A new study using satellite evidence confirms that a rock and ice avalanche caused the Chamoli disaster in India earlier this year. The resulting mud and debris flood led to massive destruction downstream.

On 7 February 2021, the Chamoli district in the Uttarakhand region of India experienced a humanitarian tragedy when a large mass of rock and ice, around 27 million cubic metres, was released from the steep mountain flank of the Ronti peak.

This collapse caused a flow of debris to barrel down the Ronti Gad, Rishiganga, and Dhauliganga river valleys, causing significant destruction along the route, killing more than 200 people and destroying two major hydropower facilities that were under construction.

In response to this, the International Charter 'Space and Major Disasters', a service that provides satellite images in response to natural and human-made emergencies, was activated. The service provides free access to very high-resolution satellite data such as from Worldview 1/2, Cartosat-1 and Pleiades.

Combined with freely available images from Landsat and the Copernicus Sentinel-2 mission, scientists analysed numerous images acquired before and after the event to quickly determine what was going on and quantify key measures of the event, for example its total volume, elevation differences and travel distances.

This analysis allowed scientists to exclude that a glacier lake outburst flood had been the cause of the disaster. Instead, the study provides satellite evidence that the disaster was caused by a large mass of ice and rock dislodged from the slopes of Ronti Peak, starting as a giant landslide that transformed into a mud and debris flow causing destruction along its path.

The team of 53 scientists and experts came together online in the days following the disaster to re-construct the event and investigate the scope and impact of the flood caused by the landslide. Their study, published on 10 June in the journal Science, not only analysed satellite imagery, but also seismic records and eye-witness 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, commented, "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."

The results of the analysis were also sent to the governmental agencies of India to help them plan and support emergency assistance to the local teams.

Two participants from ESA's Climate Change Initiative, specifically the Glaciers_cci and Permafrost_cci projects, helped with the retrieval and analysis of satellite images which included Copernicus Sentinel-2, PlanetLab and Corona.

Andreas Kaab, from the University of Oslo, was able to determine the volume and ice/rock mixing ratios based on his experience with such events from earlier studies. He explains, "The calculated 80% rock in the avalanche completely converted the 20% glacier ice into water over the 3200 m 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."

Among many other technical details, 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 already some years ago and that an ice avalanche from a neighbouring glacier occurred in 2016. The images from 2016-2020 show the ice avalanche deposit largely melting away over this period.

Frank Paul, from the University of Zurich is science lead of the Glaciers_cci project, and commented: "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."

Andreas 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."

The researchers suggest that in a warmer climate such events might be happening more frequently, and that the full potential of satellite data and knowledge should be utilised to identify possibly dangerous regions.

Related Links
Observing the Earth at ESA
Bringing Order To A World Of Disasters
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When the Earth Quakes

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