Massive ice-rock fall, not earthquake or GLOF, triggered Nepal floods, experts say

The devastating flash floods that swept through parts of Nepal on August 26 were triggered by a massive fall of ice, rock and debris in the high Himalayas and not by an earthquake or glacial lake outburst flood (GLOF), according to experts.

The floods, which originated near the Nepal-Tibet border and swept through northern and central Nepal, killed hundreds, left thousands missing and caused widespread damage to homes, roads, bridges and hydropower infrastructure.

A study commissioned by the Nepal Environment Society (NES) found that a huge mass of ice, rock and debris fell from an altitude of around 5,200 metres in the Langtang Lirung area of Rasuwa before entering the Lhende River.

The debris generated a powerful, sediment-laden flood that travelled downstream through the Bhotekoshi and Trishuli rivers, according to the study.

Bishal Nath Upreti, president of the Nepal Centre for Disaster Management, said initial theories that the floods were caused by a GLOF or an earthquake had subsequently 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.

An earthquake was then considered a possible trigger for the massive ice and rock fall. However, the US Geological Survey later clarified that the sequence was reversed, he said.

"A massive rock and ice fall into the valley generated the earthquake," Upreti said, adding that the tremor was initially 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 ruled out a GLOF as the cause of the disaster.

Researchers analysed satellite imagery, hydrological and meteorological data, terrain characteristics, as well as changes in elevation and slope along the river system. Their analysis found no evidence of a glacial lake outburst.

According to the study, the ice-rock mass began falling from the northern section of Langtang Lirung at around 8:37 am on Wednesday.

It descended steep terrain from approximately 5,200 metres before entering the Lhende River at around 2,930 metres.

Satellite images indicated major geographical changes across an area of about 10 square kilometres, while a glacier section measuring approximately 0.56 square kilometres had broken away.

The study estimated that the ice-rock mass travelled about 22 km to Rasuwagadhi in just over seven minutes, reaching an average speed of 46.3 metres per second, or about 167 kmph.

The flow slowed to around 73 kmph between Rasuwagadhi and Betrabati and to approximately 22 kmph farther downstream. Despite losing speed, it continued to transport large amounts of rock, sediment and soil, the report said.

The study also found no evidence of exceptionally heavy rainfall in the upper watershed on the day of the disaster, weakening the theory that extreme rainfall or a cloudburst alone triggered the floods.

Upreti said a second and smaller flood occurred about three hours and eight minutes later after another landslide created a temporary dam that subsequently breached.

"Fortunately, it flowed slowly and did not result in a major disaster," he said.

The disaster also caused extensive damage to Nepal's hydropower infrastructure.

Citing preliminary data from the Nepal Electricity Authority, the NES report said 13 hydropower projects and one solar project were affected, removing around 431 MW of generation capacity from the system.

The study said further field investigations were required to establish the precise geological mechanism behind the event, determine the contribution of melting ice and assess whether the river had been temporarily blocked upstream.

Upreti cautioned that the exact cause of the flash floods remained subject to scientific investigation, with interpretations evolving as more information becomes available.

"New ideas are coming up. So as a scientist, I cannot just confirm right now what is happening. But these are the developing hypotheses or developing theories or developing information that is coming up right now," he said.

India steps up relief support

Upreti said India had consistently extended support to Nepal during major natural disasters, including the 2015 earthquake.

"India has been always supportive whenever we have big earthquakes or these kind of disasters," he said, recalling that India was the first country to send relief materials and search-and-rescue teams following the 2015 earthquake.

He said India had again responded quickly to the latest disaster, with the first consignment of relief material arriving within hours.

Upreti also called for stronger scientific cooperation between India and Nepal, saying the neighbouring countries had considerable scope for joint disaster-related research.

"Our countries are so close, we are friendly countries, adjacent to each other, share the boundaries. I can say millions of people crossover on a daily basis. But scientific cooperation between India and Nepal almost doesn't exist," he said.

"We can do a lot, together for the people. Research is so important. Scientific cooperation, collaborations, joint research are very, very important," Upreti added.