Earlier in February, as the US was in the icy grip of an Arctic blast of cold air, a tragedy was unfolding on the other side of the planet, in northern India. On February 7, 2021, a large torrent of water and debris wiped out multiple hydroelectric power plants in the northern state of Uttarakhand, killed dozens, and resulted in hundreds of missing people. What happened?
Given the location in the remote mountains, it was thought that perhaps a GLOF (glacial lake outburst flood) or glacial collapse of some sort had occurred. And then conspiracy theories started to spread, including that of Chinese setting off dynamite in the area!
As far as scientists can piece together, using people on the ground, in the air (helicopters) and satellite imagery from Planet Labs (https://www.planet.com), this was a large landslide from a mountainside that eventually transitioned into a debris flow. For a few months, a large crack had been developing on the flank of a 6000-meter mountain called Nanda Ghunti. On the morning of February 7, the block finally let loose and collapsed in a large landslide – cascading down nearly 2000 meters downslope.
The landslide entrained ice and glacial debris as it came down, so it included a large amount of water and sediment, allowing the landslide to transition to a debris flow that surged down-valley, leading to massive flooding. While in this case, the disaster was not due to GLOFs, which are becoming an increasing threat in a time of climate change, erosion and collapse of rock slopes that are becoming unstable from thawing of glacier ice is likely to increase in the future. Here’s a Science Short I made on mountain erosion:
There is concern that there wasn’t adequate monitoring or early warning systems in the valley, especially with multiple hydropower plant projects that were all damaged. This tragedy has brought to light how much more work there needs to be done to mitigate the coming disasters in mountain communities.
More here on how scientists pieced together what happened in a matter of hours (versus days) using satellite imagery: https://blogs.agu.org/landslideblog/2021/02/08/chamoli-2/