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Estuaire de la Vilaine : des dragages d’entretien innovants et sous contrôle

Document du 05 février 2006 de l’IFREMER

The laboratory Ifremer Environment Resources Morbihan-Pays de Loire (LER / MPL) of the Trinity-sur-Mer is currently involved in environmental monitoring of maintenance dredging of the estuary of the Vilaine (Morbihan). All these operations dredging is conducted in a coordinated manner that might prove exemplary.
The estuaries are home to heavy sedimentation in the estuary of the Vilaine is no exception. The construction of the dam Arzal in the late 60 has accelerated the phenomenon by blocking the migration of muddy cap at the dam. The silting has profoundly altered the ecosystem and disrupts port activities. To maintain access to the ports of the estuary and marine activity (fishing, mussel breeding, yachting) of the work of dredging is essential. They are produced in contracting and financing of the institution Alteration of the Vilaine (IAV), which manages the dam Arzal.
An active role of the various players
It is recognized that the dumping of dredged sediments are a channel for the transfer of contaminants into the marine environment. The operations of dredging and dumping can therefore be achieved by taking into account the constraints of preservation of coastal ecosystems. As part of the estuary of the Vilaine, these works are the subject of a broad dialogue among the various actors: government agencies, elected officials, professional associations, scientists gathered in a Committee Estuary, an adjunct Scheme Development and Management of Water (SAGE) of the Vilaine. This consultation has resulted in a dredging program that meets the two goals: interest of users and limiting impacts to the coastal ecosystem and customs attached (fishing and mussel breeding).
Volumes smaller but more often
To limit the impact on the marine environment and customs, the solution adopted for the dévasage ports Camoël (hold Old Roche) and Tréhiguier (common Pénestin) is the technique of rotodévaseur (see photo), borrowed from charentais oyster farmers. The principle is to sustain an annual access to port works. Volumes set games will be low with a significant impact on the marine environment. The sediments are re-suspended in the strong tidal waters (coefficients greater than 70) and are released into the estuary at ebb tide by tidal currents. The re-suspension of sediments is possible because it is not contaminated sediments. Volumes set games are low (5000 m3 in Vielle Roche and 25000 m3 to Tréhiguier the first year) and marginal compared to the volumes of sediment passing through the estuary.
Work under control
The period of work has been defined together with the various users and Ifremer to limit their impact. Work has started on October 15, 2005 and will end on February 28, 2006, at a time when biological activity is slowed. The work is supervised by different followed made by the LER / MPL: continuous monitoring of turbidity at the port of Tréhiguier, weekly monitoring stations on 4 microbiological contamination, a state before and after work of the chemical contamination of mussels 'livestock.
And for the sandy sediment?
More spectacularly, the technique used for the dredging of the channel access port Billiers, another port on the estuary. The dredge Fort Boyard "ownership" of the department of Charente Maritime uses the technique of overflow to disperse the fine sediment at ebb tide, but retains the sands, which are then returned for the reloading of the beaches close. Tests reloading of direct Bétahon beach sand dredged from the port of Billiers were conducted in the presence of members of the working group "siltation" Estuary Committee on January 16, 2006 and proved to be convincing. Each shuttle carries the bucket and poured on the beach, with a gun, 250 m3 of sand (see photo) who takes an early color more sympathetic!


The dredge "Fort Boyard" in action: reloading sand on the beach at Bétahon - photo Jean-Pierre Allenou / Ifremer


An initial assessment satisfactory
The rotodévaseur "Roche Vilaine" fulfills its mission and mussel are pleased to be able to regain access to the hold of the post and Tréhiguier fuel. "The drag Fort Boyard" continues his ballet between the port of Billiers and Bétahon beach. The results on contaminated mussels are very satisfactory and monitoring of turbidity shows no effect of the work on its development

Category : PENESTIN
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