Immediately after the landslide, there was a debris flow that had progressed to Bondo. Experts spoke of a unique phenomenon. Now there are first explanations.
In order for a so-called "dry landslide" to develop a debris flow, it needs water – lots of water. Because water reduces the friction and mixes with the debris tree into a soapy sliding mass, which then forms a debris flow.
Muddles develop only after intensive rain
After the landslide it only took 30 seconds for the rock slide to fall through the Val Bondasca and eventually Bondo. Eight people died in this natural disaster, parts of Bondo were destroyed. Neither the residents of Bondo, nor the authorities or the experts expected that.
But where did the water that was needed for that came from? At the time of the landslide, there was dry weather in Bondo and steel blue skies.
Experience has shown that debris only flows after intensive rainfall. The authorities responded quickly and appointed a top group of experts to solve this puzzle.
First hypothesis: Glacier liquefied
The scientists immediately set off after the event, measured the landslide and calculated the debris flows in the computer. Last December the group of experts came to the media and presented a first explanation: the landslide fell on a glacier.
The impact released energy that melted the glacier ice. The water was built into the earth-shifting mass and caused a debris flow. "It was an obvious statement and our first working hypothesis," says Florian Amann.
The geologist and his team of scientists, however, did not agree with this yet, continued their research, analyzed data and images and carefully evaluated all testimonies. Then they found the first evidence that spoke against the original hypothesis – including in video images of the landslide.
New theory: water-saturated sediments
"We saw on the video images of the landslide that the glacier was literally blown away and flew through the air like a kind of ice mist," explains Amann. The powdered ice was settled on the mass of the rock mass and was not mixed with the lower layers of rubble.
The new theory of Amann reads: The rock mass fell on sediments, in whose pores before a lot of water was stored. "You can imagine it as a sponge that is saturated with water," explains Amann. Due to the pressure of the earth-shifting mass, the water was squeezed out of the pores.
According to Amann, eyewitnesses of "geyser-like" water fountains reported: "That is typical of such events". The extruded pore water formed a sliding mass which produced the waste stream.
Consequences for future risk assessments
For the geologist this new theory has consequences for future risk assessments of landslides. "If such an event would happen again, I would concentrate less on the mountain, more on what lies in the valley, whether the water has saturated sediments."
Time and again it was suspected that climate change was partly responsible for the disaster. But what role does climate change actually play? Such events would only take place every 30 years, the geologist said. "We simply lack the data to make contact with the greenhouse effect."