Scientists find clues in satellite imagery to the cause of catastrophic floods in Nepal and Tibet

As rescue teams searched Thursday for missing people following catastrophic flooding along the Nepal-China border, earth scientists reviewed satellite imagery for clues to the cause and assessed the devastation.

The flooding killed hundreds of people in Nepal and Tibet on Wednesday. Many more are still missing, including scores of foreigners and pilgrims.

Geomorphologists Kristen Cook and Dan Shugar said initial images from Nepal on Wednesday were obscured by clouds and dust from the debris, so they could only see that part of a glacier had broken off from the Langtang Lirung peak, near the border with Tibet. In clearer images from Thursday, they said they could see that the bedrock underneath the glacier had collapsed, taking the massive chunk of ice down with it and sending boulders and ice into the valley.

“It shows the whole area without anything hidden, and we could see that, yes, it looked like there was not only this glacier that had failed, but a much larger chunk of the bedrock of the mountainside itself that had given way,” said Shugar, associate professor at the University of Calgary. “You have this big bedrock failure that took part of the glacier with it.”

Shugar said it’s heart-wrenching to look downstream from there on the images, to where people were living, and see how the debris has buried villages.

After slamming to the valley floor, the debris maintained its momentum through the steep, narrow valley as ice melted, said Cook, at the Université Grenoble Alpes in France. Water and mud several stories high swept through communities.

The Trishuli River surged during the flooding. Before and after satellite images of the Kolpurtar, Betrawati and Syapru Besi areas of Nepal show that the river is now much wider, and brown and black, because it overtopped its banks in some areas and filled with debris in others. The change in the color of the water shows how much sediment the flow was carrying, turning from greenish to very brown because it's full of mud, Cook said.

Towns and villages that were devastated along the river banks are submerged in debris and mud. They appear as areas of brown. Some houses are still poking through in an image of the Betrawati area.

The flooding toppled and swept away buildings in Syapru Besi, a popular tourist spot where a famous trek to Langtang begins. In before and after images, a side river is now wider too, because the flow was so powerful it pushed upstream.

The flooding “left a wasteland in its wake” that will take many years to recover from, said Alton Byers, a mountain geographer at the University of Colorado at Boulder. Byers lived in Nepal and now travels there yearly to study glacial hazards and work with communities to prepare. This is the largest glacier flood he has seen, by far.

Byers, faculty research scientist at the university's Institute of Arctic and Alpine Research, said he thinks Wednesday's flooding is related to climate change. The Earth's warming is melting glaciers and permafrost, causing instability and the breakage of massive amounts of snow and ice at high altitudes. That can be a catalyst for a series of cascading events, he said.

“All the indicators, to me, point to the impacts of warming trends,” he said. “In this case, the permafrost that for millennia has helped to hold together high-altitude rock and ice and soil and glaciers is now weakening and becoming unstable."

Cook and Shugar said that at this point, they can't say for sure there's a direct link to climate change. Slopes fail and landslides happen, Cook said, though permafrost is melting and that is destabilizing mountainsides.

“While we cannot at this point directly link this collapse to climate change, it is not unexpected to see more events of this type with a warming climate,” Cook said. “Not this big, hopefully not this big.”

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