This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A catastrophic flood in Nepal that left thousands missing began with a collapse of ice and rock high in the mountains—a warning that disaster planning must prepare for chains of hazards, not isolated events, experts say. The cascade of ice, rock and debris that tumbled through the Nepal–China border region last week probably blocked a tributary of the Bhotekoshi River, forming a dam before bursting downstream with devastating force, according to disaster analysis.
The torrent arrived in Nepal's Rasuwa district with little warning, ripping through settlements along the Bhotekoshi River system, destroying bridges, roads and hydropower infrastructure and cutting a vital trade route with Tibet. More than 1,000 people have been confirmed dead, with almost 4,000 still missing and around 11,800 rescued, according to UN figures on Tuesday (Sept. 1). Thousands are in need of humanitarian assistance, hindered by blocked roads and damaged bridges.
Experts say the disaster illustrates a growing problem across the Hindu Kush Himalaya: hazards that once might have been assessed separately—glacier collapse, landslides, avalanches, river blockages, floods and even earthquakes—can interact within minutes or hours, creating an event much larger than any one of them. "Most people think of flash floods as being caused by heavy rainfall," said Liz Stephens, professor of climate risks and resilience at the University of Reading in the UK. But in high-mountain regions, such floods can result from "complex chains of hazards, including landslides, avalanches and glacial lake outburst floods," she added.
That complexity, Stephens says, makes early warning particularly difficult. Analysis of satellite imagery from U.S. aerospace and data analytics company Planet Labs suggests that a substantial section of glacier and surrounding rock collapsed at high altitude near the Nepal–China border, sending an enormous mass of ice, snow and debris toward the valley. The collapse appears to have affected the Lhende Khola, a tributary that becomes part of the Bhotekoshi River system.
A rapid-analysis report released by the Integrated Research on Disaster Risk (IRDR) on Friday (Aug. 28) said the event was likely caused by an "ice, rock and soil mass failure" rather than intense rainfall. The material probably blocked the river for about 18 or 19 hours, forming a natural dam that then gave way on the morning of Aug. 26, unleashing masses of water, ice, rocks and sediment, the report said. Funneled by the steep Himalayan terrain, the surge became a fast-moving mixture of water and debris capable of carrying huge boulders, destroying bridges and erasing riverbanks.
The rapid-analysis report said the steep gradient and narrow valley accelerated and concentrated the flow, while the heavy debris load increased its destructive power. Chinese state broadcaster CCTV said the flood was "caused by glacier instability under the long-term effects of global warming" and described this as a "new and prominent feature of cryosphere disasters" on the Tibetan Plateau. Discover the latest in science, tech, and space with over 100,000 subscribers who rely on Phys.org for daily insights. d research that matter—daily or weekly.
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