Reference, completed project

SIBLES2: Characterisation of the dune geosystem in Pays de La Loire

What lies under the dunes? As part of the SIBLES2 project, co-funded by the Pays de la Loire Regional Environment, Planning and Housing Directorate (DREAL), BRGM has developed a methodology for characterising the region's dune systems.

13 October 2025

Geophysical acquisition by georadar at La Turballe (44) for the SIBLES2 project (April 2023).

Challenges and needs

The coastline itself is the most widely used indicator for studying coastal change. But this boundary between land and sea is part of a complex coastal environment that is often little-known. Although sandy coasts share common characteristics, they also have noteworthy local particularities. These environments pose a major challenge for coastal managers, particularly when it comes to assessing the vulnerability of dune belts to erosion and the risk of them breaking-up during storms, or anticipating their evolution in response to global change. A better understanding of these environments is also essential to encourage local inhabitants and elected representatives to accept and take responsibility for management options (including 'soft' methods).

The approach developed in SIBLES2 goes beyond landscape analysis and monitoring the evolution of the coastline. It provides an in-depth understanding of the coastal geosystem, looking at the history of its formation, its internal organisation and sedimentary and geotechnical properties. Improving our knowledge of this environment also means acquiring the keys needed to understand the current dynamics of coastal areas and being in a better position to anticipate future developments on the coast in response to the challenges posed by global change.

The SIBLES 2 project has given us a better understanding of the geological structure that lies beneath our coastal dunes, which is a useful complement to coastline data. This innovative and reproducible methodological approach completes the range of tools available for understanding the evolution of our coastline in the context of climate change. The reliable results obtained at the three sites in the Pays de la Loire region confirm the value of this scientific approach. The method developed, which can be transposed to other sites, is a valuable asset for improving the management of our coastal heritage.

Thibaut Novarese, Head of the Natural and Technological Risks Department, DREAL Pays de la Loire

Synthetic geological models of coastal geosystems along a sandy coast made up of dune systems: difference between the conventional view of a sandy coast and the actual situation for 3 sites in Pays de La Loire.

Results and use

The sandy coast is traditionally considered to be a homogeneous environment. The results of this study highlighted the unsuspected geo-diversity of the sandy coast of the Loire. Three sites in the Pays de Monts (85) and La Turballe (44) were explored using a combination of geological, geomorphological, geophysical and geotechnical techniques. Although they have strong points in common, such as being located on a sandy coast close to marshes and characterised by dune systems, investigation of these three coastal geosystems revealed their individual past dynamics and specificities. Each site has thus been characterised in detail, by integrating it in an ocean-land-marsh continuum, allowing its specific features to be linked to those of the nearshore zone, the substratum, and the backshore marshes.

Taken together, this knowledge made it possible to draw up a specific "identity card" for each site, facilitating comparisons and highlighting their singularities, as well as developing a synthetic geological model. In particular, this has enabled us to identify under the dune landscapes (i) ancient paleo-shoreline marks, (ii) the actual transition zone between the marsh and the sandy coast, (iii) a clay-filled meander under the coastal dune a few metres from the current shoreline, (iv) to understand the stages in the formation of a sedimentary spit, etc., and (v) to study the resilience of a beach/dune system in an urbanised area.

The results of SIBLES2 underline the importance of an integrated and interdisciplinary approach in furthering our understanding of the diversity of coastal geosystems and then adapting their management to take account of environmental challenges and climate change. SIBLES2 is also a tried and tested methodology that can be deployed on any new site on the sandy coast. Managers of coastal municipalities in the region have already shown an interest in using this methodology in their areas, and the strategy is now being applied to other parts of France (French Guiana, New Aquitaine, Corsica).

BRGM’s role

This research project was led and carried out by BRGM, and co-financed by DREAL Pays de La Loire. BRGM's interdisciplinary team helped to develop an innovative methodology for investigating the coast, thereby advancing our knowledge of the sandy coastal areas of the Loire. Through its work and reports, BRGM has also been able to pass on this knowledge and the new approach to the managers of the Vendée and Loire Atlantique coastal sectors (DREAL, managers, DDTM, regional actors, etc.) as well as to the scientific community (through conferences and publications).