Deciphering the sequence of events that lead to the terrible debris flow \/ flood in Chamoli yesterday<\/a> was quite a challenge, especially on a Sunday.\u00a0 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<\/a>, meant that we had the sequence determined within 8 hours of the event.\u00a0 This is remarkable – the landslide occurred in a very remote location, and the infrastructure was severely damaged.\u00a0 It took us a few days to understand the 2012 Seti landslide in Nepal<\/a>, which was similar to this one.\u00a0 Now we can do this in a few hours.<\/p>\n So, for clarity, I thought it would be helpful to lay out the sequence events.\u00a0 I am going to lean heavily on Twitter, but these tweets come from a combination of world experts and citizen scientists.\u00a0 I have no doubt that this is the true story of what happened, but there are details to understand still.<\/p>\n 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.\u00a0 https:\/\/twitter.com\/pyjeo\/status\/1358480152682987523<\/p>\n .<\/p>\n These cracks defined the block that failed.<\/p>\n The failure of this block shows up very clearly in a satellite image collected later in the day by Planet Labs<\/a>.\u00a0 It was first identified by Dr Dan Shugar<\/a>:<\/p>\n https:\/\/twitter.com\/WaterSHEDLab\/status\/1358437031802593280<\/p>\n .<\/p>\n The best view of this has been provided by Scott Watson<\/a>:-<\/p>\n 3D rendering of @planetlabs image collected 7th Feb showing the source of the Uttarakhand disaster located by @WaterSHEDLab<\/a>. Appears to be a complete detachment of a previously glaciated slope #Chamoli<\/a> #Disaster<\/a> #Landslide<\/a> pic.twitter.com\/SElrZh36kH<\/a><\/p>\n — Scott Watson (@CScottWatson) February 7, 2021<\/a><\/p><\/blockquote>\n1. For a few months a large failure had been developing in the high mountains<\/h4>\n
I believe that this is Trisul, but there is some uncertainty about the names of the peaks<\/del>.\u00a0 It is now clear that this mountain was Nanda Ghunti.\u00a0 This crack was detected after the event in a sequence of Sentinel satellite images by Julien Seguinot<\/a>.\u00a0 At the end of the sequence the unstable block is clearly visible:<\/p>\n2. On the morning of 7 February 2021 the block collapsed in an enormous landslide<\/h4>\n
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