Projet Mont Saint Michel - Hydrosedimentary surveys

Whilst the Bay is silting up in a natural process on a scale beyond any counteracting human endeavour, we can at least remove the manmade objects responsible for the most undesirable effects.

The combined action of the sea and the Couesnon will flush the sediment out from around the Mont, isolating it amid a vast expanse of sands.
80% of this target will be reached within 8 years of commissioning the dam.

Trials on scale models at the SOGREAH laboratory have gradually suggested ways of recreating in the vicinity of Mont St Michel this environment of sands constantly redrawn by the tides so characteristic of the everchanging marine landscape in the Bay.

A large scale model of the Inner Bay

-> Hydrosedimentary Brochure

In its impact study on the site of the planned works, SOGREAH worked from digital and physical models of the Outer Bay, the Couesnon and the dam and from a large (900 m²) 3D scale model of the Inner Bay (43 km²) in order to simulate the silting up process there.
Using this model, a "reference trial" covering 45 annual cycles showed what would happen to the Inner Bay were nothing to be done to it. By 2042, the salt marshes would have gained 50 hectares as compared with 1997, and the site's marine environment would have been irretrievably lost.

A series of short trials to try out various types of modifications finally led to an effective long-term solution being found.

Recovering the sea bed

Outcome: by 2042 around the Mont, the sea bed's mean level will be 70 centimetres lower; the marine area (under 5m above sea level) will gain 50 ha.
In a way this will take several decades off the site's age!

Figures aside, the Couesnon will open out into a broad deep water estuary. Its future will no longer depend on the Sée and the Sélune. Its wandering course will take it over a vast expanse of sands.
At last Mont St Michel will recover its much sought-after marine landscape and stay that way for generations to come.

The modifications to be carried out will have little or no effect on the Inner Bay.
The scientific committee took the view that the long-term future of the Inner Bay was in no way connected with the planned works on the dam and around the Mont.

Traditional activities in the area (oyster farms, shrimping etc.) will not be affected. Likewise, walkers will continue to stroll across the sands and take in the natural beauty of the Bay.

-> Maps