In the river La Sélune in France, TenCate Geotube® technology from TenCate Geosynthetics, a Solmax Company, was used to effectively and ecologically dewater mud which had accumulated over the years in front of two dams. Migratory fish, such as Atlantic salmon and eels, have returned to the Sélune river following the deconstruction of the dams.
Old dams dismantled in favor of a UNESCO Word Heritage site
In line with an EU directive and a decision from the French Environmental Forum “Grenelle”, the coastal river of La Sélune in Normandy (France), which empties into the bay of the famous tidal island of Mont-Saint-Michel, was to be completely restored.
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Plans were made to demolish two almost century old dams in order to restore the hydrological and biological functions of the river. Following the demolition of the first dam at Vezins in 2019, all sediments accumulated in front of the second, smaller dam at la Roche-Qui-Boit. The bay of Mont Saint Michel risked silting up further.
The Mont Saint Michel island is located only a few hundred meters from the mainland, to which it has been permanently connected since 2014 via a pedestrian bridge.
Prior to the removal of the last dam, additional measures were required to prevent the accumulation of sediments further downstream and protect the tidal landscape.
The Sélune project is an impressive demonstration of how the flexibility of TenCate Geotube® dewatering technology can significantly contribute to sediment removal in a socially, economically and environmentally responsible manner.
Gerben van den Berg, BDM Environmental Remediation EMEA
Efficient and environmentally-friendly drainage
It goes without saying that transporting 140,000 m3 of dredged wet sludge over many kilometers of narrow country roads would have been extremely costly. It would also have left a huge carbon footprint and a significant impact on the local population.
A Dutch dredging company, Van den Herik, suggested a different approach recommending dewatering and consolidating the sediments along the semi-restored river on the former bed of the Vezins reservoir.
Experts from TenCate Geotube® and the dredging company joined forces. Together they designed seven dewatering platforms, tested the dewaterability of the sediments and determined the need for flocculants. Using an eco-friendly 16-inch cutter suction dredger, the mud was pumped from the bottom of the basin straight into the tubes laid out on the platforms.
The specially selected TenCate Geotube® GT500D fabric ensured continuous dewatering as well as high-quality filtration. Using backhoes, removing the dewatered mass from the site after a few weeks in a clean manner was an easy thing to do. This efficient way of dewatering saved thousands of truck journeys in the environmentally friendly disposal of the material.
Low carbon footprint: Thousands of truck journeys prevented
The demolition of the last dam and the desludging of the river have spurned an increase in biodiversity, as once-lost spawning areas have now become accessible again, while the abbey of Mont Saint Michel silently dominates the bay.
Flexible drainage restores storage capacity
Reservoirs often accumulate massive amounts of sediment which impedes water storage resulting in the significant and costly loss of production capacity. On-site TenCate Geotube® dewatering and storage can provide unexpected solutions for active dams as well in recovering storage capacity lost due to massive sediment deposition.
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Gerben van den Berg, BDM Environmental Remediation EMEA