Degraded over half of its route, the Rhône-Sète canal will undergo major renovation

Le contexte des étangs rend complexe le réaménagement du canal du Rhône à Sète (ici à Frontignan). Midi Libre – PHILIPPE MALRIC

Les entrées maritimes et les ruissellements ont fortement dégradé les berges et remplissent le canal de sédiments. 60 M€ seront mobilisés pour conserver la navigabilité de l’ouvrage.

The Rhône-Sète canal is not new and it shows. Designed in the 17th century, this heritage structure provides a river-sea link between the port of Sète and the Rhône-Saône basin. In 2010, a very ambitious modernisation programme aimed to make it capable of accommodating large-scale boats (2,500 tonnes). Bridges were raised (in Carnon), areas were developed, but the major 100 million euro project was not completed. “We've done half of it”, does not hesitate to affirm Christophe Wendling, territorial director at VNF, Voies navigables de France, the public establishment which manages, under the supervision of the State, the canal. “Operating costs have exploded. This canal crosses watersheds with diverse influences, it is connected to the Palavasian ponds, and the project of digging it deep to 3.50 m/4 meters was no longer feasible.”

Undesirable sediments

Maritime incursions during increasingly frequent storms deposit an increasing quantity of sediment. More or less salty waste that can no longer be used as before. “The guidelines have changed, and the uses that allowed these sediments to be recovered to fill a quarry, carry out work, etc., are no longer authorized”. Currently, VNF has on its hands (or more precisely in dedicated “bins”) 350,000 tonnes of unwanted sediment. The collapse of the banks is making the problem worse: from an average of 60,000 tonnes of sediment accumulated per year, the figure has risen to 100,000 tonnes. VNF estimates that over the 70 km of linear stretch, the banks are now degraded over more than half. In Palavas-les-Flots, the canal has simply “melted” in the Arnel pond.

In Palavas, in the Arnel pond, the banks of the Rhône-Sète canal were disappearing. VNF – VNF

In 2020: stop or encore?

In 2019, VNF sounded the alarm with its supervisory ministry (the Environment), which launched an inspection mission. The year 2020 brought together all the stakeholders around a “channel conference”. The stakes are colossal because the question that arises is simply stop or continue.

The sediments are accumulating: dredging of increasingly large volumes is necessary (here in Palavas). VNF – VNF

"We could have decided to abandon it. But local stakeholders are keen on this work. The Region, which owns the port of Sète, is committed to multimodality, the canal is freight transport but also a tourist activity". Surprisingly, the spin-offs from the two sectors are equivalent: 17 M€ per year of freight, as much for tourism while no action is really taken to promote it. “We assume that our infrastructure will accommodate “only” 1,500-ton ships. The current average is one freight ship per day. We are sticking to conservative estimates while taking into account the growth needs of the port of Sète”, continues Christophe Wendling.

A new project

Once the decision was made to keep the structure operational, everything remained to be done. The State-Rhône Saône Region interregional plan contract, the CPIER 2021-2027, has therefore included in its objectives 60 million euros of work on the canal. 25 million euros come from VNF, the same from the Region, 2 million euros will be financed by Europe and the 8 million euros are to be mobilized from the communities crossed by the structure.

An experimental technique, with the use of sediments inside the “curtains” of boards. VNF – VNF

Of this sum, 30 M€ will be devoted to the restoration of the banks, the other half to the management of sediments. This "grey debt&quot present all along the canal requires finding the evacuation channel. The Zifmar 2 project, or fluvio-maritime industrial zone, of Sète, undertaken by the Region (for 7 M€) will use part of it to create a platform, like a polder.“In the Arnel pond in Palavas, we are finishing the first of the major renovation projects with a double curtain in wood cladding”, assures Marie Capitaine, from VNF.

Two rows of untreated oak piles are driven into the ground, boards are fixed to them while the space between these wooden curtains is filled with sediment. A technique that, by VNF's own admission “will surely not last 200 years ! It's experimental”. The rest concerns a 8 km section (on both sides) between Palavas and Les Aresquiers. A preliminary study has just been launched, it will last one year. The context of the ponds will require specific techniques.

Agnès Langevine: “It's a whole balance to find!”

What is the role of the Conservatoire du littoral here ?

VNF as owner has the project management. The Conservatoire du littoral, which I chair, is concerned as a neighbor of the canal and as part of the overall protection of the coastline. We are faced with a heritage object, forged by man, which needs to be maintained and renovated in order to be navigable.

How to anticipate sea level rise ?

It's all about finding a balance! Extending your lifespan with what budgetary means: how much to invest these sums to act without regret ? You have to prioritize, keep in mind your vulnerability to climate change because if it's to do like the Shadoks, it's useless! The canal conference, in 2020, chose the intermediate scenario: tourist boats and freight but of average sizes.

The conservatory has its opinion to give on the construction site?

We support anything that allows a light footprint on the environment like these reversible wooden constructions in the Arnel pond. The chosen approach crosses the issues, and it is important to have a complex approach. Transporting goods by water is better than by road, but what is the current demand? ? Making this canal a tourist route, yes why not, but for slow tourism. Our software can no longer be the same as before.

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