Preliminary assessments suggest that an ice-and-rock avalanche may have blocked the Lhende River, creating a temporary barrier before releasing a powerful surge downstream.
The flood carried mud, boulders and other debris at exceptional speed, producing a dense and destructive flow that has been likened to “liquid concrete”.
Scientists are still investigating the precise trigger, while a glacial lake outburst flood remains one of the explanations initially considered.
Scientists are examining how a possible glacial collapse and ice-rock avalanche may have triggered the devastating flash floods along Nepal's Bhote Koshi River, with preliminary assessments indicating that debris may have temporarily blocked the Lhende River and created a landslide dam that subsequently gave way.
The catastrophic flood swept down the Bhote Koshi in Rasuwa district on Wednesday, killing hundreds and destroying bridges and other infrastructure along the river corridor, as it gushed downstream, causing havoc in multiple districts.
How Did The Glacier Collapse Trigger A Flood?
Initial speculation after the sudden surge near the Nepal-Tibet border crossing in Rasuwagadhi included a glacial lake outburst flood (GLOF) and an avalanche.
However, scientists analysing satellite imagery now believe a large mass of ice and rock may have fallen into the Lhende River, temporarily blocking its flow and creating a landslide dam that subsequently gave way, The Kathmandu Post reported. The National Disaster Risk Reduction and Management Authority has also identified an ice-and-rock avalanche as the leading explanation, according to the report.
Satellite imagery analysed by scientists showed what appeared to be a large avalanche about 20 km northeast of the Rasuwagadhi border crossing. The debris appears to have entered the Lhende River, a tributary of the Bhote Koshi, triggering a powerful, debris-laden flood, the report said.
Rijan Bhakta Kayastha, a climate researcher and professor at Kathmandu University, was quoted as saying by The Kathmandu Post that an avalanche appeared to have blocked the Lhende River before the flood broke through.
“We suspect an avalanche on the Nepal side blocked the Lhende,” Kayastha said. “An earthquake occurred at around the same time, but we do not yet know whether it helped trigger the avalanche.” The US Geological Survey (USGS) initially recorded what it classified as a magnitude 4.4 earthquake in Tibet at 8:37 am on Wednesday, about 47 km north of Gosainkunda.
It later revised its assessment, saying analysis of long-period seismic waves showed that the seismic energy had been generated by a glacial collapse and debris flow, not an earthquake. The event was subsequently classified as a magnitude 5.2 seismic event.
Why Did The Flood Behave Like ‘Liquid Concrete’?
The debris reportedly blocked the Lhende River, causing a temporary lake to form before the blockage gave way and sent a torrent of mud and boulders towards the Bhote Koshi.
Sarthak Shrestha, a remote sensing and geoinformation associate at the International Centre for Integrated Mountain Development (ICIMOD), said the evidence so far pointed to an avalanche on the Nepal side as the main trigger, according to a PTI report.
“So far, the evidence indicates that an avalanche above Syaphrubesi blocked the river and the flood followed,” Shrestha said, adding that it could take another week or two to establish the full picture. ICIMOD is working with Chinese institutions to determine precisely what happened, PTI reported.
The institute said its preliminary assessment, based on video footage and reports from the affected area, indicated that a huge volume of ice and rock debris may have fallen into the Lhende River. The resulting flood was unusually fast and powerful.
At Galchhi, the Trishuli River rose by as much as nine metres in just 30 minutes, while the water level at Malekhu climbed about seven metres during the same period, according to ICIMOD.
Another possible explanation initially considered was a glacial lake outburst. The same region witnessed a sudden surge in the Bhote Koshi on July 8 last year, which Nepal's Department of Hydrology and Meteorology attributed to a glacial lake outburst in the Lhende River near the Nepal-Tibet border.
A study by ICIMOD and the UN Development Programme has identified 3,624 glacial lakes across Nepal, India and Tibet, of which 47 were classified as potentially dangerous, including 21 in Nepal, as per PTI.
Scientists have also warned that the risk of further flooding has not been ruled out.
Unlike a conventional flood dominated by water, the surge carried a heavy mixture of mud, rocks and ice. The high concentration of debris made the flow denser and more destructive, allowing it to carry large boulders and batter infrastructure with enormous force. This helps explain why the torrent has been likened to “liquid concrete”.
