4 min readPuneFeb 11, 2021 03:16 AM IST
First published on: Feb 11, 2021 at 03:16 AM IST
Sunday’s flash flood in Uttarakhand was triggered by a large mass of rock and snow falling from a hanging glacier, a preliminary investigation has found.
The sliding mass of rock and snow, along with soil, earth and vegetation, fell into a narrow stream, blocking the flow of water. After a few hours, under the pressure of accumulated water, this blockade gave way, resulting in large amounts of water and debris gushing downstream.
It was not a case of a breach in the glacial lake, which results in an event called Glacial Lake Outburst Flood, or GLOF, as was initially suspected.
A five-member team from the Dehradun-based Wadia Institute of Himalayan Geology made aerial surveys of the location on Tuesday and Wednesday, and had has collected samples of water, snow and debris from the area to make further investigations. A preliminary report detailing the likely sequence of events has been submitted to the Department of Science and Technology, Kalachand Sain, director of Wadia Institute told The Indian Express.
“The team has spotted the scars of a very large rock mass that fell off. This rock was under a hanging glacier. Rocks below glaciers sometimes get weakened due to repeated freezing and melting of snow above them, and develop cracks. It could have happened here as well. When the rock finally slipped, the snow over it also came off, and all this fell down the steep slope, of about 40 degree. This scar of the rock is located at the height of about 5,600 metres above the sea-level,” Sain said.
“The rock scar looks very large. It would be difficult to estimate the size, but it would be several tons of rock and snow coming down. As it sloped down, it carried everything else with it, vegetation, soil, earth, debris. All of this rolled almost 2,000 metres down, before falling into a narrow stream of water whose flow is maintained only by the melting of snow in the glacier. This happened at a site that is about 3,500 metres above sea level,” he said.
“This large mass of rock, snow and everything else blocked the course of the stream, and this situation would have remained for several hours before a breach would have happened because of accumulating water, and other material. This would have burst open, leading to a sudden discharge of a large amount of water mixed with rocks, debris and all sorts of other things. From what the team has been able to observe, this is what seems to have happened,” Sain said.
Sain said the team had completed its initial investigations and was likely to return tomorrow. “They can stay there another day if they have to pick up more samples from the area, but otherwise they can return. They have got very good pictures from close distances, and collected all the samples they need. This is not the time where we can reach the spot, and carry out any further observations. But we would return to the area sometime after March, when it would be easier to access. Right now, roads, bridges are all gone,” he said.
A preliminary report detailing the likely sequence of events has been submitted to the Department of Science and Technology, Kalachand Sain, director of Wadia Institute told The Indian Express. 
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