From Glacier To Floodwave: Why Nepal's Flash Flood Was So Destructive

An ice-and-rock collapse high in the Himalayas set off a chain of events that rapidly turned into a deadly flood in Nepal. The collapse blocked the Lhende River, creating a temporary barrier that was overwhelmed by a surge of water and debris. The resulting torrent swept downstream through the Bhotekoshi and Trishuli river systems, causing widespread destruction.

The death toll in the massive flash floods in Nepal rose to 165, leaving many, including more than 300 Indian tourists, missing in one of the country’s worst disasters in recent years, authorities said on Thursday, as per PTI. The flash floods were caused after an avalanche blocked the flow of the Lhende tributary of the Bhotekoshi and first hit Timure, near Nepal’s border with China at Rasuwagadhi.

How Does A Glacier Collapse Become A Flash Flood?

Local media reports said officials were caught unprepared for the flooding on Wednesday, as there had been no rainfall in the area. The disaster appears to have originated high in the Himalayas, where a section of glacier collapsed, triggering a chain of events that sent a powerful surge of water and debris downstream, according to Al Jazeera.

Satellite images indicated that a large portion of the glacier's lower section had detached at an altitude of roughly 5,200 metres (17,000 feet). It then fell approximately 1,200 metres (3,900 feet) to the valley below, picking up rocks and sediment during the descent.

The ice and rock avalanche subsequently hit the Lhende River in Tibet, around 20 km (12 miles) northeast of the Nepal-China border crossing. The debris formed a temporary barrier across the river, holding back the water until the blockage collapsed and unleashed a strong flood downstream.

Initial accounts attributed the glacier collapse to a magnitude 4.4 earthquake. However, the United States Geological Survey (USGS) later changed its assessment, determining that the seismic signal came from the landslide itself rather than an earthquake.

The landslide was ultimately recorded as a magnitude 5.2 seismic event.

Why Nepal Flood Was More Destructive Than Chamoli

A preliminary assessment on Thursday suggested that the flash flood was caused by an ice avalanche upstream blocking a river on the Nepal–China border, Disaster Management authorities were quoted as saying by My Republica.

The assessment suggests that the flood in the Trishuli River was caused by an ice avalanche upstream blocking the Lendhe River. The NDRRMA said that a debris-laden flood occurred in the Lendhe River following an ice-rock landslide at the Nepal–China border, about 20 kilometres northeast of Rasuwagadhi, according to PTI.

"The same flood appears to have entered the Trishuli River after passing through Timure,” the authority said, amid various theories and assessments regarding the cause of the flood, which has now become a recurring issue. Disaster expert Basanta Raj Adhikari also argues that an ice avalanche occurred in the Lendhe River, about 20 kilometres upstream of Rasuwagadhi, blocking the river and causing the flooding, My Republica reported on Thursday.

On 7 February 2021, the Chamoli district in the Uttarakhand region of India experienced a humanitarian tragedy when a large mass of rock and ice, around 27 million cubic metres, was released from the steep mountain flank of the Ronti peak.

This collapse caused a flow of debris to barrel down the Ronti Gad, Rishiganga, and Dhauliganga river valleys, causing significant destruction along the route, killing more than 200 people and destroying two major hydropower facilities that were under construction.

The timing may have amplified the disaster. Unlike the Chamoli event, which occurred in February, Nepal's flash flood struck during the monsoon, when Himalayan rivers were already carrying large volumes of water.

Why Himalayan Disasters Are Becoming Cascading Events

The precise cause of the glacier collapse has not yet been established, with scientists continuing to examine what may have triggered the event. It is also too early to say what role, if any, climate change played in this particular incident, according to Al Jazeera.

The collapse occurred, however, amid significant warming and widespread glacier loss across the Himalayas. Across the Hindu Kush Himalaya region, which covers Afghanistan, Bangladesh, Bhutan, China, India, Myanmar, Nepal and Pakistan, the rate of ice loss from glaciers between 2011 and 2020 was 65 per cent higher than in the preceding decade.

In Nepal, glaciers have lost nearly one-third of their ice volume over approximately 30 years. The retreat of glaciers can contribute to greater instability on mountain slopes, while meltwater may collect within or around glaciers, creating conditions that can lead to sudden flooding.

Satellite data recorded unusually high temperatures near the glacier before the collapse. The observations also showed a marked reduction in snow cover, leaving more exposed ice visible.

Scientists are examining whether melting played a role in weakening the glacier and ultimately contributed to its collapse. However, researchers have not established a direct connection between climate change and the event that occurred this week, Al Jazeera reported.