victim of corrosion, the Arzal dam under construction for several months

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Piles, a crane, embankment, a temporary platform built on the Vilaine estuary, at the Arzal dam, in the south of Morbihan, a major project was launched in September 2024. It will extend to in June 2025. Cost of this operation financed by Eaux & Vilaine: €4.35 million. Objective: to consolidate this 65-year-old structure, victim of corrosion.

Replace corroded gabions

Road traffic on the dam is not interrupted, the locks continue to operate, as do the valves to supply the drinking water plant located on the banks of the Vilaine in Férel. However, the dam is under construction. With one objective: to replace the corroded gabions with a “combined” wall, made up of piles, drilled 5 m into the rock, and sheet piles. In charge, the Eiffage maritime and river works company.

A weakened “belt”

The dam dates from the 1960s. It was built in two parts: one in concrete, on the Arzal bank, on which the 5 high valves are placed, another, on the Camoël side, made up of a dike made on riprap and mud. Between the two, gabions, in other words large metal cylinders 20 m in diameter filled with rubble. They form a sort of belt, an interface, between the two constructions.

Problem is, over time, these gabions have been attacked by corrosion. To the point of weakening their structures. The most exposed are located on the ocean side. Eaux & Vilaine, which manages the dam, therefore decided to change them.

“Intervening in an estuary is always complex”

Four years of studies (geotechnical, topographical, bathymetric, public survey, etc.) were necessary to choose the operating method with the least impact on the environment.

Intervening in an estuary is always complex and highly regulated.

Eaux & Vilaine took the opportunity of this project to extend the siphons (which pump the salt water to discharge it downstream) installed on the new enclosure.

A new glass eel pass

Another novelty made possible with this project, the creation of an glass eel pass, the third on this structure in addition to the migratory fish pass. Since 1995, the scientific committee of the managing establishment has studied these migrations. If the current fish pass is no longer accessible to the public for security reasons, Eaux & Vilaine is working on new equipment to welcome visitors.

Soon a €14 million project

This wall, currently under construction, “should last 50 years,” adds Sébastien Baron. But Eaux & Vilaine knows that the gabions, on the fresh water side, will also have to be replaced in the medium term.

Not to mention the XXL investment in the anti-salinity lock, work on which should start in 2025 with the consequence of closing the lock for two winters.

This equipment, at €14 million, will no longer allow salt water to enter and thus protect the drinking water supply.

We remember that in the summer of 2022, due to the drought, Eaux & Vilaine had to drastically reduce, or even close, the airlocks to limit the intrusion of salt water into the drinking water production process which remains the primary purpose of the dam.

What is the Arzal dam used for?

• It will celebrate its 65th anniversary in 2025. Recognizable by its five high colored floodgates and its footbridge overlooking the Vilaine estuary, the Arzal dam has two priority purposes: to constitute a fresh water reserve of 50 million m3 transformed at the Férel plant into drinking water for 1 million people (on the Guérande peninsula and in the Saint-Nazaire region) and prevent the tidal wave going up to Redon.

• The work also made it possible to develop boating and related economic activities. It also plays an important role in the scientific monitoring of fish migration.

  The public territorial basin establishment, Eaux & Vilaine, owns and manages the dam, as part of its water policy mission in the Vilaine watershed (515 municipalities in several departments).

  We cannot do without this dam. It is essential to combat flooding and to secure the distribution of drinking water.

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