Dalloz Weir demolition: a multifaceted approach to restore river connectivity, fish populations and improve industrial processes


River Tacon

River Tacon is the main tributary of River Bienne in Jura Province in Eastern-central France (Figure 1). It’s a typical and representative example of freshwater watercourse in this karstic French region. River Tacon is of great ecological importance and since 1995 is included in the Natura2000 Network (site number: FR4301331). Several valuable fish species inhabit this river, such as Bullhead (Cottus gobio) and Brown trout (Salmo trutta). However, the populations of these fish species have been decreased dramatically during the last decades, mainly due to manmade infrastructure, like dams and other river barriers, and the industrial usage of freshwater. The River Bienne watershed is highly fragmented, since more than 300 river barriers have been identified by the Parc Naturel Régional du Haut-jura. All these barriers have been classified based on their degree of connectivity relative to the existing hydrological regime using a river connectivity index (ICE).

Figure 1. River Tacon watershed in Jura Province, Eastern-central France

Dalloz Weir was one of the barriers fragmenting the watercourses in the River Bienne watershed. It was built in the 1900s along the corridor of River Grosdar (latitude: 46.37603, longitude: 5.879205), a tributary of River Tacon (Figure 2). It was a 3 m high and 10 m wide weir initially built to supply water to a sawmill and later for other industrial purposes (Figure 3). The years before its removal, Dalloz Weir was used to supply water to Dalloz Créations, a sun lenses producing company situated right by the riverbank, for various processes (e.g., for cleaning tools). For the water to be accessible to the company, an artificial channel had been created to redirect the watercourse, diminishing drastically the water amount and flow in a 400 m natural river stretch. In addition, the water quality was degraded by the chemicals used by the company during the cleaning processes.

Figure 2. Location of the Dalloz Weir
Figure 3. The Dalloz Weir (photo by S. Ghidini)

Dalloz Weir had also been considered as having a substantial negative impact on fish migration. Targeted studies suggested that this barrier was unsurpassable to more than 70% of the resident trout population. This result was based on the analysis of fish movements through Radio Frequency Identification (RFID) tracers aiming to evaluate the impact of this low-head dam on the trout population.

“We aim to continue collaborating with Parc Naturel Régional du Haut-jura who helps us in finding solutions for our processes and financial needs so we can have zero impact on the river”

Frédéric Drivet, Director of Dalloz Créations

Barrier Removal Process

Parc Naturel Régional du Haut-jura follows a manifold approach in designing and implementing its projects. Their rationale is to solve many problems at once in a single area. The removal of the Dalloz Weir was part of the “Work together to restore our rivers” project that also incorporated solutions to change the industrial processes of Dalloz Créations and enhancement of the stability of the bridge over the barrier.

Figure 4. Removal operations of the Dalloz Weir (photo by Romain Bellier)
Figure 5. Removal operations of the Dalloz Weir (photo by Romain Bellier)
Figure 6. Removal operations of the Dalloz Weir (photo by Romain Bellier)

In 2019, Parc Naturel Régional du Haut-jura prepared the engineering design for the project, which was then presented to Dalloz Créations and the State. The implementation phases, including the barrier removal (Figures 4-6), the remodelling of the riverbanks (Figures 7-9) and the changes in the industrial processes of the company were successfully carried out from 2020 to 2021. The project costed 110.000 euro and was co-financed by the French Water Agency (70%) and the Region (10%).

Figure 7. Reconstruction of the riverbanks following the removal of the Dalloz Weir (photo by Romain Bellier)
Figure 8. Reconstruction of the riverbanks following the removal of the Dalloz Weir (photo by Romain Bellier)
Figure 9. Reconstruction of the riverbanks following the removal of the Dalloz Weir (photo by Romain Bellier)

Ecological and community benefits

The Dalloz Weir demolition reopened 3 km of the river stretch that are now accessible to all resident fish species, regardless of their size or the hydrological conditions (Figure 10). Additionally, the water has returned to its original course, and it’s no longer redirected to the artificial channel to serve industrial purposes. The unhindered passage of trout has already been documented through surveys using RFID tracers (Figure 11) and such monitoring will continue up to one year after the removal of the barrier. Dalloz Créations is also satisfied with the newly implemented processes and is now paying more attention to the river status, the water quality and the local biodiversity.

Figure 10. River Grosdar upstream the removal site after the completion of the barrier demolition (photo by Romain Bellier)

Before & After photo pair

The removal site at River Grosdar: (left) before and (right) after the removal operations of Dalloz Weir (photos by S. Ghidini)

With many thanks to Romain Bellier for providing the information and photographs presented herein.

Written by Foivos A. Mouchlianitis

  • Name: Dalloz Weir
  • Location: River Grosdar, Jura, France 
  • Type: Weir
  • Dimensions: Height 3 m; Length 10 m
  • Aim of removal: To restore river connectivity, to improve water quality and quantity and to eliminate security risks
  • Year of removal: 2021