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Scientists have found the origins of a mysterious and deadly flood in India


On February 7, a massive flood swept through a valley in the Uttarakhand state of the Himalayas in India, washing away two hydropower plants and leaving at least 200 dead or missing. What caused the deadly flood was a mystery, but after accumulating evidence from satellite imagery, seismic records and eyewitness accounts, a team of more than 50 scientists claim to have solved the case.

The final culprit was a massive avalanche of rocks and glaciers that fell 1,800 meters down a steep slope of Ronti Peak, causing a cascade of events that led to the disaster, according to online researchers on June 10 in Science.

It was not an ordinary landslide, says Daniel Shugar, a geomorphologist at the University of Calgary in Canada. "This was a multi-risk scenario where it was much more fluid and mobile than would be expected to be a landslide. It was the worst of the rock and ice cases and (the) height of the fall."

This animation shows the progress of the avalanche, as it rushes down towards the two Tapovan hydroelectric dams. The colors show what material dominated in the avalanche as it flowed. At first, where the avalanche is red, mostly (about 80 percent) was rock and 20 percent ice. The green is mostly ice, as the rocks begin to splash or settle. By the time the flow reached Tapovan, it had already turned into water (blue).

Initially, the culprit was suspected to be a well-known high mountain hazard called glacial lake flooding, in which dammed water suddenly spills over its boundaries and rushes down the mountain slope (SN: 2/9/21). But the few data available immediately afterwards pointed to a possible landslide, Shugar says.

In the months that followed, he and his colleagues used numerous data sources and computer simulations to thoroughly reconstruct what happened that day.

This is what the data shows:

As of 10:21 a.m. local time on Feb. 7, about 27 million cubic feet of rock and ice had fallen from the steep northbound of Ronti Peak, which sits at 6,063 feet above sea level. The landslide, consisting of about 80 percent rock and 20 percent ice, originated at a height of about 5,500 meters and fell down about 1,800 meters, traveling at a speed of up to 60 meters per second.

On February 9, two days after the disastrous flood, French Pleiades satellites made this high-resolution image of Ronti Peak, showing a scar on the steep north face of the mountain spanning 500 meters. That scar, a novelty, marks the origin of the avalanche of rock and ice that caused the flood. Using stereo images from these satellites, the researchers also built high-resolution 3-D models of the site, which allowed them to estimate the volume of the avalanche.© CNES, Airbus DS

Digital elevation models now reveal a rock scar on the slope that didn’t exist before. Previous images from the site suggest that a very long, wide fracture was opened in the overhanging glacier in 2018.

When the landslide rushed down the valley of the Ronti Gad stream, wet material splashed the sides of the valley depositing sediment and large stones on the valley walls. Satellite images also captured thick blankets of dust in the air – the first indicators that a landslide could be the culprit.

As the detachment continued to descend, the ice began to melt due to friction, helping to accelerate it. Then the landslide found a sharp curve in the valley and much of the solid material fell, changing it from a thick, viscous flow to a smoother, faster-moving flow. These precipitous waters were now on their way to the two hydroelectric power stations that were on their way downstream. Eyewitness accounts have only seen this part of the flood.

On February 7, the day of the disaster, a constellation of miniature satellite cubes belonging to the private imaging company Planet Labs made these images of huge deposits of dust scattered across the valley below Ronti Peak but above the hydroelectric dams. These images were one of the first clues that a landslide, rather than a glacial lake flood, was responsible for the disaster.

There are no easy answers to whether or how people can prepare for such a disaster, but the first step is a better understanding of the possible causes, Shugar says. That’s where he hopes this study can help. "We need to do a better job in hazard assessments and not examine hazards in uniqueness."

As for the role that climate change may have played, it is also difficult to say. There are no weather stations near the site of the original slope fault that could provide temperature or precipitation measurements to assess climate change in that region. But “we can say that climate change is increasing the severity and frequency of natural disasters,” by thinning glaciers and their foundations, Shugar says.

And it is also clear that increased development in the mountains increases the likelihood of disaster, adding to the urgency of understanding the potential dangers, he points out. "If the mountains themselves are getting more and more dangerous and we're also advancing in those mountains, that becomes a dangerous mix."

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