Nepal's deadly debris flow: How a glacier collapse sent a wall of mud racing at 167 kph

As a glacier melted and gave way, the bedrock beneath it collapsed as well. Frictional heat generated by sediment carried along by the melting ice accelerated the glacier's disintegration, and the debris flow that followed gained speed at an unstoppable rate — reaching 167 kilometers per hour. No one in its path could have escaped. That is the conclusion of Japanese research into the cause of Nepal's catastrophic floods and landslides.

The Yomiuri Shimbun reported Wednesday, drawing on research by Niigata University professor Kawashima Katsuhisa, a specialist in snow-and-ice disaster research, that the floods and landslides unfolded in precisely this way. "When a glacier melts, water seeps in and weakens the bedrock," Kawashima said. "There is a possibility it had already been weathered by exposure to wind and rain." He added that after the bedrock collapsed, frictional heat between the debris and the ice would have caused the glacier to melt even faster, further intensifying the debris flow.

The debris flow that struck the affected areas traveled at up to 167 kph, meaning it would have reached a border checkpoint 22 kilometers from the point of origin in roughly eight minutes. Irazawa Michiya, an emeritus professor of erosion control at Iwate University, said the speed was "five to ten times that of debris flows that cause damage in Japan — it is on an entirely different scale."

According to a report by the Nepal Environmental Society, the floods and landslides that struck on Aug. 26 were triggered when a glacier and bedrock on a slope at roughly 5,200 meters above sea level collapsed and plunged 1,200 meters. The energy released was equivalent to a magnitude-5.2 earthquake — enough that the US Geological Survey initially mistook the event for a glacier collapse caused by a seismic event.

The floods originated near the border between Nepal and China's Tibet, and a massive debris flow also struck the Tibetan side. The South China Morning Post reported Tuesday (local time) on the initial findings of the Institute of Mountain Hazards and Environment under the Chinese Academy of Sciences, which found that the debris flow was traveling at 19 meters per second — about 68 kph — when it struck the border checkpoint at Gyirong Port in Tibet. Using digital modeling and frame-by-frame analysis, the research team concluded that "the disaster involved a cascading sequence of transitions — from an ice avalanche to a debris flow, then a mudslide, and finally a flood." The team added that "the time from the onset of the ice avalanche to its arrival at Gyirong Port was extremely short, leaving very limited time for an effective response."

The research team calculated that even when the floodwaters reached the Trishuli River, 52 kilometers from the border checkpoint, the surface velocity was still 11.7 meters per second. No person, however fast, could have outrun it, the team concluded.

Meanwhile, the disaster has renewed attention to a 2019 report by the UN's Intergovernmental Panel on Climate Change, which made strikingly similar predictions. That report warned that warming temperatures would cause high-altitude glaciers to melt, triggering cascading avalanches and floods. Enomoto Hiroyuki, a specially appointed professor at Japan's National Institute of Polar Research who contributed to the report, said the disaster looked "as if it had reproduced the report" and warned that "dangerous sediment disasters will continue to occur in cryosphere regions around the world."

As of Wednesday — the eighth day since the floods struck — the death toll in Nepal had risen to 1,058, with 4,606 people still missing. In China's Tibet, at least 16 people were confirmed dead and 546 remained missing. Combined, the two countries have recorded 1,074 deaths and 5,152 missing.