(Guéthary) Every spring, they survey beaches, dunes and cliffs with their measuring tools: the technicians from the Nouvelle-Aquitaine Coast Observatory map the retreat of the coastline, which threatens homes and economic activities, to help define development strategies.
For twenty years, data has been collected from April to June, from “transects”, virtual lines perpendicular to the coastline, which make it possible to evaluate the evolution of the sand stock, but also that of the dunes and cliffs, using a GPS with centimeter precision.
From the mouth of the Gironde, in the north, to the Spanish border in the Basque Country, in the south, 185 profiles have been drawn up after having been battered by winter storms.
“The natural hazard is erosion or marine submersion, and the issue is the house that will be located above the beach: the two associated constitute the risk,” explains Lisa Martins, coastal risks engineer for the Geological and Mining Research Bureau (BRGM), associated with the National Forestry Office within the Observatory.
This work aims in particular to inform public decision-makers on their development strategies for a territory in which 6,000 housing units could be threatened by coastal erosion by 2050.
Thus, the BRGM is associated with various works in progress, like that carried out in Bidart, in the Basque Country, around a golf course on the side of a cliff, part of the course of which must be relocated. Or even a development project in Saint-Jean-de-Luz, which must, in the coming years, renature part of its seaside, move economic activities back there and relocate a wastewater treatment plant.
With its 840 kilometers of coastline, New Aquitaine is faced with different problems. Its sandy coast, in Gironde and Landes, is retreating by 1.7 to 2.5 meters per year. In Charente-Maritime, over the last decade, the west coast of the island of Oléron has retreated by 20 meters per year on average, making it the highest rate of coastline retreat in Europe, according to figures. of the Observatory.
For its rocky part, in the Basque Country, the evolution is measured in landslides and landslides. “The sandy coastline has a resilience that the rocky coast does not have because in the summer, the beach can refill with sand, either naturally or through anthropogenic action by communities,” emphasizes Lisa Martins.
By 2100, IPCC experts predict a rise in sea levels of between 60cm and 1m. This phenomenon will cause a rise in the high tide limit on the shores, particularly in the estuaries. During high tides or strong swells, the impact of waves will be stronger on the coast and will reinforce erosion and submersion processes.
“There is also an important role for precipitation,” adds the engineer, “with the runoff of water which will infiltrate from the top of the cliff” and weaken it.
“We know today that there will be an impact on all weather variables such as swell or precipitation, which will affect the intensity and frequency of events,” recalls Lisa Martins. “What is not yet known is the impact on erosion. What effect if the rainy episodes are more or less intense, or more or less numerous? We don’t have the answers. »
