The Tapovan Vishnugad hydropower plant, in the aftermath of the devasting flash flood of 7 February 2021

© Irfan Rashid, Department of Geoinformatics, University of Kashmir

India: origin of the flood that devasted Chamoli


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Networks of researchers, an alliance of experts from various disciplines and a series of international collaborations have identified the cause of the geological disaster that occurred in India in February 2021. More than 200 people were killed or reported missing, following an avalanche of rock and ice. What preventative measures can be put in place to ensure that such a disaster does not happen again?

On Sunday 7 February, in India, a torrent of water, rocks and ice gushed down the valleys of the Chamoli district, in the north of the country, resulting in the death or disappearance of more than 200 people and the destruction of two hydroelectric power stations. That afternoon, Pascal Lacroix, a geophysicist with the Institute of Earth Sciences (ISTerre) in Grenoble, was contacted by colleagues. Within a few hours, a network of around 50 scientists had formed, with the key aim of pooling their complementary skills to determine – practically live, as things were happening – the cause, significance and impact of the event.
“There was already a network of remote-sensing experts, seismologists, hydrologists, geologists, glaciologists and sociologists working in the region on glaciology issues,” says Pascal Lacroix. “A network that several of my colleagues at LEGOS and CESBIO in Toulouse and Norway were part of.” The geophysicist is a specialist in landslides and was on assignment in Norway, working on a project studying the links between between landslides and deglaciation. He joined the team immediately.

Satellite images

What caused this “devastating flood”? “The initial hypotheses implicated a flood caused by an overflowing glacial lake,” recalls Dan Shugar, a geomorphologist at Canada’s University of Calgary and lead author of the study, “but there wasn’t a glacial lake big enough in the vicinity of the site to have caused such a flood.” Thanks to high-resolution satellite images, obtained with the support of the French Space Agency (CNES), the scientists were able to determine the source of the disaster: “an enormous mass of rock, and the ice on top of it, became detached from a peak on the north face of Mount Roti and caused the devastating flood,” explains Pascal Lacroix. “It flowed a great distance, destroying the hydroelectric dams.”

  • 27

    million cubic meters of rock and ice

  • More than


    meters of elevation

  • At the average speed of



More than

At the average speed of




million cubic meters of rock and ice

meters of elevation


In fact, the flow generated a wave measuring more than 200 metres high, which cascaded down the initial slopes at 200 km/h! It was an exceptional event in terms of the distance covered. Just two other ground movements have covered a longer distance, both of them in Peru, from the summit of Huascaran (in 1962 and 1970). This propagation distance can be explained by “optimum” proportions in the mix of rock and ice, such that the combination of speed and friction caused the ice to melt, thus creating this devastating torrent.

What about future risks?

As well as identifying the cause of this particular event, the satellite imaging reveals that previous large masses of ice had become dislodged from the same crest and struck the same area of the valley in recent years. “That places a question mark over the durability of the hydroelectric dams that India has developed recently,” warns Pascal Lacroix. How do we know that future avalanches won’t result in the same scale of disaster? Particularly as it’s difficult to determine why this landslide occurred. Although climate change is suspected of having played a role, there’s no clear proof, mainly because not enough scientific measurements have been taken in the region.

Seismology and collaborating on prevention

“However, seismometers are recording what is happening on the roof of the world, as Pascal Lacroix points out. Data from those set up in Nepal showed clear indications of avalanche activity”. 

The seismometers at the Bayn and Ghan stations, in Nepal, located around 200 km from the avalanche, clearly recorded the points at which the rock broke off (1st arrow) and when the valley was hit (2nd arrow).

© D. H. Shugar et al., Science 10.1126/science.abh4455 (2021)

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Even though they were located 200 km from the site of the disaster. “With a view to prevention, one solution would be to network the seismometers in the region,” suggests Pascal Lacroix. “That would require a lot of effort, but it’s likely that the early detection of avalanche signals would enable the areas downstream to be evacuated. In the Chamoli case, it took between 10 and 15 minutes from the appearance of signals for the torrent to reach the hydroelectric dams.” 

The scientist also acknowledged the collaboration that had already existed for some time between the French Alternative Energies and Atomic Energy Commission (CEA) and Nepal, via the country’s National Centre for Research and Monitoring of Earthquakes. “Without this longstanding partnership, we would not have been able to access the data,” Lacroix emphasises. He is therefore calling for more robust and wider discussions around the issue of prevention, in particular. His desire for collaboration is shared by Monika Jha, head of the National Centre for Research and Monitoring of Earthquakes within Nepal’s Department of Mines and Geology. She says: “It is really important that we continue to conduct this type of research in order to develop an early-warning system.”