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

On February 7, a massive flood broke through a valley in the Indian state of Uttarakhand in the Himalayas, flushing out two hydropower plants and leaving at least 200 dead or missing. What sparked the deadly flood was a mystery – but after collecting evidence from satellite imagery, seismic records and eyewitness accounts, a team of over 50 scientists now say they have solved the case.

The ultimate culprit was a huge avalanche of rock and glacier ice which tumbled 1,800 meters down a steep slope of Ronti Peak and triggered a cascade of events that led to the disaster, the researchers report online on June 10 in science.

That was no ordinary landslide, says Daniel Shugar, a geomorphologist at the University of Calgary in Canada. “This was a multi-hazard scenario where it was much more fluid and mobile than would be expected from a landslide. It was a rock and ice worst case scenario [the] Height of fall. “

This animation shows the progress of the avalanche as it rushes down the slope to the two hydropower plants at Tapovan. Colors show which material was dominant when the avalanche flowed away. At the beginning, where the avalanche is colored red, it consisted mainly (approx. 80 percent) of rock and 20 percent of ice. Green is mostly ice as the rocks begin to splash or settle. By the time the river reached Tapovan, it had mainly turned to water (blue).

The perpetrator was initially suspected to be a known high mountain danger called the glacial lake outbreak flood, in which dammed water suddenly pours over its borders and falls down the mountainside (SN: 2/9/21). But the little data available immediately afterwards instead pointed to a possible landslide, Shugar says.

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

The following is what the data shows:

From around 10:21 a.m. local time on February 7, around 27 million cubic meters of rock and ice fell from the steep north face of Ronti Peak, which is 6,063 meters above sea level. The landslide, consisting of around 80 percent rock and 20 percent ice, originated at a height of around 5,500 meters and fell down the slope around 1,800 meters at a speed of up to 60 meters per second.

On February 9, two days after the devastating flood, the French Pléiades satellites took this high-resolution image of the Ronti summit, showing a 500-meter-high scar in the steep north face of the mountain. This scar, a new feature, marks the origin of the rock and ice avalanche that caused the flood. Using stereo images from these satellites, the researchers also constructed high-resolution 3D models of the terrain with which they could estimate the volume of the avalanche.© CNES, Airbus DS

Digital elevation models now show a scar on the slope that was not there before. Earlier images of the site suggest that a very long and wide fracture had opened in the overhanging glacier in 2018.

When the landslide then tumbled down the valley of the Ronti Gad stream, wet material splashed up on the valley walls and deposited sediments and large boulders on the valley walls. Satellite imagery also captured thick layers of dust in the air – the first signs that a landslide could be the culprit.

As the landslide continued downhill, the friction began to melt the ice, which helped to accelerate it. Then the landslide hit a sharp bend in the valley and much of the solid material fell out, turning it from a thick, viscous stream to a faster-flowing, more fluid stream. These rushing waters were now on their way to the two hydroelectric plants on their way downstream. Eyewitness accounts only saw this part of the flood.

On February 7th, the day of the disaster, a constellation of miniature cube satellites from private imaging company Planet Labs took these pictures of huge dust deposits scattered across the valley below Ronti Peak but above the hydroelectric power plants. These images were one of the first indications that a landslide, rather than an erupted flood of a glacial lake, was responsible for the disaster.

There are no simple answers as to whether and how people can prepare for such a disaster – but the first step is a better understanding of the possible causes, says Shugar. Here he hopes this study can help. “We have to process risk assessments better and are not allowed to investigate risks individually.”

It is also difficult to say what role climate change may have played. There are no weather stations near the original slope failure that could provide temperature or rainfall measurements to assess climate change in that region. But “we can say that climate change increases the severity and frequency of natural disasters” by making glaciers and their underpinnings thinner, says Shugar.

And it’s also clear that increasing development in the mountains increases the likelihood of disasters and increases the urgency of understanding the potential dangers, he notes. “When the mountains themselves become more dangerous and we push further into these mountains, it will be a dangerous mixture.”