The Croissy dyke, which links the “Île des impressionnistes” in Chatou and the “Île de la Grenouillière” in Croissy-sur-Seine, is an earth hydraulic structure that dates back to the time of Louis XIV. It maintains a water difference of 3.2 metres between the two arms of the Seine and is part of a system of dams (Bougival and Chatou) which is important for navigation and water uses (equipment, water intakes and discharges, stability of quays). It is managed by Voies navigables de France (VNF), a member of the Seine-Scheldt EEIG.
Weathered by time and the river, the dyke has lost its historic width and was reinforced on the Marly arm side in 2007. Following a major flood in 2016, major collapses occurred and the condition of the dyke on the Rivière Neuve branch has worsened, with the risk of breaches during the flood and receding waters phases of the Seine, which might cause a wave surge and the loss of the 3-metre water depth over 30 kilometres.
The operation therefore consists of reinforcing the entire side of the dyke on the Rivière Neuve branch with rockfill in order to ensure the long-term stability of the structure. The project was subject to environmental authorisation and the public inquiry, held in the 4 riverside towns, enabled VNF to improve the project in order to preserve the trees at the top of the bank as much as possible.
The first tree felling works on the dam were completed in March 2023, under the supervision of an arboricultural expert. The downstream half of the dam will be treated this year, with earthworks and the installation of riprap from late May to early December. Work on the upstream part will take place in 2024, after another phase of tree felling. An ecologist will accompany the various phases of the work to ensure the protection of protected species.
The cost of the work amounts over €10m, of which €4.7m is being co-financed by the European Commission (as part of the Connecting Europe Facility – CEF) and the Region Ile-de-France.