11 February 2021
High resolution Planet Labs imagery of the Chamoli landslide in Uttarakhand
Planet Labs has very kindly captured a set of high resolution SkySat imagery of the Chamoli landslide, which they have made available to the research community. Once again I must note my appreciation and thanks for this – it is invaluable. The dataset is so rich that I hardly no where to start with it; at this stage I don’t really have a plan as to how I will post it here. But a good place to start is with the block that detached to create the disaster.
I must also add here that all my interpretations on this blog are initial – they are not intended to be definitive. Proper peer reviewed papers will follow; I hope that I will be involved in some of them. They will provide the definitive story. My posts here are intended to provide an initial interpretation and to give insight into how our thinking evolves over time. As with previous posts, I will seek to correct the posts as new information becomes available.
This is the a Planet Labs SkySat image of the landslide source (part of a much larger image). The image is fantastic, and alongside the Pleaides image that is also available, it gives a really clear impression of what has happened:-
For orientation, the valley floor is to the north, and the block that has dropped off is about 550 m wide. This is a very steep slope, so the block is very large indeed.
My interpretation is that this is a wedge failure – note the inclined surfaces on the east and west side, although the basal shear plane (the darker rock in the image) may be another, slope parallel, joint. The block has dropped out of this wedge-shaped source, and fallen almost vertically about 1800 metres.
It is interesting to compare the image above with one from the days before the collapse. The image below of the site of the Chamoli landslide was captured by Planet Labs using their lower resolution PlanetScope instrument on 30 January 2021:-
A couple of things to note here. First, a large tension crack at the crown of the landslide is clearly visible in the January image. To have been captured on imagery with a 3 m spatial resolution means that this was a really substantial feature. Second, whilst there have been some suggestions that this was a hanging glacier. There was some thick ice in the upper reaches, but I don’t believe that this could be described as a hanging glacier. Further down the slope, on the block that failed, bare rock is visible.
Thus, I continue to believe that this is the failure of a rock block not of a hanging glacier. It is a landslide not a glacial event, in my opinion at this time.
Planet Team (2021). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/