The River Tardoire is located in the heart of the Périgord-Limousin Regional Natural Park in south-western France (Figure 1). It sources in Châlus, in the Haute-Vienne département and runs for 114 km before flowing into Rive Bonnieure near Saint-Ciers-sur-Bonnieure. It drains a watershed of 1200 km2 and has been identified as a waterbody of ecological significance.
Several transversal barriers have been fragmenting the River Tardoire. Four of those barriers were identified as the most impactful on the river’s ecological flow (Figures 2-3) and their removal was chosen as the optimal solution to restore river continuity.
The weirs were previously used to provide water to mills (Figure 4), but they remained obsolete and in poor condition for decades. Prior to the removal works, archaeological surveys were conducted in 2021 and 2023 to assess the heritage significance of the structures (Video 1). Probes were also installed upstream from each weir to monitor the water quality. Other parameters, like flow, substrate, erosion and vegetation were also assessed before the demolition of the barriers. Fishes, including eels, trouts, bleaks, chubs and minnows, were caught through electrofishing and transferred to a different site before any in-river works occur (Video 1).
Demolition works (Figure 5; Videos 1-3) were completed between late 2022 and mid-2023. All four weirs were levelled to a maximum head of 20 cm. In general, the materials from the removal works were sorted on site and reused during the site restoration process as much as possible, for example to reform the riverbed.
The removal of the Seuil des Planches was completed in January 2023 (Figure 6a) and of Seuil du Moulin de Raux the following month (Figure 6b). The materials produced during the reconstruction of the latter weir were placed in the riverbed to diversify it and create currents. The banks on this site were partially filled in to reduce the river width and facilitate concentration of water at low water flow conditions. Concentrated sediment was also removed at this location and the slope of the right bank was slightly modified to become less steep. Seuil de la forge du Buisson was gone by June 2023 (Figure 6c). This barrier was completely unpassable to fishes and significant amount of sediment had been entrappted right upstream, which during the demolition was placed on the right bank. The removal of Seuil du moulin de Peyrassoulat was also complete in mid-June (Figure 6d; Video 3). This stone weir was filled with fine sediment and considerably damaged.
European Rivers Network (ERN) and Syndicat Mixte Bandiat Tardoire (SYMBA) partnered in this project which was funded 100% by Open Rivers Programme. The work was carried out by SYMBA. The total cost of the project was ~310000 €.
”“Thanks to upstream consultation and the technical choices made, this project proves that it is possible to reconcile the mill's historical heritage with the removal of a structure. These sites have not been disfigured but, on the contrary, enhanced”
Corinne RonotProject manager at ERN
The fours projects were prepared in cooperation with the local communities and the owners, by taking into account the benefits for the inhabitants (e.g., leisure activities on site) and their and concerns (e.g., attachment to the structures of heritage value). The barriers have been completely removed, but the artificial channels (which are short in length) continued to be fed with water as required by the owners (sentimental attachment). This was achieved by using ecological techniques of hydraulic calibration and will not pose any detrimental effects on the restored river continuity.
The removal of the four barriers reopened 18.6 km. The hydromorphological aspects of the river are expected to be restored along with its natural functions, like sedimentation. The aquatic habitats in the restored watercourse are also expected to be benefited, to become more diversified, better oxygenated, and water quality to be improved. In addition, fishes are anticipated to take advantage of the now available river stretch. The impact of these removals is being monitored, especially on the European eel and pearl mussel populations. The project was of major importance to the success of the whole watershed management plan. The barriers that were removed were located between 320 and 340 km from the sea, so they affected the migration of the European eel throughout the upstream part of the River Tardoire.
The benefits from this project will be cumulative with those from previous barrier removals carried out within a multi-year action program focused on the upstream part of the River Tardoire since 2017. Six barriers have been removed by the SYMBA within that project. The monitoring shows that the morphology and the natural functions of the river have been restored on these sites, with better quality habitats (oxygenation, diversity) and a better capacity for self-purification. In total, 35 km have been reopened on River Tardoire.
Removing the weirs has allowed the section of channels in which they were located to naturalise (Video 3-4), with further habitat improvements likely to develop over future high flow events, through the reinstatement of natural riverine processes and sediment transport.
More information is available on the following sites:
https://www.ern.org/en/effacement-de-4-seuils-sur-la-tardoire-bv-charente/
https://syndicat-bandiat-tardoire.fr/
Before & After photo pair
With many thanks to Corinne Ronot from ERN and David Marhein from SYMBA for providing the information and the visuals presented herein.
Prepared by Foivos A. Mouchlianitis