India’s Uttarakhand region experienced a humanitarian tragedy on February 7, 2021, when a wall of debris and water washed down the river valleys of Ronti Gad, Rishiganga and Dhauliganga.
The event began when a wedge of rock bearing a glacier broke off a steep ridge in the Himalayan mountain range. The resulting debris flow destroyed two hydroelectric facilities and left more than 200 dead or missing.
A self-organized coalition of 53 scientists met in the days following the disaster to investigate the cause, extent and impacts. The team determined that the flooding was caused by falling rocks and glacier ice that melted on its descent – not a diverted lake or river – which will help researchers and policymakers better identify emerging dangers in the region.
The study, which used satellite imagery, seismic recordings and eyewitness videos to produce computer models of the flow, was published on June 10 in Science.
“On the morning of the event, I was reading the news over coffee and saw a headline about a disaster in the Himalayas,” said co-author David Shean, assistant professor of civil and environmental engineering at the University of Washington. “I sat down at the computer and retrieved the satellite images that had been acquired that morning. When I saw the cloud of dust moving through the valley, I started writing emails to other scientists to ask if they were working on it. A thread quickly grew to five, then ten, and the response effort consumed most of our waking hours for the next two weeks. “
Initial hypotheses about the cause of the event suggested overflowing glacial lake flooding. But there aren’t any glacial lakes large enough to produce flooding near the site, the team determined.
“Our access to high-resolution satellite imagery and research software, as well as our expertise in satellite remote sensing were crucial in gaining an overview of how the event unfolded,” said co-author Shashank Bhushan , doctoral student at UW in civil and environmental engineering. “We worked with our French collaborators to coordinate the satellite collections in the days following the event and quickly process the images to derive detailed topographic maps of the site.”
The researchers compared the images and topographic maps before and after the event to document any changes and reconstruct the sequence of events.
“We followed a plume of dust and water to a prominent dark spot on a steep slope,” said lead author Dan Shugar, associate professor at the University of Calgary.
The dark spot turned out to be the scar left by the missing 35 million cubic meters of rock and glacier ice – enough material to cover Washington, DC, with a layer 3 feet deep.
“It was the source of a giant landslide that started the cascade of events and caused immense death and destruction,” said Shugar, who was previously an assistant professor at UW Tacoma.
The researchers also used the maps to determine how far the block of ice and rock fell.
“The failed block fell over a mile before impacting the valley floor. To put that height in context, imagine vertically stacking 11 Space Needles or six Eiffel Towers,” Bhushan said.
Then the larger team was able to quantify how the pulverized rock and ice was redistributed to the downstream areas.
“When the boulder fell, most of the glacier’s ice melted within minutes. This resulted in a huge volume of water associated with the flooding,” Bhushan said. “This is very unusual – a normal rocky landslide or snow / ice avalanche could not have produced such huge volumes of water.”
For Bhushan, the work was personal.
“In general, doctoral research projects are very specialized. Sometimes I have a hard time explaining to my parents why it is important to measure the dynamics of glaciers,” Bhushan said. “But due to the magnitude of this disaster, my family and friends in India were very curious about how this event unfolded, and they expected me to find an answer. provided a sense of belonging and motivation. that some of my research can be of so immediate benefit to society. “
The team also used archives of satellite images to show that ancient masses of ice had been dislodged from the same ridge and struck the same valley in recent years. The researchers suggest that climate change is likely to increase the frequency of such events and that the greater magnitude of the latest disaster should be taken into account before further infrastructure development in the region.
“These high mountain rivers are attractive for hydropower projects, and we need to better understand the full range of potential high mountain hazards,” said Shean. “We hope that the lessons learned from this effort will improve our ability to respond to future disasters and guide policy decisions that will save lives.”