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A stream of debris again misses the village of Brienz by a hair’s breadth. Rock masses stopped at the edge of the village as early as 1877. Worse was feared this time, but Brienz was lucky again. Because the current landslide also has its own character.
Only a few meters were missing, and the rock masses would not only have buried the cantonal road, but also parts of Brienz. A scenario that the geologists previously considered likely. But the part of the so-called island that slipped off in the middle of the night stopped in front of the first building, the former school building.
On the basis of a historically documented landslide, the geologists have predicted how dangerous the debris flow that has now fallen could become. However, this did not behave as calculated.
Faster than the debris flow of 1877
In 1877, the so-called “Igl Slide” fell just a few dozen meters east of the current slide. Like the current slide, geologists describe this as a debris flow. Means: Rock masses slowly move downhill, similar to honey would do. In the event of 1877, the sliding speed was around eight meters per day.
With the current flow of debris, however, it was 40 meters per day in the last measurements before the last measuring points collapsed. If the “Igl slide” slid over several weeks, the current event lasted only a few hours. According to a model from March, the current flow of debris with the same dynamics as the “Igl slide” would have partially destroyed the village with a probability of 65 percent and completely with a probability of 30 percent.
Fast – but also braked quickly
At first glance it may seem paradoxical: the current slide was faster than the “Igl slide”, but still came to a standstill earlier than calculated. The reason may lie in the rock type of the debris streams. The current debris flow consists mainly of brittle Vallatscha rock. The “Iglrutsch” on the other hand consists of a clayey Allgäu formation. This glided down the valley, so to speak.
The Vallatscha formation is anything but slippery. Rather, it is a heap of rough boulders, which have a high friction among each other. This high friction braked the slide correspondingly faster. Luckily for the village. The “Igl slide” from 1877 also stopped at the edge of the village. But he went down in much flatter terrain.
Does the slide continue?
What the “Igl slide” also teaches the geologists: The current island slide may continue to slide. That will be more of a creep though. Because the “Igl slide” is still moving downhill by 10 to 20 centimeters a year. And that on the village plateau, which itself slides down into the valley. Especially with a lot of rain or heavy snowmelt, it cannot be ruled out that the current slide will continue. However, probably also at most in crawling gear.