Floods: cleaning rivers, “a false good idea”, is debated

Cleaning, namely the extraction of sediments accumulated at the bottom of a river or canal, nevertheless gives rise to disagreements within the scientific community.

It's a "false good idea" which can lead to low profits, says Philippe Lagauterie, member of France Nature Environnement, who knows the area as a former regional environment director in Pas-de-Calais.

"If you clean out part of the river upstream, you can probably make some benefits locally, but you're going to send water downstream faster and in greater quantities. You can't develop a place without thinking about the consequences of this development a few kilometers further," he explains.

On the environmental side, cleaning can have “catastrophic” consequences: “cleaning will destroy the biological bottom of the river, which we call the biotope, which has a purifying power”.

“The risk would be to transform rivers into anti-tank ditches, and to shift the burden of flooding to areas downstream,” adds Emmanuel Soncourt, independent hydrogeologist.

But the practice should not be banned: experts insist on the need to clean watercourses on a case-by-case basis, depending on the region.

For example, Jean-Marie Aversenq, director of the mixed union of Aude rivers (Smmar), compares the Mediterranean arc to Pas-de-Calais: "In our case, cleaning is not relevant because we are subject to to intense rainy episodes, with accumulations over very short periods of time. The whole protection strategy consists of slowing down the flows, while cleaning rivers accelerates these flows," he says.

"The floods in the north of France are so-called 'lowland' floods with a much slower submergence duration. The water rises more slowly, but also remains much longer, so cleaning the rivers can be an opportunity to drain flooded areas more quickly,” he believes.

Locally, several canals in Pas-de-Calais are regularly cleaned by the Voies navigables de France.

Prevention

Ecologists and hydrologists must in any case be involved in any development project.

It is also the responsibility of public authorities to “teach people to live with this permanent risk” and “to increase the resilience of what is built when possible”, for example, “not to put a boiler in the basement”. floor of the house", or even "raise the floors of homes", continues Philippe Lagauterie.

The creation of flood extension zones is another possible solution.

In the Var, a costly development plan is underway until 2026 on the 4 km of the Nartuby which crosses Draguignan and Trans-en-Provence. In June 2010, torrential rains left 23 dead and two missing: villages were devastated, the Draguignan prison destroyed, industrial zones wiped out...

The work aims to widen the river bed to increase its capacity to 180 m3/second, the equivalent of a thirty-year flood.

“It is essentially a matter of removing human constructions,” Alain Caymaris, mayor of Trans-en-Provence, explains to AFP. Eleven bridges will be modified and 150.000 m3 of materials moved, mainly to widen and strengthen the banks.

But this valley river is not threatened by the siltation which affects the watercourses of the plains: due to drought, it is regularly dry.

For Charlène Descollonges, independent hydrologist, "we need to change strategy: pumping water and releasing it into the sea is very energy-intensive and will not be enough. Raising the dikes will be very expensive and will not be enough either."

The engineer wants France to take the example of the Netherlands, a country "much more exposed to flooding than us", which has chosen to "leave room for rivers".

An argument shared by Nicolas Camphuis, co-director of the European Center for Flood Risk Prevention (Cepri), according to whom "preventing nature from behaving normally requires colossal investments, which can hardly be without effects".