It is a popular place for tourists, who like to sail on the Mayenne and admire its bucolic shores. The Grez-Neuville canal, north of Angers (Maine-et-Loire), has been the subject of rather rare experimentation on this scale for two years: restoring its banks by preferring plants to stones. “We had observed their degradation during periods of low water, says Gwennaël Cordier, technician in aquatic environments in the department. To restore them, two options were available to us: resort to civil engineering, with masonry or riprap, or else to plant engineering, by focusing on the capacities of plants to fix and consolidate the banks. We chose the second. “
To select the best species to implant, the department relied on the expertise of the Fresne Agricultural High School, near Angers. “They had to resist the water, the waves caused by the boats, the erosion and the nutria which dig galleries, explains the technician. They also had to develop slowly and require little maintenance. “
The choice fell on the plantation of earth sausages surrounded by coconut fibers, sown with helophyte plants such as irises or rushes. “They have the particularity of having their feet in the water and their heads in the sun”, describes André Evette, engineer and researcher at the National Research Institute for Agriculture, Food and the Environment (Inrae), which closely monitors plant engineering projects in France (1). “These nature-based solutions are not new and date back to Antiquity, he recalls. But we lost sight of them from the 1950s and the transition to all-concrete. It was the era of the Thirty Glorious Years, and we didn’t have the same way of seeing the world as today … “ In the 1970s and 1980s, Austria and Switzerland brought these techniques up to date, followed by Germany and the Netherlands. In France, they have reappeared for ten years.
“Their major advantage is to favor the ecological restoration of the banks, high places of biodiversity, explains the researcher. The plants filter the pollutants that arrive in the water, provide a welcome shade and recreate a whole habitat in the air as in the water: insects, dragonflies, crayfish, freshwater shrimps, fish … “ A virtuous circle, which saves money. “Not only is it better to work with living things, but it’s also cheaper, confirms Gwennaël Cordier. Our plant site costs around € 40,000, when a “hard” development can amount to millions of euros, taking into account the difficulties of access to the site for construction machinery. “ Finally, if its effectiveness is not as immediate as a concrete installation, over time, “Plant engineering is becoming stronger than civil engineering”, he continues.
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Work on the Grez-Neuville canal was carried out in October 2018, testing two techniques. For the first 75 meters, a strip of local herb seeds has been planted above the helophyte sausages in geotextile. “It is a good alternative to the plastic sheet because it degrades naturally. “ Over the next 75 meters, a bed of willow plants has been added. The most effective technique will be reproduced on the 650 meters of remaining banks. A year and a half later, the first assessment is already positive: “The plants are well anchored to the ground and the willows have grown very well, greet the technician. But we must wait until spring 2021 to draw our conclusions. “
Only 5% of rivers in good ecological condition
In this department concentrating 7,000 kilometers of rivers, of which only 5% are in good ecological condition, the restoration of the banks and, ultimately, the recovery of the quality of the water prove to be crucial. “With plant engineering, we try to repair the mistakes of the past”, explains Marc Helbert, head of the water service in the department, which coordinates an Observatory of water quality in Anjou. “What weighs most on us is the fish index, he comments. It will replenish when they can circulate more freely in our waterways. But these actions will take time to produce their effects… ”
Especially since these techniques inspired by nature are not so simple to implement. “They require multidisciplinary skills and strong field experience, comments André Evette. It is not enough to follow technical rules like in civil engineering. Living things are impossible to model. How to predict that a beaver will eat the willow that has been planted? “ If plants can solve a multitude of problems – maintaining agricultural land in the face of erosion, preventing avalanches in the mountains, cleaning up soil, phyto-purification, wind or noise barrier – they cannot always be enough . “When you have a torrent near a school, you may not be going without concrete to secure the site, comments the researcher. In an urban environment, you don’t always have a choice. But mixed solutions are developing. “
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Nor is it easy to convince of the economic profitability of these developments. The establishment of hedges or buffer zones not cultivated in agricultural zones, for example, requires educational work. “Obviously, the financial gain is not necessarily demonstrated for a productivist agricultural model”, recognizes André Evette.
Last merit of plant genius: embellishing the landscape. “In front of our work on the canal, we have a very busy towpath, notes Gwennaël Cordier. Walkers are delighted with the new plantations. “ What hardly surprises André Evette: “Plants have a positive influence on humans: our blood pressure decreases in the forest, seeing a tree from its window facilitates healing in the hospital like concentration in a classroom. Putting plants back, in the end, is no longer disjoining man and nature … “
Not enough fish, too many pesticides
The balance sheet drawn up in 2018 by the Observatory of water quality in Anjou over the last ten years draws up an unquestionable observation: too much nitrates, presence of 151 pesticide molecules (glyphosate, atrazine…), average or even bad fish population.
However, two indicators experience improvement (phosphorus, nitrogen) linked to
progress in wastewater treatment.