Torrents of mud that carry away everything in their path, houses destroyed and a hamlet completely buried in a few minutes. During the night from Thursday to Friday, the Vénéon violently came out of its bed and cut off the inhabitants of La Bérarde, in the commune of Saint-Christophe-en-Oisans, in Isère, from the world. Thanks to the rescuers, all the people were sheltered and no casualties were reported but “a lot of material damage was noted on homes and roads”, indicated the prefecture of Isère .
How could such a meteorological episode have occurred? What is this type of phenomenon? Why should we expect to be able to relive it? 20 minutes make the point.
What happened during the night from Thursday to Friday?
During the night from Thursday to Friday last, torrential floods hit Savoie and Isère, in the Oisans and Ecrins massifs, causing significant damage. “A torrential lava, that is to say, a flow of debris, a mixture of mud, sand, pebbles and boulders several meters long, came down from the mountains and covered the hamlet of La Bérarde, in the central part of the Ecrins national park”, describes Thibaud Simon-Labric, geologist and member of the Oisans geology center.
“This area is very wild and is surrounded by peaks reaching an altitude of up to 4,000 m, with extremely steep slopes on which there can be significant accumulations of precipitation, particularly in these stormy contexts,” he explains. -he. According to Christophe Béchu, Minister of Ecological Transition, some “200,000 m3 of water flowed down the Vénéon valley”. “These elements descended into the valleys with steep slopes and slopes, hence the violence of the torrents,” indicates the geologist.
What do we call a “burst lava phenomenon”?
We describe the phenomenon as “torrential lava” when “we no longer have a flow associated with water but with mud,” explains Thibaud Simon-Labric. Before developing: “We then have a much more viscous flow which will transport large blocks of rock. The debris flow will also be more powerful than transparent water. »
Thus, in the Oisans and Ecrins massifs, the flow rates and speed of the flow were “very significant”. “The water was loaded with mud and the transport capacity of the flow was so powerful that it was able to tear down houses,” supports the specialist.
How could such an event happen?
Several factors are responsible for the intensity of the phenomenon: heavy storm precipitation and the melting of the snow cover. “For several weeks, we have had continuous precipitation which means that the rivers and soils are waterlogged,” underlines the geologist. We also had significant precipitation this winter. Which means that at altitude, at more than 2,500 m, there was a lot of snow stock. The extreme phenomenon that we experienced is therefore due to the additional episode of brutal stormy rains coupled with the atmospheric warming of recent days,” explains the expert.
Before adding: “All the mountain areas where this precipitation fell and which are at significant altitudes have seen all their rivers and torrents increase in power in a very impressive manner in recent days. In the Ecrins, it broke out, all the rivers were in flood in the area. »
Should we expect other similar events?
According to the geologist, it is possible to experience this type of phenomenon again if other brutal episodes occur in the coming days. “Otherwise, things should calm down,” he said. Traditionally, in these places and at these times, in late spring and early summer, there is an increase in river levels due to the melting of the snowpack. The risk of this happening again decreases during the summer. »
But Thibaud Simon-Labric has also noted a “multiplication of extreme phenomena” in recent years, linked to global warming. “There are seasonal parameters, such as this snow cover coupled with intense precipitation which explain the phenomenon but there are also the effects of global warming which accentuate it,” he analyzes.
Before developing: “The potential increase in this type of extreme precipitation will occur in very short periods of time, in wild places where the consequences are brutal. Already for a year and a half, there seems to be an increase in these flood events and associated damage. »
On July 29, 2023, the slopes of the Etançons valley, in the Ecrins, were affected by 200,000 m3 of rocks and mud. The Ecrins National Park then dedicated June 11 and 12 to an operation to “rehabilitate the La Bérarde hiking trails”. “From July to October, the various floods and debris flows washed away kilometers of trails and 37 of the 44 footbridges maintained by the Park in the valley,” explains the park on its website.
How to adapt to global warming and its consequences in the mountains?
As the Oisans geologist observes, “this type of extreme phenomenon, originally extremely rare, will be likely to recur on a more regular basis due to global warming”. Thus, according to him, it is necessary for “territorial stakeholders to take into account the evolution of the climate and its consequences, particularly on the transformation of the landscape”.
He asks: “Perhaps there are road networks that we thought would be sustainable which are no longer so if these kinds of events are repeated? » He also thinks about access to the territory, the tourist and economic activity of the territory which must be considered accordingly. Christophe Béchu announced on Saturday “a damage assessment mission” in this disaster area to “very quickly release an emergency fund for local authorities” and allow the start of work “in the coming days”.