Scientists traced Nepal’s devastating flash floods to an ice-rock avalanche.
Satellite imagery found no evidence supporting the initial GLOF explanation.
Temporary lakes and altered rivers have raised fresh concerns about Himalayan hazards.
The flash floods that hit Nepal and Tibet last week were triggered by an ice-rock avalanche, not a glacial lake outburst flood, according to a preliminary report commissioned by the Nepal Environment Society.
The report, released on Sunday, said satellite imagery found no sign of a GLOF, the explanation first given for the disaster.
The distinction matters because both hazards are linked to climate change in the Himalayas. A GLOF is a sudden release of water from a glacial lake. Mass wasting is the downhill movement of rock, ice and soil under gravity and can build up over time before a collapse.
As per Reuters, death toll has climbed past 900. More than 3,000 people, including many Indians, remain unaccounted for.
Avalanche Details
Scientists traced the trigger to Langtang-Lirung. The report was prepared by Dr Binod Baniya, Dr Nitesh Khadka, Dr Amrit Prasad Sharma, Bishnu Maharjan and Dr Jayaram Karki. It stated that ice and rock began breaking away from the north side of Langtang–Lirung peak at about 8:37 AM local time on Wednesday, Aug. 26.
From there, the material came down steep slopes from about 5,200 metres and reached the Lhende River. It then coursed through the Bhotekoshi and Trishuli rivers, turning into a debris-heavy flood.
"The avalanche triggered ground vibration created a strong seismic signal (M5.2). The ice-rock avalanche with debris descended to ~2930 at Lhende Khola (border between China and Nepal) from 5200, sharply falling 2270 m, generating speed debris flow," the study said in remarks reported by Reuters.
Reuters reported that the ice-rock mass covered about 22 kilometres to Ashwagandha in a little over seven minutes. The average pace was estimated at 167 kilometres per hour.
River Changes And Warnings
The flood also rerouted the river and left behind two temporary lakes that need continued monitoring. Before the disaster, river levels at Rasuwaghadi and Saprubesi had already dropped. Scientists said that could indicate an upstream blockage in the Lhende River.
Some questions remain unresolved. Reuters reported that the size of the blocked dam and the extent of the river obstruction still require further study.
Satellite imagery was used to dismiss a GLOF. The report said the floods were linked instead to glacial mass wasting. It cited major surface changes, including less snow and ice cover and more exposed terrain. It also said a large glacier mass and bedrock sheared off and fell into the Lhende River basin.
The image review was a key part of that conclusion. "Analysis of several satellite images, including Gaofen-1 (GF-1), SAR, Sentinel, PlanetScope, and Landsat images, indicates the presence of three glacial lakes upstream in the Tibet region. However, no evidence and indicators of a glacial lake outburst flood were identified," the study notes, as Reuters reported.
The report added that there was no major cloudburst on the day of the floods. Scientists said such an event would likely have shown up in river levels, which had already fallen before the water arrived.
The broader warning extends beyond this single disaster. The study said both GLOF events and mass wasting are tied to climate change in high-mountain regions. It described a GLOF as a sudden, catastrophic release of water, while mass wasting is the downhill movement of solid rock, ice and soil under gravity and a longer-term process.
It said glacial mass wasting can include glacial calving, ice-rock avalanches and glacial thinning and retreat, and that it often acts as the direct trigger for a GLOF. On that basis, the scientists warned that flash-flood risk may rise, especially across the Himalayan region. give healdines
