The Project
In 1878-18794, a dike-road was created to access Mont. A railway line from Pontorson to Mont was installed in 1872, with a steam tram as equipment. These infrastructures promote tourism to Mont Saint-Michel which rose from 100,000 in 1908 to 150,000 in 1930, which did not prevent the General Council of La Manche from closing the line, which had become in deficit, in 1938. Later, car parks will be built below the dike, mainly to the east, for parking many passenger vehicles.
In the 19th century, to improve the irrigation of agricultural land, canals were dug, diverting water from the Couesnon. This practice leads to a "reclamation" of the Bay, favoring its silting up. The dike-road, breaking the flow of the tide which revolved around the Mount, also accentuates the natural phenomenon of the bays silting up. Indeed, the flow of the rising tide is more important than the flow of the falling tide: the sediments are deposited at the bottom of the bays.
A first project to level the dike connecting the island to the mainland was launched in 1938 following debates opened in 1910 between the Department of Fine Arts and the Ministry of Public Works. The railroad was cut before World War II shut down the project.
In addition, a dam on the Couesnon was built from 1966 to 1969 to protect Pontorson mainly from rapid level rises. This dam, equipped with 6 gates that opened manually, contributed to the silting of the Couesnon, upstream and downstream of the gates. Before its construction, two million m3 could move up the Couesnon to the water in its channeled channel of a hundred meters wide and four kilometers long.
We could cite other reasons which led to the silting up of the Mont, but these are the main ones.
Since the 1970s, numerous studies, consultations, surveys, tests and models have been carried out (in particular the forecasts established from studies on a model from the SOGREAH Laboratory in Grenoble between 1997 and 20019, according to which the Mount should have lost all maritime character. in 1995 and that it would be entirely surrounded by grass in 2042), proof of the interest that the State had in the project. These documents aimed to determine the means of ensuring the restoration of the maritime landscape around the Mont.
In 1979, the Mont-Saint-Michel site was listed as a UNESCO World Heritage Site, and the bay in 2007.
Several prime ministers supported the project, but it was not until 1995 that the project to restore the maritime character of Mont-Saint-Michel was officially launched by Prime Minister Édouard Balladur.
The project was declared of public utility in 2003, after an impact study and 2 months of public inquiry. The sole designer of the project, the State participates in the financing of the project to the tune of 85 million euros out of a total of 184 million in public money, but in 2006 it withdrew from the operational implementation of the works which were entrusted exclusively to local authorities grouped together since 1997 in the Baie du Mont-Saint-Michel mixed union.
From June 2006 to June 2009, a new dam was built (piers, raft, equipment for the eight gates11 and the lock with two fish passes, promenade bridge with "maritime balcony", the old dam (20 meters downstream of the current one) being destroyed in November 2008) on the Couesnon. Designed by architect Luc Weizmann, it was commissioned in September 2009 and provides a hunting effect13 at the ebb tide with a maximum flow of 100 m3 / s, ten times the average natural flow of the Couesnon. Hydraulic works were carried out downstream from 2012 to 2015 to allow the erosive straying of the Couesnon: creation of two channels separated by a riprap partition sill, fitted with groynes, dismantling of historic riprap lines. The upstream hydraulic works consist, after the preliminary cleaning and pruning of the banks, in dredging the Couesnon and digging the Moidrey Cove with nine kilometers of canals.
The 2013 report of the Court of Auditors finds this project “badly conducted” and questions its relevance20. The precariousness of the new conditions of access to the site and the relocation of the car parks to the coast are also judged by catering professionals and traders, "unsuited to the tourist offer".
After 9 years of study, 3 years of work and many controversies, the Mont-Saint-Michel footbridge was inaugurated: on July 22, 2014, visitors can now get to the Mont by this access structure created by the architect Dietmar Feichtinger. A new dike and a bridge on stilts letting the water pass below will now serve the island. The old dike-road will be demolished as the new one is built.
On October 29, 2015, the President of the Republic François Hollande officially inaugurated the dam on the Couesnon, an essential element of the maritime reconquest of Mont-Saint-Michel.
The Dam
The 80-meter-long dam, designed by the architect Luc Weizmann, has a functional and aesthetic role: covered with a wooden deck evoking the deck of a ship, surmounted on the side by a console 60 cm wide, it also offers an unprecedented panoramic view of the bay and the rock. Visitors can also "wander and sit for contemplation."
To remedy the dysfunctions caused by the old dam, the Syndicat Mixte decided to build a new one with a hunting lock. Its function is no longer to protect only the municipalities located upstream from flooding, but also to regulate the Couesnon coast and to remove the sand upstream and downstream of the dam by an emptying effect. This new dam is located immediately downstream of the old one, which was destroyed in November 2008. The new dam was filled in spring 2009, put into service in May 2009, and it has been open to the public since June. 2010. It is designed differently: there are 8 passes instead of 6 previously. Each gate measures 9 m by 7.40 m and weighs 30 tonnes and is of the “sector” type, instead of flush doors operated by the pressure of the water. This new structure is automated, with several control modes. The dam operates according to the tide schedules, the water height at the foot of the dam and the turbidity of the water. It allows a flow rate of 100 m3 / s.
The dam is designed to optimize the flushing effect of the ebb tide and thus "clean" the immediate estuary of Couesnon of the 570,000 m3 of sediment that clutters it. The principle of operation is as follows:
At rising tide: the valves close (rising to the maximum), the tidal water is blocked at the foot of the valves and settles as and when. Once the high tide is reached, the gates gradually lower and the settled water enters through the top of the gates into the Couesnon. The sediments are deposited at the foot of the sluices. This water will spread in the Couesnon and fill (according to the tidal coefficient) the Anse de Moidrey. This storage of clarified seawater can reach up to 1.2 million cubic meters.
At ebb tide: when the flow reverses and low tide begins, the valves close. The ebbing tide begins to remove the sediments that have settled at the foot of the sluices. When the force of the flow begins to weaken, the gates are raised and the water from the Couesnon, previously stored, is released and flows into the estuary, through the bottom of the gates this time, creating a flushing effect under the gates. which will amplify the work of sand removal initiated by the tide.
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