Misox devastated What caused the devastating landslide?

Severe storms have caused devastating damage in Misox. A mudslide destroyed three houses in Sorte GR. How can such a debris avalanche even occur? An expert explains.

In the canton of Graubünden, nature has shown all its power: Heavy rainfall triggered a debris flow and caused the River Moesa to overflow. The hamlet of Sorte GR was largely destroyed, and parts of the A13 national road were also swept away by the flood. One person has since been rescued dead, another woman was rescued from the debris.

In an interview with blue News, Prof. Dr. Alexander Puzrin from ETH Zurich explains how such a violent debris avalanche is triggered. He is an expert in geomechanics and geosystems. Puzrin also knows how climate change affects such events.

How does a debris flow actually occur?

Sediment moves down a slope due to gravity. However, friction ensures that the soil stays in place. Gravity is the driving force and friction or shear strength is the restraining force. As the latter depends on the groundwater flow and this rises during heavy rain, resistance is reduced.

Highway gone, village buriedPictures show the extent of the destruction in Misox

So if there is too much water, the friction is no longer sufficient - and the sediment loosens and rolls downwards.

Is the affected region prone to this kind of disaster?

Absolutely. The terrain here is difficult. You need a slope for a debris avalanche. And there are many, many slopes in Graubünden. And if the material that makes up the slope is not solid rock, but consists of different types of sediment, the ground is unstable. So if there are different types of soil, from large to small stones to gravel, sand and clay, this can be particularly risky if there is heavy rainfall.

In Graubünden and many other regions of Switzerland, such combinations of steep slopes and relatively weak deposits occur, so that the risk of landslides and debris avalanches is high.

Was it foreseeable that a debris avalanche could occur in the current weather conditions?

It has rained very, very heavily in a short space of time. I think it was about 125 millimetres in one day, whereas the monthly norm in June is 154 millimetres. If the same amount of rain falls over a longer period of time, it doesn't have the same effect, because then the water has time to drain away and there wouldn't have been such a strong flow within the very permeable sediment. So the heavy and intense rain was the problem.

Through extensive monitoring and analysis, experts could make predictions, but it is impossible to cover the whole country with so many mountains.

Will climate change lead to more disasters like this in Switzerland in the future?

Climate change has a twofold effect in mountain regions: on the one hand, temperatures are rising and, on the other, more concentrated and intense events are occurring more frequently. This means that it probably won't rain more throughout the year, but there will be more heavy rain events instead. And in the periods in between, it is relatively dry.

Weather in the northern hemisphere"What was once extreme but rare is becoming increasingly frequent"

These intense events - as we are currently experiencing - are not good. On the other hand, they are not equally bad for all soils, because with some types of sediments that are not very permeable, a large part of the water simply runs off the slope during heavy rain without penetrating the soil. The higher temperatures can in turn have the effect that water evaporates from the surface instead of penetrating the slope, and thus the slope stability is not weakened.

So we cannot directly say that climate change will favor landslides. However, for the type of debris avalanche we have seen in Graubünden, climate change is not a good thing, but rather increases the risk.

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