The sea rises and the coastline changes face. Sometimes it retreats, storm after storm, sometimes it advances, constructions after constructions. To understand how the French coasts are evolving, including those near you, several data allow us to compare the current situation with that of several decades ago.
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The data in question are those of the Center for Studies and Expertise on Risks, the Environment, Mobility and Planning (Cerema). Their teams analyzed aerial photos from the 1930s to 1950s, and compared them with more recent images. Their objective: to observe whether the coastline, materialized by the edge of a cliff or a line of vegetation on the sand, has receded or advanced.
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What are their conclusions, made public in 2018? Nationally, 18.6% of the coast has seen its coastline retreat, or 900 km, sometimes several meters per year, over periods of 50 to 70 years. The institute also calculated that 11.7% of the territory is experiencing an advancement of the coastline, and that a large majority of the linear, 69.7%, has not seen any noticeable change.
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According to Cerema, “523 municipalities have at least one sector affected by a decline in the coastline”. And for 59 of them, the retreat in some places is more than 1.5 meters per year on average. Are the coasts near which you live, or that you know, concerned?
Using our interactive map and its search engine (the magnifying glass at the bottom of the map), you can search for a town and observe whether the coastline has retreated (in red), advanced (in blue) or n has not moved (in gray), as is often the case in Brittany for example. Or click on a department to see the evolution of its coastline.
The overseas coasts are well represented in this map: you will be taken there after entering the name of an overseas municipality or department in the map’s search engine.
The French coasts are unequally affected by the phenomenon, depending on their geological profile. The cliffs and rocky coasts, which represent 54% of the French coastline, are experiencing a decline in the coastline on only 6% of their length. While the sandy coasts, which represent 41% of the linear, experience a decline in the coastline for 37% and an advance in 23% of cases.
Result: the departments with long sandy beaches are the most affected by the retreat of the coastline. The most affected are the Gironde, Manche and Charente-Maritime. In fifty years, the areas in decline correspond to a loss of surface of 30 km2, mainly in Gironde and Charente-Maritime, in the south of the island of Oléron.
On the Atlantic coast of the town of La Tremblade, some dunes experienced a record decline of 7.9 m per year between 1945 and 2010. The photos below, taken in 1957 (right) and 2018 (left), allow you to see how the ocean is advancing over the vegetated dunes.
At this precise location, the retreat of the coast has no impact on inhabited areas. On the other hand, this is the case a few kilometers further south, in Soulac-sur-Mer and its residential building “Le Signal”, built in the 1970s 200 m from the sea, and which has become one of the symbols of erosion. coastal. According to data from Cerema, the coastline declined 4.3 m per year between 1957 and 2013. And more than 30 m during the winter storm 2013-2014. Emptied of its inhabitants, it is the subject of a long judicial soap opera, recently punctuated by the decision of the Senate, in July 2020, to compensate the co-owners of the premises. The first compensation was paid in March 2021.
Sometimes, the coastline shows little or no setback, but the consequences do not fail to worry residents and researchers. At the entrance to the Gâvres peninsula, not far from Lorient, erosion threatens the only road leading to the town. And increases the risk of submersion. In 2008, a protective wall had given way, leaving the way open to the ocean to flood part of the town.
But the place where the coast recedes the most is undoubtedly in Guyana. On the coast of the town of Awala-Yalimapo, between 1950 and 2006, the coastline retreated in some places by more than 60 m per year, according to data from Cerema. An exceptional phenomenon linked to the displacement of a mud bank along the coast, well known to the inhabitants.
In other places, on the contrary, the coast gains ground on the sea. Most of the time, this advance occurs after the construction of structures such as dikes, which tends to modify the sedimentation in the surroundings, causing for example an accumulation of sand on one side of the dike.
It is in Grau-du-Roi (Gard) that the coastline has advanced the most. In some places in this area, the coastline gained more than 15 m per year between 1937 and 2011, according to data from Cerema. This is due to the construction of a marina and a dike, which blocked the circulation of sand, which accumulated in the south.
The comparison of aerial photographs, taken between 1963 (right) and 2018 (left), allows to visualize the advance of the port on the sea.
What does the future hold for us? In the coming decades, the coastline is forecast to continue to decline in some places and the sandy coasts will always be the most affected. In a study published in 2016, the Observatory of the Aquitaine coast envisaged an erosion of 50 m of the coast by 2050, “to which is added a setback linked to a major event in general of the order of 20 m”. The Observatory has calculated that the retreat of the coastline could reach higher values than previously expected, namely “average setbacks of 2.5 m per year in Gironde and 1.7 m per year in the Landes”.
And the consequences could be serious. Cerema has attempted to quantify the results of the various scenarios of a more or less marked decline in the coastline. By 2100, between 5,000 and 47,300 homes could be victims of erosion, depending on the various hypotheses studied, with a cost of 0.8 to 8 billion euros. According to a forthcoming report, it would also be between 120 and 1,000 km of roads that would be affected. Certainly wide ranges, but which allow us to assess what awaits the Hexagon.