A few days apart, two landslides cut the RD 66 and the RN 20, isolating Haut-Conflent, Cerdagne and Capcir from their main axes. This is reminiscent of the landslide which fractured the roadway of the RD 66, (RN 116 at the time) following storm Gloria in 2020, and many other smaller events.
Table of Contents
- A few days apart, two landslides cut the RD 66 and the RN 20, isolating Haut-Conflent, Cerdagne and Capcir from their main axes. This is reminiscent of the landslide which fractured the roadway of the RD 66, (RN 116 at the time) following storm Gloria in 2020, and many other smaller events.
- Rain, frost or root: who threw the first stone?
- “On a European scale, we are the first”
- Fragile like a rock
- When the tree protects and triggers at the same time
- An unstable mass, 120 meters above the road
- 6 million to protect RD 66
Will climate change accelerate the phenomenon? Not so sure, and not everywhere, according to the scientific elements collected by Made in Perpignan. Photos: work in progress in Thuès-les-Bains © Michel Jauzac – CD 66
The RD66 reopens this Friday, February 6. The Department announces in a press release that the work has progressed more quickly than expected. The road will reopen at 6 p.m., without traffic restrictions.
Rain, frost or root: who threw the first stone?
Counterintuitive. This is undoubtedly the term that best suits the realities of landslides, despite preconceived ideas. The plants known to hold the ground are also those which can trigger the detachment of rock. The most intense showers sometimes prove less detrimental than long rains. Warming could reduce the frequency of landslides by affecting frost cycles, but accelerate it where permafrost reigned. Above all, there is an urgent need to collect data. Immersed in a world where causality cannot be reduced to simplistic sequences.
“We do not have enough information to say that we have an increase in the frequency of rock slides, or their magnitude,” assures Bastien Colas, geotechnical engineer at the Occitanie Geological and Mining Research Bureau (BRGM). He deplores a lack of feedback from communities and other witnesses to develop reliable statistics. “Rock falls are governed by a mathematical law which means that the smaller the rocks are, the more frequently they break away, whatever the geomorphological contexts. »
“On a European scale, we are the first”
An airborne lidar (radar for topography) developed by the IGN results in 3D modeling down to the meter which should enable better risk mapping. Furthermore, the BRGM has just joined the European Spiral program in conjunction with Spanish institutions and Andorra, which will focus on the Pyrenees to define a “movement risk” strategy. This project will conclude this year, and it’s unprecedented. “We aim to provide vigilance cards. On a European scale we are the first. » This will make it possible to target protective measures.
The predisposition of the massif obviously plays a role, for example with a fragility on the metamorphic rocks of the schist type, fractured like a mille-feuille. Along the RD 66, the geology is layered. We observe more earth slides at the foot of the massif, then, as soon as we climb a little, passages with shale more conducive to falling rocks. This is the case of the Thuès-les-Bains sector where the last landslide occurred, even if this specific passage had not yet experienced a similar event. Granite only dominates much higher up, when we arrive on the Cerdan plateau.
Fragile like a rock
Climate change is undeniably bringing more frequent intense weather phenomena. There will be other Gloria storms. But the forces oppose each other. According to an April 2025 study carried out by the Ministry of Ecological Transition, and in conjunction with the ONF and INRAE, the reduction in freezing and thawing cycles linked to global warming could compensate for the increase in intense precipitation. In fact, the freezing of water infiltrated into the cracks creates a wedge effect which pushes the blocks away from the wall. Less frost is therefore rather good news in mid-mountains.
“In the absence of permafrost, there does not seem to be a significant trend in the frequency of rock falls and landslides linked to the context of ongoing climate change” concludes the study.
It’s a different story in very high mountains, where the withdrawal of permafrost, that is to say the earth remaining frozen all year round, is clearly linked to the increase in landslides. These are altitudes above 2700 meters; the Pyrénées-Orientales are only slightly affected.
There remains the question of precipitation. While heavy rain can have a ripple effect, it will run off more than light but persistent rain which will infiltrate deeply.
“If you fill a crack with water,” explains Bastien Colas, “even if it is only a few millimeters wide, you have the weight of the entire column of water. This applies pore pressure. What is very harmful is repeated rain, sufficient to saturate the soil. »
When the tree protects and triggers at the same time
Among the other triggering factors, the roots. Here again, it’s a double-edged sword. In the Pyrénées-Orientales, climate change has favored forest growth at altitudes unthinkable a few decades ago.
“The forest is a natural protection in the landslide propagation zone. Where you plant a forest, it is very rare for a block to go all the way down. »
On the other hand, the plant is a trigger at the level of the wall, with roots which swell in the cracks and in turn produce the wedge effect. “The roots can go down up to six meters into the cracks. It is not rare that, on the scars of a rock slide, we observe rootlets, or a little earth, because roots were there. » The theme is the subject of contradictory views. CNRS researchers, on the contrary, mention cracks undergoing a drying phenomenon and becoming more sensitive to rain due to lack of vegetation.
“You have to admit that on a mountain road, you are going to be exposed to a rock fall. » explains Bastien Colas. “There will always be a car that gets hit by a rock. We are not going to consolidate the mountains, that makes no sense. » However, targeted prevention and protection measures are possible.
An unstable mass, 120 meters above the road
David Richard, director of roads at the Pyrénées-Orientales Departmental Council, talks about the numerous inspections along the main roads.
“Since the start of the year we have had quite a few rock falls in the foothills areas, whether in Vallespir or even in Fenouillèdes. Usually these falls occur throughout the winter season and blocks fall in spring or at the end of winter with the freezing and thawing cycles. But the two years of drought blocked the process. »
We then note a delaying effect with a brutal purging of rocks which should have fallen earlier. “It all came at once with the rains at the end of the year and in January. » According to David Richard, the Conflent sector is more sensitive to rain than to thaw cycles.
On the Thuès landslide of January 23, there was no warning sign. “Generally where the rocks are fractured we regularly pick up stones on the road, this was not the case. » The unstable mass is located approximately 120 meters above the departmental road, with imposing blocks which have mainly stopped on the overhanging railway line. “It is this mass that we must stabilize, therefore purge. » Several explosive shots are planned to evacuate the rocks. After the emergency measures, more lasting protections can be put in place.
6 million to protect RD 66
“In June 2024, we voted for a multi-year investment program on the RD 66, with an amount of 6 million euros to protect it against rocky hazards” recalls David Richard. “We have already put 400,000 euros into it in 2025 and we plan to put 500,000 euros into it in 2026, regardless of the shift that has taken place. »
The measurements depend on the topography of the place. Nets can be placed directly on risk areas, or screens along the road. It is this type of system that will be put in place near Thuès-les-Bains. More rarely, on furniture parts, masonry of the wall is possible. Regarding the road itself, almost half of the RD 66 is installed on retaining walls, which are monitored every year to prevent events similar to that of 2020.
To date, drones are only used to observe road damage, but David Richard imagines it for the prevention of rocky disorders. “It’s not there yet, but I think that in the years to come it could happen. »
A journalist in the Pyrénées-Orientales since 2011, Philippe joined the editorial staff of Made In Perpignan in the summer of 2025.
