast of France, at the mouth of the Couesnon River near Avranches. For centuries pilgrims to its abbey, founded in the 8th century AD, crossed the sands at low tide.

Over the years the tides and coastal rivers the Couesnon, the Sée and the Sélune have carried silt into the Bay of Mont St-Michel. By the 1990s the sediment accumulating at the foot of the rock was colonised by vegetation and there were fears that it would not be long before the Mont became attached to the mainland. The French State and local authorities joined forces with a massive reclamation project to construct a dam on the Couesnon River, as well as a new pedestrian bridge, new parking facilities and a transport shuttle to service the three million visitors to the historic site each year.

The new hydraulic dam controls the release of water to limit the build-up of silt in the bay and help flush the sediment out to sea. The eight sets of sluice gates clad with 36 tonnes of nickel-containing duplex Type 2205 (UNS S32205) stainless steel, selected for its corrosion and abrasion resistance, face out to sea towards Mont St-Michel leaving a clear view of the landscape. To the south on the land side can be seen the channelled river with the mechanical efficiency of the equipment producing a steady water flow.

The barrage also offers new public spaces over the water with viewing points on the promenade deck and marine balcony. And the new footbridge which replaced the old causeway allows waters from the sea and the Couesnon River to flow freely around Mont St-Michel as they originally did. In the spirit of the pilgrims of old, an almost invisible bridge which combines technical water management functions with its role as a public space for exploring and sightseeing, makes the crossing to the Mont St-Michel from the mainland a pleasurable experience.

As dam architect Luc Weizmann put it, “In this environment of churning water and energies, but also where people from all horizons can meet and explore a place steeped in history and culture, in the silence of the bay these spaces will encourage people to discover or rediscover how Mont St-Michel relates to nature, the water, the land and the sky”.

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