'Hanging' glacier broke off to trigger India flood
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A general view of the damaged Dhauliganga hydro power project in Chamoli district, Uttarakhand, India, 09 February 2021
image captionA huge surge of water, rocks, snow and sediment surged downhill, breaching the dam
A devastating flash flood in India's northern state of Uttarakhand killed at least 32 people and trapped workers in underground tunnels. It also sparked informed speculation on what caused Sunday's deluge.
Did an avalanche cause a glacial lake to burst? Or could an avalanche or landslide have dammed a river for some time, causing it to overflow? Or did ice blocks break off a glacier due to a rise in temperature?
Rescue efforts are still continuing, with more than 170 people missing - 12 workers were pulled out from a tunnel after being trapped in the freezing dark.
Now a team of scientists investigating the incident believe a piece of a Himalayan glacier did fall into water and trigger the huge flood in Uttarakhand state.
A glacier is a slowly flowing river of ice, accumulating snow, rock, sediment and often liquid water.
Five scientists flew over the glacier in a helicopter, took pictures, looked at satellite photographs and collected data from the devastated area.
The origin of the incident, they say, was a glacier on a remote and inaccessible 5,600m (18,372ft) peak called Raunthi.
The scientists believe that a part of this glacier was wrapped around and hanging on a slope, held up by a mass of rock. This part weakened over a period of time due to freezing and melting.
This chunk of ice and rock broke off on Sunday, and hurtled down a steep mountain slope, they say.
When it reached the floor of a valley they believe its rocks, snow and sediment fell into a relatively narrow mountain stream at a height of 3,600m, blocking it.
As the water level increased in the stream, it burst. A surge of water and rubble then rushed further down breaching the dam and causing massive floods downstream. Water, gravity and geology conspired to create havoc, leading to numerous deaths and leaving hundreds in danger.
"What we have found is actually not very common. The weakening of the 'hanging' glacier must have happened over a long period of time as the ice froze and thawed," Dr Kalachand Sain, director of the Dehradun-based Wadia Institute of Himalayan Geology, which sent the team of the scientists, told the BBC.
Climate change, he said, is the "main factor" for rapid temperature changes leading to freezing and thawing of ice, and glacial fractures over time. Glaciers collect snow in winter, and lose snow in summer. But when temperatures rise, they lose ice faster than they can collect it. Ice melts can lead to the retreat of glaciers.
There are some 10,000 glaciers in the Indian Himalayas. Uttarakhand itself has up to 1,495 glaciers and many are receding due to the warming climate. "If something as uncommon as what happened on Sunday even occurs in 10% of our glaciers, the danger to life and property is immense," Dr Sain said.
Clearly, India needs more surveillance of glaciers.