Removal of the Moulin du Plain dam, Franco-Swiss Doubs watershed, France

Introduction

The River Doubs, with a total length of 450 km, forms the border between France and Switzerland for around 50 kilometres shortly after its source, then flows through Swiss territory for approximately 30 km before returning to France.

This section of the Franco-Swiss Doubs is renowned, both regionally and beyond, for the quality of its landscapes and its abundant populations of trout and other mid-mountain river fish.

Project context and opportunity

Despite the river’s ecological and scenic qualities, for centuries, the use of hydraulic power has led to the river being constricted, with three dams producing hydroelectricity and many others that are no longer in use. Among the latter, four structures, which are a priority for both French and Swiss authorities, offer significant potential for reopening the watercourse if they are removed.

This is the case for the Moulin du Plain dam, which alone would allow 24.5 km of river to be reopened in the first phase (6.5 km upstream). In a second phase, the two structures upstream (Theusseret dam in France) and downstream (Bellefontaine weir in Switzerland) of the Moulin du Plain dam are also included in a programme to restore ecological continuity. The removal of these two structures in the coming years will then allow the reopening of 54.5 km of river in total.

Project aims

The objectives of the mission are to remove the Moulin du Plain weir in order to restore lateral and longitudinal dynamics on the Doubs River, to rediversify flows and thus aquatic habitats, and finally to restore fish continuity.

More specifically, completely removing the Moulin du Plain weir will make it possible to:

• Restore fish continuity, as the weir is currently impassable;
• Restore diversified flows within the hydraulic eddy of the weir (lentic channel) to restore the “natural” flows of the Doubs (lotic channel);
• Restore a diverse aquatic fauna by restoring diversified flows.

The drop height of the threshold is estimated at approximately 1.2 metres for a total threshold height of 1.70 metres. This drop classifies the threshold as impassable for the objective of continuity for “all species.”

The operational objectives of the mission are:

• Levelling the threshold and creating a breach;
• Filling the channel and creating a pool;
• Stabilising the upstream bedrock to contain excessive regressive erosion;
• Rehabilitating the threshold blocks on the downstream bedrock to diversify aquatic habitats and features.