Guwahati: The massive glacial collapse that triggered catastrophic flash floods along the Nepal-Tibet border on Wednesday has killed at least 162 people, with hundreds still missing. Among those unaccounted for are 133 Indians, many of them travelling to Tibet on pilgrimage to Mount Kailash.
The flash floods were triggered at 8:37 am Nepal time when a huge section of a glacier in Langtang National Park, about 60 km north of Kathmandu, broke away and crashed into the valley below.
The force of the event was initially mistaken for an earthquake. The United States Geological Survey (USGS) first recorded a magnitude 4.4 seismic event in the region.
It later revised that assessment.
After analysing seismic data, long-period waves and satellite imagery, the USGS said the seismic energy detected by its instruments had been generated by a glacial collapse and debris flow, and not an earthquake.
“This event was initially reported as a magnitude 4.4 earthquake,” the USGS said in its update. “Additional analysis of nearby seismic stations, long period seismic waves and satellite imagery led to the determination that the seismic event was instead a glacial collapse and debris flow, and that no earthquake had occurred.”
So, what turned a glacial collapse high in the Himalayas into a devastating flash flood?
How did the glacial collapse trigger the floods?
The collapse unleashed a huge volume of water, mud, rock and ice into the Lhende Khola, a high-altitude tributary of the Bhote Koshi river.
The torrent gathered speed as it moved through the steep Himalayan terrain, carrying more debris downstream. It then surged into the Bhote Koshi and Trishuli river systems, sending floodwaters towards settlements and infrastructure in northern Nepal.
The Lhende Khola is a transboundary river that flows through Nepal and China. It feeds into the Bhote Koshi, which forms part of the wider Trishuli-Narayani river system.
The interconnected river network links the high Himalayan valleys with the plains downstream. It also explains why flood alerts were issued in Indian states such as Bihar and Uttar Pradesh.
Videos from the Gyirong border post, which connects Nepal and Tibet, showed people running as a wall of debris swept through the area, destroying buildings and carrying away buses.
In Nepal, houses and roads were washed away within minutes. The flood also damaged critical infrastructure, cutting power and communications and crippling Gyirong Port, a major crossing point between Tibet and Nepal.
Why was the flood so destructive?
The answer lies partly in the way water and debris behave in steep Himalayan terrain.
When a large mass of ice, rock and snow enters a mountain river, it can dramatically increase the flow’s energy. The debris can also obstruct the river channel, temporarily holding back water before an unstable blockage gives way.
The resulting surge can carry huge quantities of mud, rocks and other sediment downstream.
Hydrologist Hatim Sharif of the University of Texas at San Antonio described such flows as being like “liquid concrete”, adding that they cannot be outrun.
Nepal’s topography further amplifies the danger.
The country has some of the highest peaks on Earth, alongside steep slopes and narrow but deep river valleys. These valleys can act as funnels, accelerating water as it descends.
As the torrent moves downhill, it can erode riverbanks and slopes, pulling more soil, mud and boulders into the flow.
This can turn an initial avalanche or glacial collapse into a cascading disaster, with the destruction increasing as the flow travels downstream.
Why the monsoon made the disaster worse
The timing of the Nepal disaster is another important factor.
The flash floods struck during the peak monsoon season, when rivers are already carrying substantial amounts of water and the soil is saturated.
That is different from the 2021 Chamoli disaster in Uttarakhand, which occurred in February.
Farooq Azam, an associate professor at IIT Indore and head of the Cryosphere group at the Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD), said the Nepal event appeared similar to Chamoli in terms of its trigger, but its downstream impact could be closer to the 2013 Kedarnath disaster.
Azam said the timing of the Nepal disaster, during the core monsoon season, made it potentially more destructive than the February 2021 Chamoli event. With the soil already saturated and rivers carrying substantial volumes of water, the sudden influx of ice, rock and debris could trigger a far more destructive cascade downstream.
A large volume of ice, snow and rock entering such a river system can therefore create much greater destruction, he said.
The Himalayan landscape also makes these events particularly dangerous.
“The Himalayan region is very fragile. Such events lead to a chain reaction,” Azam said. A surging river carrying large rocks and ice blocks can erode banks and slopes, bringing more soil, mud and rocks into the river.
“It becomes a cascading disaster because the waters do not have any other way to spread out,” he said.
Was this like the Chamoli disaster?
There are similarities, but the mechanisms are different.
The 2021 Chamoli disaster was caused by an ice and rock avalanche. A large mass of ice and rock broke away and travelled downhill, generating a destructive surge that killed about 200 people, many of them workers at two hydroelectric power projects.
The 2013 Kedarnath disaster, on the other hand, was initiated by what is known as a glacial lake outburst flood, or GLOF.
A GLOF occurs when a glacial lake overflows or breaches its natural boundary, releasing a large volume of water downstream.
Both events produced major surges in Himalayan rivers and caused widespread destruction downstream.
The Nepal disaster appears to have been caused by an ice-and-rock avalanche, but the downstream damage has been amplified by the monsoon and the interconnected river system.
What caused the initial collapse?
This remains under investigation.
Authorities and experts initially considered the possibility that an earthquake had triggered the avalanche. The USGS had reported a magnitude 4.4 earthquake in the region, which was widely believed to have set off the landslide.
The USGS has since ruled out that explanation.
Its updated analysis found that the seismic energy detected by its instruments came from the glacial collapse and debris flow itself.
The precise sequence that caused the glacier and surrounding rock to collapse is still being examined.
What does the Sikkim disaster tell us?
The 2023 disaster in Sikkim offers another example of how a disturbance in the high mountains can develop into a much larger downstream catastrophe.
A study published in Science examined the collapse of a large mass of rock, soil and snow into South Lhonak Lake in northern Sikkim.
The collapse involved nearly 14.7 million cubic metres of material and triggered a glacial lake outburst flood.
The resulting displacement wave overtopped and breached the lake’s natural barrier, releasing about 50 million cubic metres of water.
As the flood moved downstream, it eroded about 270 million cubic metres of sediment and triggered 45 secondary landslides.
Most of the erosion occurred within about 67.5 km of the lake, the study found.