The need
The commune of Pénestin in Brittany is regularly subject to cracking phenomena both in the walls of houses following episodes of drought and in its coastal cliffs (the Mine d'Or), causing the latter to retreat. Between 2012 and 2020, BRGM carried out detailed geological mapping (on a scale of 1:10,000) to understand the phenomena involved and provide the knowledge needed to take them into account in spatial management and planning.
As a result of several episodes of cracking of the walls of individual homes and recurrent erosion of the coastal cliffs, the commune of Pénestin, which is the most affected in Brittany by the phenomenon of coastal cliff retreat and the second most affected by the clay shrinking-swelling hazard, wanted to gain a better understanding of all the natural hazards and predict how they would develop in the context of climate change. To meet this need, the Morbihan Departmental Directorate for Territories and the Sea asked BRGM to carry out a study comprising several parts, designed, on the one hand, to identify the causes of these cracks (coastal or shrinking-swelling of clay formations) and, on the other, to make projections of the coastline for 2030 and 2100.
Mechanism behind the retreat of the Mine d'Or cliff (Pénestin, Brittany).
© BRGM
The results
Between 2012 and 2020, BRGM initially carried out high-resolution geological mapping on a scale of 1:10,000, by installing eight underground monitoring structures (including six piezometers and two inclinometers) and equipping ten homes with Saugnac gauges to monitor cracks. This initial study clearly showed that the cracks in houses in the commune of Pénestin are related to a clay formation, which had not been identified on the 1:50,000 geological map, and which has a more than 80% content of swelling clay minerals (smectites).
BRGM then carried out a diachronic analysis of the retreat of the Mine d'Or cliff, over as many periods as possible (11 in total from 1952 to 2018, with an average time step of 6.6 years, including four decadal time steps, four time steps of six years and two time steps of two to three years). This enabled BRGM to determine the rates of retreat of the Mine d'Or cliff: instantaneous rates of 1 to more than 2 metres, in jolts, and an average semi-secular rate of 0.3 metres per year, whereas the rate previously calculated was 1 metre per year. The result is a map of the projected coastline for 2030 and 2100, taking into account the effects of climate change.
Using the results
These two parts of the study, which were carried out successively to ensure that they complemented each other more effectively, enabled BRGM not only to identify more precisely the areas of the municipality at risk of severe clay shrinking-swelling and to inform real-estate purchasers of the risk, but also to analyse the retreat of the cliff and make a long-term projection. All the results of this study provide the commune of Pénestin with reliable knowledge on which to base its policy of prevention and management of these natural hazards as part of its regional spatial planning.
The partners
- Direction Départemental des Territoires et de la Mer du Morbihan (56)
- Commune of Pénestin
