how is the coastline measured each year after winter, to the nearest centimeter

L’Differential GPS control screen delivers its verdict. Some distance from the waves which die on the slope of Pointe beach, south of Capbreton, it has an altitude of 2.96 meters. “The measurement is centimeter precise,” assures Nicolas Bernon, the project manager of the New Aquitaine Coast Observatory (OCNA) who works at BRGM, the Geological and Mining Research Bureau.

This April 26, a battery of information is stored on this profile of the coast of the south of the Landes. A profile ? In NACO language, it is a straight line perpendicular to the shore which starts from the edge of the ocean and ends in the forest, some two hundred meters further. Meter by meter, Marie Branelec and Zoé Bleunven, the two BRGM technicians in charge, record the data from the satellite signal. The foot of the coastal dune is carefully marked. It is he who marks the location of the coastline. On rocky shores, it is the cliff top.

Every year at the end of winter, the Observatory documents 185 coastal profiles, from the Spanish border in the south to the Gironde estuary in the north: around sixty for the sandy coast, the same number for the rocky coast, and no less than 66 for the Arcachon basin alone, including eleven on Bird Island. This year, the campaign runs from April 8 to early June, with a few profiles per day. In Charente-Maritime, the University of La Rochelle produces similar work, in partnership with the Department.

Rear equipment

The OCNA started systematic field surveys in 1998. They are posted online on the Observatory’s website, intended for research teams, decision-makers and the general public. With a retrospective view of twenty-five years of data from the same profiles, experts can determine the movements of the coastline in the region since the beginning of the century. Unsurprisingly, he is moving backwards, at La Pointe as elsewhere.

“The erosion was strong during the winter of 2013-2014. There was a calm phase, then the decline has resumed since 2019-2020. This winter, the combination of storms and high tidal coefficients had a significant impact,” assesses Nicolas Bernon, turning his gaze towards the coastal dune which in places has a cliff-like profile, the sign of undermining work on the the ocean whose waves hit the obstacle and tore away cubic meters of sand.

Coastal mission manager at the municipality of Capbreton, Cyrille Gayer follows the operations with interest. The exact measurement of the erosion phenomenon is of capital importance for local communities. At the Point, the meters eaten away by the ocean are all the more scrutinized as a wastewater treatment plant is installed a short distance from the beach. It treats the majority of effluent from Capbreton and Soorts-Hossegor. “One day it will have to be moved. We give ourselves a 2040 horizon,” he says. Studies have been launched on the subject since 2019. Further back, a housing estate and a campsite will one day be affected by the inexorable decline of the coastline, which will undoubtedly worsen in the decades to come, in relation to the rise from sea level.

Visual observations

Once the work was completed at La Pointe, the Observatory team migrated a few kilometers further north, to La Piste beach, to repeat the operations there. There, the coastal cliff is even more evident. To satellite measurements, Marie Branelec and Zoé Bleunven add visual observations. It may be a berm, a ridge of sand, collected there by the swells, which thickens at the bottom of the beach. From a sand bank at the back of the beach where a meager vegetation will try to take root, sheltered from the waves. The condition of the white dune, subject to incessant sand transport, is noted. Then that of the gray dune, more stable and more vegetated.

In parallel with the spring inventory, the OCNA has carried out a LIDAR survey of the region’s sandy coast every fall since 2016. This laser beam remote sensing tool is carried on board an aircraft. “It has the advantage of covering the entire coastal zone but it does not provide us with the geomorphological information that we obtain by going into the field,” summarizes Nicolas Bernon. Every two years at the end of spring, photos are also taken from a microlight whose flight follows the coastal ribbon. Equipped with this arsenal, the OCNA can follow the evolution of the coastline year after year and season by season.

This long-term monitoring is indexed to the sand cycle, which obeys a few main principles. Beach stock is scoured by the very energetic swells of winter, then it is returned to shore by the calmer sea conditions of spring and summer. The storms that occur throughout the bad season disrupt the game. Suddenly, they can submerge the beaches, cut into the coastal dunes over several meters and threaten property. The OCNA storm network is monitoring them closely. It is now capable of calculating an “erosion index” over five days, in anticipation of the damage that an event will produce on the coastal strip. “Here too, it is a decision-making tool. To issue a danger order on a building or to prohibit access to beaches before the arrival of a storm, for example,” we explain to the Observatory.

An Observatory since 1996

The Aquitaine Coast Observatory was created in 1996. Now on the scale of the Nouvelle-Aquitaine region, the coastal area studied extends to the Bay of Aiguillon, the northern limit of Charente-Maritime . The scientific network is financed by Europe, the State, the Region, the four coastal departments (Charente-Maritime, Gironde, Landes, Pyrénées-Atlantiques), the Intercommunal Union of the Arcachon Basin (Siba), the BRGM and the National Forestry Office (ONF). The BRGM and the ONF are the two technical operators. The Observatory works with universities in the region and public organizations such as the Conservatoire du littoral.