22 February 2021
The valley blocking landslide at Rishi Ganga created by the Chamoli landslide
The recovery operations in the Tapovan area of Chamoli continue. As of yesterday, the remains of 68 people had been recovered, with another 135 still missing. Operations continue in the tunnel, where more victims are thought to be located, but conditions are clearly extremely challenging. Meanwhile, there is some interest in a valley blocking landslide that has been formed on the Rishi Ganga river by the landslide.
In the last few days an Indian Air Force helicopter winched navy divers onto the dam so that they could check the depth of the water. This was determined to be 8 to 9 metres, giving the lake a reported volume of 50,000 cubic metres.  Note however that other reports indicate that the depth of the lake may be 60 metres. This report indicates that it is “400 metres in length, 25 metres wide and 60 metres deep”. If so this would give a much higher volume.
Planet Labs has managed to capture some high quality satellite images of this lake. The image below provides a really good, oblique view onto the landslide dam:-
The debris flow came from the right of the image, following the main channel to exit at the bottom of the image. The dam is left of centre, with the lake just visible.
Below is a more detailed view of the dam itself:-
The good news is that there is a well-developed channel draining the lake, which has a modest volume. The risk downstream should not be ignored, but this does not pose the sort of threat that we saw at, for example, Attabad.
These images tell us something about the flow. I remain a little perplexed by the ongoing discussion about the source of the water in the flow, which seems to ignore the well-known observation that granular flows can occur with little or no water present. Large, dry granular flows give the appearance of having water present,even where they are dry, but they do generate considerable volumes of dust (as was the case here). Where they sweep down the valley, the front may become saturated as water is incorporated.
I am not arguing that the events on the Rishi Ganga were completely dry – they were not – but I do not think that we need to find a source for very large quantities of water. There was enough water present in the form of ice on the block that failed, and in the valley in the form of dead ice, snow and saturated sediments to explain the flow, in my view.
By the time the landslide reached the point shown above it was well-contained within the channel, as demonstrated by the intact trees on either side. There is some evidence of super-elevation on the outside of the bend. The landslide has dumped a large volume of sediment in this valley, which has been retained. The latter part of the flow appears to have eroded the toe.
I would be interested in the views of sedimentologists as to what the presence of this dam tells us about the characteristics of the flow. Comments?
Planet Team (2021). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/