A satellite study has revealed the cause of February's Chamoli disaster in India that saw a flood of mud and debris killed more than 200 people.
The flood flowed down the Ronti Gad, Rishiganga and Dhauliganga river valleys and destroyed two hydro power plants.
On 7 February, a large mass of rock and ice, around 953 million cubic feet, was released from the steep mountain flank in the Chamoli district in the Uttarakhand region of India.
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Researchers said that their study, which used multiple sources of data, could help scientists to predict future disasters.
A team of 53 scientists and experts came together online in the days following the disaster to re-construct the event.
Their study, published on 10 June in the journal Science, not only analysed satellite imagery, but also seismic records and eyewitness videos to determine the timing of the event and produce computer models of the flow.
Lead author Dan Shugar, associate professor in the department of geoscience at the University of Calgary, said: "The rapid increase in the number of satellites orbiting Earth allowed our team to understand the basics of what happened in a matter of hours.
"We now have access to satellites that image every part of Earth every day – sometimes even multiple times per day – and this has really revolutionised how we do this sort of science."
Andreas Kab, from the University of Oslo, said: "The calculated 80% rock in the avalanche completely converted the 20% glacier ice into water over the 10,500ft elevation difference from Ronti Peak to the Tapovan hydropower plant.
"This conversion is largely responsible for the devastating impact of the resulting mud and debris flood wave."
Kaab added: "This specific event was extreme and basically unpredictable. However, rock avalanches are known to be highly mobile, far-reaching and devastating when they mix with snow and ice."
Copernicus Sentinel-2 images revealed that the crack near the bergschrund (a crevasse that forms where moving glacier ice separates from stagnant ice) of the steep hanging glacier opened some years ago.
An ice avalanche from a neighbouring glacier occurred in 2016.
The images from 2016-2020 showed the ice avalanche deposit largely melting away over this period.
Frank Paul, from the University of Zurich, said: "This study clearly shows that satellite data could play a larger role in future high mountain hazard assessments, in particular for evaluating large and inaccessible areas."
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