Is the Poitevin marsh threatened by extreme weather phenomena? The Xinthia storm which swept across Western Europe between February 26 and 28, 2010 caused more than 50 deaths in France, including 35 in Vendée (29 deaths in the town of La Faute-sur-mer alone).
The violent winds, mixed with the open sea and a strong tidal coefficient (coefficient 102), led to a brutal and intensive episode of marine submersion.
The material damage is estimated at 1.5 billion euros (800 million euros for the consequences of the wind and 700 million for the floods).
“Xinthia was a turning point in the perception of coastal hazards and risks.”
Chaumillon, professor of marine and coastal geologyat franceinfo
The Xinthia storm made it possible to observe and model the abnormal rise in sea level. The surge reached 2 meters in the bay of Aiguillon, located opposite the Poitevin marshes. Land more than 10 kilometers away was flooded.
Éric Chaumillon specifies that the Poitevin marsh was once a territory of polders, an area which was then reclaimed from the ocean. In fact, it is the man who domesticated the water, to make breeding and culture in this region. The bays located in the marsh area are places of natural sedimentation.
“These are places where sediment accumulates and replaces water in a way”, explains the researcher. This phenomenon has been accentuated and amplified by human activity, which has consisted in digging drains and canals to drain the marshes, and in building dykes with the sediments, in order to prevent the water of the ocean from enter the polders.
“The vulnerability of the Poitevin marshes is its location below sea level.”
Eric Chaumillonat franceinfo
It must be understood that the site is completely artificial. If the marsh were left to evolve naturally, the ground would adapt to rising sea levels and rise at the same time.
Today, this is no longer possible because the dykes that have been built prevent the water from entering but they also block the arrival of sediments. This situation weakens the marsh, because some sectors are 2 to 3 meters below sea level, which means that the situation will be even more dangerous in 2100!
Some scientific work tends to show that by letting the sea penetrate controlled areas, we could regain sediments and therefore raise the ground naturally to reach sea level.
Éric Chaumillon believes that it is possible to preserve the marsh by promoting nature-based solutions. Coastal ecosystems have a real capacity for resilience. This is also true for terrestrial ecosystems, which have the ability to adapt and return to a certain state of equilibrium, after disturbed episodes.
Human activity is also involved. Eric Chaumillon would like man to start making a real transition towards other cultivation methods, for example. According to him, it is necessary to favor plants and crops that are more resistant to flooding.
Climate change involves not only temperature changes, but also changes in river flow.
“Now we have to get used to these more frequent and intense floods”, concludes the researcher from La Rochelle.