8 February 2021

The catastrophic landslide and flood in Chamoli in Uttarakhand: the sequence of events

Posted by Dave Petley

The catastrophic landslide and flood in Chamoli in Uttarakhand: the sequence of events

Deciphering the sequence of events that lead to the terrible debris flow / flood in Chamoli yesterday was quite a challenge, especially on a Sunday.  However, a combination of the willingness of a number of people to use their expertise, and the availability of daily satellite imagery from Planet Labs, meant that we had the sequence determined within 8 hours of the event.  This is remarkable – the landslide occurred in a very remote location, and the infrastructure was severely damaged.  It took us a few days to understand the 2012 Seti landslide in Nepal, which was similar to this one.  Now we can do this in a few hours.

So, for clarity, I thought it would be helpful to lay out the sequence events.  I am going to lean heavily on Twitter, but these tweets come from a combination of world experts and citizen scientists.  I have no doubt that this is the true story of what happened, but there are details to understand still.

1. For a few months a large failure had been developing in the high mountains

Although we did not detect it, high in the mountains at 30.339, 79.731 a large crack was developing on the flank of one of the high peaks over a few months.  I believe that this is Trisul, but there is some uncertainty about the names of the peaks.  It is now clear that this mountain was Nanda Ghunti.  This crack was detected after the event in a sequence of Sentinel satellite images by Julien Seguinot.  At the end of the sequence the unstable block is clearly visible:


These cracks defined the block that failed.

2. On the morning of 7 February 2021 the block collapsed in an enormous landslide

The failure of this block shows up very clearly in a satellite image collected later in the day by Planet Labs.  It was first identified by Dr Dan Shugar:


The best view of this has been provided by Scott Watson:-


The 3D diagram shows the events brilliantly.  The block of rock, with some ice, dropped from about 5,600 m to about 3,800 m, a fall of almost two kilometres, before impacting on the valley floor.  It will have instantly fragmented to generate a huge rock and ice avalanche, which travelled down the glacier.  This would have been extremely fast and very energetic.  On the way it generated a vast quantity of dust, which is smeared on the valley side to the west of the glacier  valley.


The landslide from Trishuli that caused the Chamoli flood

The landslide from Trisul that caused the Chamoli debris flow and flood. Image copyright Planet Labs, used with permission.


3. The landslide entrained stagnant ice and glacial debris

Down valley (to the north) the rock avalanche was able to entrain a vast amount of water and sediment.  The most likely source for this was identified by Matt Westoby:


It seems likely that much of this material was the remains of previous landslides and some glacial material.  The rock avalanche will have generated a huge amount of heat and will have induced fragmentation.  The ice will have been entrained into the flow, crushed and melted, allowing the landslide to transition to a debris flow.  The volume will have continued to increase as the landslide rushed to the north.


4. The flow followed the valley to the west, and struck the populated areas

The Planet Labs images show that the landslide swung round to the west, and raced down the valley. The Planet Labs image below, taken shortly after, shows the aftermath:-

The area of the landslide downstream from Trishul

The area of the landslide downstream from Trisul that caused the Chamoli debris flow and flood. Image copyright Planet Labs, used with permission.


Note that dust still in the air when the image was collected.

The movement of the landslide was so rapid that the debris flow pushed water from the river ahead of it before incorporating it. You can see this very clearly in one of the videos:



Unfortunately there were many people working on the river floor, and in tunnels, as part of the two hydroelectric power schemes in the Chamoli area.  This appears to account for the majority of the human losses.  Rescue operations continue.


Misinformation and conspiracies

Whilst the cause of this event is clearly established, the media is continuing to publish stories that this was a glacial collapse, or an avalanche, or a GLOF.  There was real uncertainty at the start, but we have known what happened for 24 hours now.  This is quite unfortunate.  I hope that this post will set the record straight.

This is important as a set of conspiracies are starting to emerge as to the cause of this landslide.  For example, there are suggestions that such an event cannot occur in winter, meaning that it was dynamite that triggered the disaster.  Some sources are linking this to activities by China.

This is not the case.  This was a classic progressive failure in mountain flank.  We have seen similar events elsewhere.  It is fair to say though that these events might be increasing in frequency as a result of climate change.



Planet Team (2021). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/