Ten years of repeated hydraulic flushing to slowly turn the clock back on over fifty years of silt building up... This is how the project is to rid the Archangel Michael’s mount of these deposits that encourage plant growth in its immediate vicinity.
Five million m3 of sediment are to be removed by the Couesnon itself, lowering the level of the sands by an average 70 cm within a 1 km radius.

To do this, the Couesnon first needs to regain its hydraulic power capacity, much of which has been lost. Cleaning the river bed up and downstream of the dam to increase the volume of water stored will help it push out the sediment constantly pouring in on every tide.

Before the dam was built at La Caserne in 1969, the sea was naturally channelled up the Couesnon on the rising tide, then swept over the sands around the Mont on the ebb tide. Today, the river is no longer doing its natural job of collecting and releasing this water. The silt building up on the river bed and downstream from the dam are causing it to lose its hydraulic power capacity as the river meanders lazily through the sediment and plant life.
Once the river bed has been cleared, the channel of the Couesnon linked to Moidrey Cove will be able to store up to 1,500,000 m3 of water. This flushing water will be partly incoming sea water managed by the future dam, plus outflowing river water.
As the Couesnon fills up over and over again with faster flowing water, it will keep itself clear through this natural mechanism, with improved flow-off, including when in spate.

Dredging of the river bed chiefly over 4 km from the dam to Moidrey Cove, with lighter work as far as Pontorson : the geometry of the river banks will remain unchanged, and their protective rockfill is to be made good.
Certain drainage outlets from the neighbouring polders will be refitted with valves or sluices. The reed marshes are to be replanted a few dozen yards away in the parking area.
Likely method (awaiting confirmation by the architect) : suction dredging and discharge as far as Moidrey Cove in a decantation chamber (80% water). The 250,000 m3 of sediment extracted from the bed of the Couesnon to be used on the spot to shape the land for the future reservoir to be created in Moidrey Cove.

For several years now, the Syndicat mixte, the Manche Chamber of Agriculture and the Mission Mont-Saint-Michel have been working on how to make best use of the biologically safe tangue extracted from Moidrey Cove to raise the land where the level is low. A 50 cm layer of silt would bring 250 hectares of farmland up to level. Contacts with farmers within a 5 km radius have accounted for all the extracted material to be reused, altogether nearly 1,250,000 m3 of sediment. The tangue is to be taken by lorry, conveyor belt or using hydraulics to cut down on lorry traffic.
Some of the salty tangue (from the initial sections of the channels in the bay) may be used to shore up the coastal dykes in the polders east of Mont St-Michel, between La Caserne and the headland at Roche Torin.

For a long time, Moidrey Cove was worked to quarry tangue* from the bay, with up to 300,000 m3 of this sediment being extracted every year. Channelling of the Couesnon (19th century), polderisation and installation of sluices (the current dam at La Caserne) have slowly helped to cut off this former bend in the river from the sea. Today, filled in and drained, it is no longer reached by tidal waters.
On its left bank, a water reservoir containing up to 770,000 m3 is to be created (35 of the 120 hectares of the cove). Added to the bed of the Couesnon, this reservoir will lower the water level in the river. This will increase the speed of the flushing currents. Once the ecosystem is stabilised, the area will also be used for public educational purposes.

The reservoir will come in the form of a network of channels (10 km of “strings” or channels split into 3 main arms) that is both in keeping with the bay’s salt meadow landscape and less liable to silt up.

Likely method (awaiting confirmation by the architect) : extraction using two combined techniques : by suction dredger and mechanical digger. The million m3 of sediment extracted in this manner – plus another 250 000 m3 from the decantation chamber - is biologically safe. It is to be recycled exclusively to fertilise local farmland, with the farmers’ and landowners’ consent.

* tangue is the sediment left by the sea and which builds up on the beaches in the bay of Mont St-Michel . It used to be spread on farmland as a limey soil improvement. This silver-grey mixture of sand and finely ground seashells gives the sands a special light quality and whitens the land in tillage.

For several years now, the Syndicat mixte, the Manche Chamber of Agriculture and the Mission Mont-Saint-Michel have been working on how to make best use of the biologically safe tangue extracted from Moidrey Cove to raise the land where the level is low. A 50 cm layer of silt would bring 250 hectares of farmland up to level. Contacts with farmers within a 5 km radius have accounted for all the extracted material to be reused, altogether nearly 1,250,000 m3 of sediment. The tangue is to be taken by lorry, conveyor belt or using hydraulics to cut down on lorry traffic.
Some of the salty tangue (from the initial sections of the channels in the bay) may be used to shore up the coastal dykes in the polders east of Mont St-Michel, between La Caserne and the headland at Roche Torin.
The contract for the architect responsible for the water management modifications also includes demolition of the parking areas and the causeway, producing material that can be reused for road building work.

Upstream from the Couesnon dam, Moidrey Cove is to be turned back into wetlands for the benefit of the flora and fauna. When the tide comes in, the water will be collected in a basin along 10 km of channels running through it, gradually flowing back out on the ebb tide.

A study on how to enhance the Moidrey Cove environment was carried out on scale models and in the field by engineers and ecology scientists. A number of reservoir variants led to the choice of a "digitate" structure to keep down sedimentation. It will work like the meandering creeks (channels) and blend in with the surrounding scenery.

A tidal reservoir, as it is called in ecology, is to be recreated.This will foster biodiversity in the cove itself. The site will doubtless remain fairly poor in terms of biomass and highly selective owing to the changing nature of the ecological parameters, but there will be nothing like it for 100 km in all directions.
This arrangement below sea level will encourage vegetation to grow naturally in tiers according to water levels and salt tolerance.

The modification keeps much of the wetlands needed by birdlife. The tidal reservoir will resemble Moidrey Cove as it was from 1850 to 1930. Two very distinct estuary areas will form again, a river one upstream from the cove, and a marine estuary downstream. The usual buffer zone between the two will be centred on Moidrey Cove.
To foster public interest, an educational enhancement of the modification is planned once the ecosystem has been stabilized.