25 August 2017

Pizzo Cengalo rock avalanche – Planet satellite imagery captured the landslide in motion

Posted by Dave Petley

Pizzo Cengalo rock avalanche – Planet satellite imagery captured the landslide in motion

The Pizzo Cengalo rock avalanche in Switzerland on Wednesday has attracted considerable attention, not least because of the two amazing videos that were captured of the event.  The first caught the initiation of the collapse and the initial development of the avalanche itself:-

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And second the extraordinarily destructive nature of the flow some 5 km downstream:-

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This is of course far from the first event on this mountain, although with eight people recorded missing it may be the most tragic.  Given that it is well-studied I will allow others to write in detail about it, but would point out one fascinating aspect of the first video.  This is an apparent shock wave (?) that appears to travel ahead of the main dust cloud immediately after the mass strikes the valley floor.  I have tried to capture it in this screenshot:-

Pizzo Cengalo rock avalanche

Possible shock wave from the Pizzo Cengalo rock avalanche? Via Youtube

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I am no expert in these events, but given the speed with which this moves downslope I am hypothesising that this might be a shock wave?  Can anyone else comment?

Perhaps even more interesting than that is a set of images from Planet Labs.  One of their constellation of satellites was overhead at the time, and remarkably has captured the landslide in motion.  This is an image taken on 4th August of the site of the Pizzo Cengalo rock avalanche:-

Pizzo Cengalo rock avalanche

Planet Labs image of the site of the Pizzo Cengalo rock avalanche site, dated 4th August.

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Pizzo Cengalo rock avalanche

Planets Labs image of the Pizzo Cengalo rock avalanche in motion.

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An even more fascinating aspect of this data is that the way in which the Planet Labs satellites collect imagery means that the near infrared (NIR) image is collected separately from the optical (RGB) imagery, with a 0.5 second gap between them.  On this basis, Bas Altena from University of Oslo suggested that the movement might be visible in the imagery.  The gap between these two images can be seen below – if you look carefully you can see that the dust clouds have moved in that 0.5 second period.  This is the NIR image (which has a lower resolution):-

Pizzo Cengalo rock avalanche

Planet Labs NIR image of the Pizzo Cengalo rock avalanche in motion.

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And this, half a second later, is the Red band of the optical imagery:

Pizzo Cengalo rock avalanche

Planet Labs Red band image of the Pizzo Cengalo rock avalanche in motion

 

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Acknowledgement

Planet Team (2017). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://api.planet.com

Thanks in particular to Joe Mascaro of Planet Labs, who pointed out that this event had been captured, Bas Altena from University of Oslo who suggested the RGB/NIR trick, and to various friends who have tweeted, left comments and emailed me about this amazing event.