Work to restore ecological continuity on the lower Risle in Pont-Audemer, France

11/29/2021

On 10 November 2021, the Risle River was reopened at the Madeleine dam in Pont-Audemer (27) after 13 months of work.

This old dam, which consisted of a gate, a flap gate and a hydroelectric power station, was the first obstacle to the upstream migration of migratory fish from the Seine estuary on the lower Risle and its tributaries.

At this point, the Risle is now cut in two by a central embankment made of sheet piles and topped with prefabricated elements:

  • on one side, three new automatic gates to manage the upstream water level and protect the town centre of Pont-Audemer against flooding;
  • on the other, in place of the power station, a 113 m long fish ramp consisting of three inclined riffles and two resting basins. The inclined riffles are fitted with concrete blocks and secondary macro-roughnesses, in order to favour the passage of species with a low swimming capacity. Against the central embankment, there is an eel ramp and then a flat section allowing trout and salmon to swim upstream.

The system complies with the expectations of the Seine-Normandy Water Agency, the French Office for Biodiversity and the Eure DDTM. The work is already bearing fruit, as many fish have been observed in the resting basins, particularly eels.

The work, carried out by the Syndicat Mixte de la Basse Vallée de la Risle on behalf of the town of Pont-Audemer, owner of the site, was carried out by the Vinci Construction Maritime et Fluvial (leader)/ Vinci CT/CMI John Cockerill/Benedetti Guelpa consortium.

Egis was responsible for the complete project management.