20 April 2020
Salkantay – what happened after the initial failure?
The gorgeous Planet Labs images of the site of the Salkantay landslide in Peru provide considerable insight into the post-failure processes, and suggest that the landslide behaved in ways that I find quite surprising.
The image below shows the foot of the slope, Salkantay Cocha Lake and the initial part of the track:-
It is worth comparing this with the Google Earth image of the same area, collected in September 2019:
Starting at the southeast corner, there is a new landslide deposit between the track/path and the moraine ridge. This deposit is mostly fine-grained but with some large boulders present too. The moraine ridge has been smoothed by the passage of the landslide. There are two possible interpretations for this. Perhaps landslide has spread laterally after failure. This is possible but feel slightly counter-intuitive as the large boulders will have had to have undergone a major change in trajectory? Or perhaps there was also a failure on the slope orientated roughly north-south above the lake, which is in line with this deposit?
The main part of the landslide has traveled almost due west into Salkantay Cocha Lake (this is sometimes termed Humantay Lake). The path that it took as it entered the depression, and the path it took as it left, are both clear. The intriguing aspect is that the amount of water in the lake appears to be essentially unchanged. This implies that the landslide skimmed across the surface of the lake without disturbing it. I find this really surprising. The alternative explanation, that the lake refilled after passage of the landslide, seems unlikely as there is no obvious source for this amount of water. The water was present in the lake on 10 March, and reports immediately after the landslide also indicated that water was present.
This is very perplexing behaviour.
The track of the landslide after leaving the Salkantay Cocha Lake is also clearly shown in the imagery. The southern margins of the track appear to me to be indicative of dust deposition:-
The implication is, I think, that this portion of the flow was mostly dry – i.e. it was a rock avalanche at this point, not a debris flow. The videos much further down the valley appear to show a much wetter flow, which I think implies that the landslide entrained substantial amounts of of water (and probably wet sediment) as it traveled downstream.
These Planet Labs SkySat images are a remarkable resource for understanding this complex landslide. Your thoughts on my interpretations are very welcome.
Reference and acknowledgement
Planet Team (2020). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/
Many thanks to Robert Simmon of Planet Labs, and to his colleagues there, for tasking the SkySat instrument and for providing the imagery. Their help and support is hugely appreciated.