Faced with floods, the limits of regional planning

"We keep hammering home this message: we can do work, but we can't eliminate the risk," emphasizes Betty Cachot, director of the Brévenne-Turdine river union (Rhône).

Since 2006, his union has carried out numerous developments on these two waterways that cross the Monts du Lyonnais. Presented as models in terms of flood prevention, they were visited at the end of October by Prime Minister Michel Barnier.

On the Turdine, two flood control dams, similar to small bridges, have been installed. "When the river flows normally, it passes underneath. As soon as it starts to swell, the water is held back" using a "funnel" system, explains Ms. Cachot. "This allows time to evacuate" the populations, "or even completely contain the flood."

The bed of the Brévenne, for its part, has seen its old dikes removed. "We are giving the river as much space as possible" so that it can swell without overflowing into habitable areas, explains Ms. Cachot.

"It's still engineering, but based on nature," explains Valérie November, a researcher at the CNRS, for whom large structures, on the other hand, can prove counterproductive by "displacing the problem," such as dikes which create erosion and increase risks downstream.

Multiplication of risk

Of the 16,8 million French people who live in flood zones in a scenario of extreme flooding in mainland France, 2,5 million live in Auvergne-Rhône-Alpes, according to estimates from the Regional Directorate for the Environment, Planning and Housing (Dreal).

The Rhône-Mediterranean basin is in fact faced with winter floods (linked to long periods of rain), spring floods (rain and snowmelt), as well as more occasional episodes of intense precipitation, such as the Cévennes events.

"No part of the basin is completely protected from the risk of flooding," notes Hervé Piégay, research director at the CNRS.

In urban areas, in addition to overflowing watercourses, water runs off artificial surfaces and flows rapidly downstream, amplifying flood peaks.

"In Lyon, there is a whole history of waterproofing," says the geographer. "Now, we are trying to reopen the asphalt wherever possible, to promote infiltration phenomena" and to install retention basins to store water.

For 50 years, the ENS researcher points out, "we have tended to increase our vulnerability" by building closer and closer to waterways, thinking we were protected by developments. But when these structures prove insufficient, the damage is disastrous.

"Destitute"

Different developments are designed for different flood levels, explains Thomas Adeline, expert consultant in flood prevention: bridges are generally designed to contain thirty-year floods (which have a one in thirty chance of occurring in a year), urban planning to withstand hundred-year floods.

When they exceed these thresholds, "we are quite helpless," summarizes Mr. Piégay.

At that point, "the only issue is forecasting and warning," he says, meaning having effective weather services and a system to warn people, minimize damage and save lives.

In Valencia, Spain, where more than 220 people died in floods on October 29, the authorities were accused of delaying sending an alert message to the population.

But climate change, which makes precipitation more intense and unpredictable, complicates the mission of those who must anticipate risks. "We don't know how to model what a 40-year flood will look like in XNUMX years," notes Romaric Vallaud of Dreal.

In the Brévenne Turdine basin, the rivers union has set up small radars which measure the height of the water in real time, and ensures that the population remains aware of the risk, in particular through awareness workshops.

"In a way, I think we were lucky to have floods" in the past, concedes Ms. Cachot, because when we talk about projects to local residents "they don't question the fact that it's useful, they remember."