Forces of nature in the mountains Debris flow, landslide or landslide? They are all devastating

Storms have devastated large parts of the southern side of the Alps: What exactly caused the destruction? One thing is clear at this stage: there were several events. It is not always easy to distinguish between them.

Several people are dead and it is currently unclear how many are still missing: the severe storms of the past few days have caused enormous devastation on the southern side of the Alps: the Maggia Valley is largely cut off from the outside world and the situation is confusing for the emergency services.

The classification of the various events is also confusing: The media often refer to a "landslide" or "slide" when what is actually meant is a debris flow. There have also been reports of "landslide" or "rockfall". blue News explains what the various forces of nature mean - and what their consequences are.

Landslide

A landslide is when entire, huge parts of a mountain fall into the valley. This last happened in Switzerland in the Bergell valley in southern Graubünden in 2018 on Piz Cengalo and last summer in the Bisistal valley in the canton of Schwyz. The rock masses often end up as debris avalanches in the valley floor and then dam up the water. As the pressure of the dammed water increases, so does the risk of debris flows: This is what happened in Bergell in 2017, where the village of Bondo was devastated by several debris flows following the landslide.

Rockfall

If individual boulders and rocks fall into the valley, it is usually a rockfall: the distinction between a rockfall and a landslide is not always clear and depends on the volumes broken off and the chronological sequence and intensity. Many roads in the mountains are at risk of rockfall: The relevant warning sign should be familiar to every road user.

Debris flow

A debris flow is an avalanche of debris triggered by immense masses of water. Debris flows often occur during or after a heavy rainfall event and frequently in several places in the same valley. Various debris flows have recently devastated the Maggia Valley, the Misox and also side valleys of the Valais.

Landslide

A landslide refers to a landslide of a particularly large amount of soil, usually after heavy rainfall events: The affected slopes are so saturated with water and heavy that the soil starts to slide and can plunge into the valley as a mudslide. The last major landslide in Switzerland before the recent events on the southern side of the Alps was the one in Schwanden GL last summer.

Landslide

Which brings us to the so-called landslide (also known as Rüfe in some parts of Switzerland): Landslide areas are already loose mountain slopes that slide down into the valley as a whole and often move slowly over a long period of time until they suddenly start moving very quickly.

The last major event in Switzerland was the Brienz GR landslide in mid-June 2023. Landslide zones are widespread in the Alps: These are zones composed of loose materials such as sediments, debris or unstable rock layers such as shale. These areas are often closely monitored. If a catastrophic event occurs, it was usually foreseeable for a long time.