Nepal floods were triggered by massive ice-rock fall, experts say

Experts and a new study say Nepal's deadly floods began after a massive ice and rock fall in the high Himalayas. The finding rules out an earthquake or glacial lake burst and shifts attention to unstable mountain slopes.Image used for representational purposes onlyExperts in Nepal have said the devastating floods that swept through the country were triggered by a massive fall of ice and rock in the high Himalayas, and not by an earthquake or a glacial lake outburst flood. The flash floods began near the Nepal-Tibet border on August 26 and tore through the Bhotekoshi river corridor, destroying homes, roads and bridges and damaging hydropower projects.The death toll crossed 900 on Monday, while more than 4,000 people remained missing, according to the National Disaster Risk Reduction and Management Authority. A study commissioned by the Nepal Environment Society found that a huge mass of ice, rock and debris fell from about 5,200 metres in the Langtang Lirung area of Rasuwa into the Lhende River, setting off a debris-filled flood that moved downstream through the Bhotekoshi and Trishuli rivers.Bishal Nath Upreti, president of the Nepal Centre for Disaster Management, said early theories that the disaster was caused by a glacial lake outburst flood, or GLOF, or by an earthquake had later been ruled out. "Initially, many thought it was a glacial lake outburst flood, or GLOF. Later, it became clear that there had been no lake burst in Tibet along that line, so that theory was discarded," Upreti told PTI Videos. He said it was then thought that an earthquake had triggered the ice and rock fall, but the US Geological Survey later said the sequence had happened the other way round. "A massive rock and ice fall into the valley generated the earthquake," Upreti said, adding that the earthquake was first recorded at magnitude 4.2 and later revised to 5.2. "So, the disaster was not caused by an earthquake; rather, the falling ice and rock generated the earthquake," he said.The NES study, released on Sunday, also concluded that the disaster was not caused by a GLOF. Researchers studied satellite images, hydrological and meteorological data, terrain features, and changes in elevation and slope along the river system, and found no sign of a lake outburst. According to the study, the ice-rock mass began falling from the northern part of Langtang Lirung at around 8.37 am on Wednesday. It moved down steep terrain from about 5,200 metres before reaching the Lhende River at around 2,930 metres. Satellite images showed major geographical changes over about 10 square kilometres, while a glacier section covering around 0.56 square kilometres had broken away. Upreti said scientists were still trying to establish the exact mechanism behind the disaster because no one had yet reached the site and much of the research was based on satellite imagery. "One emerging hypothesis is that it was actually a massive rockslide, with the ice riding over the falling rock, rather than the glacier itself sliding down," he said, while cautioning that this could not yet be confirmed. The study said the ice-rock mass travelled about 22 km to Rasuwagadhi in a little over seven minutes, at an estimated average speed of 46.3 metres per second, or 167 kmph. Its speed later fell to around 73 kmph between Rasuwagadhi and Betrabati, and to about 22 kmph farther downstream. Even as it slowed, the flow kept carrying large quantities of rock, sediment and soil, the report said.The study also found no sign of exceptionally heavy rainfall in the upper watershed that day, weakening the assumption that the disaster was caused only by extreme rainfall or a cloudburst. Upreti said a second, smaller flood took place after another landslide around three hours and eight minutes later created a temporary dam that later breached. "Fortunately, it flowed slowly and did not result in a major disaster," he said. The floods also caused heavy damage to hydropower infrastructure. Citing preliminary Nepal Electricity Authority data, the NES report said 13 hydropower projects and one solar project were affected, taking about 431 MW of generation capacity out of the system. Overall, the study said more field investigation was needed to determine the exact geological mechanism, the role of melting ice, and whether the river had been temporarily blocked upstream. With PTI Inputs- EndsPublished By: India Today Web Desk Published On: Aug 31, 2026 18:36 IST