Explained: No Rain Or Cloudburst, How Cryospheric Change Triggered Deadly Nepal Flood

It was a day of tragedy, chaos, and helpless witnessing of nature's fury along the Nepal-China border on Wednesday. Millions of gallons of water and a massive debris flow charged down from Tibet into central Nepal, devastating everything in its path.

Nepal's disaster management authority, on Thursday morning, said 826 people were missing, including 300 Indians, as it put the death toll at 165. Nuwakot and Rasuwa districts were the worst affected. China's state broadcaster CCTV reported that 558 people were missing in Tibet, including many foreigners.

Twenty-four hours into the tragedy, a haunting question remains: with hardly any rain or cloudbursts in the area, what caused such a monstrous flash flood?

Initial assessment by the authorities in Nepal pointed towards an earthquake that could have led to the disaster since tremors were felt in and around the affected area minutes before hundreds of people were swept away by a massive wall of mud water.

The earthquake theory, however, was soon set aside by the US Geological Survey as it pointed out that the tremors were actually caused by a landslide.

On its website, the USGS said the 5.2 magnitude seismic energy was generated by a glacial collapse and debris flow.

Damaged houses stand precariously near the swollen, debris-choked banks of the Trishuli River in Nepal. (AP)

The glacial collapse, most likely a glacial or ice-rock failure, sent a surge of water, mud, and debris racing down the Lhende Khola River (a tributary of the Bhote Koshi). The river originates in the high-altitude mountains of Tibet and flows southward where it enters Nepal.

According to the International Centre for Integrated Mountain Development (ICIMOD) headquartered in Kathmandu, water levels in the Bhote Koshi basin began rising rapidly around 9 am local time. The flood breached riverbanks and travelled downstream through the Bhote Koshi into the Trishuli River.

Amid a massive operation to trace the missing and recover the dead bodies buried under several feet of mud and debris, scientists are trying to piece together evidence to understand the catastrophic event in the Himalayas.

GRAPHIC CONTENT: A body of a victim lies on a stretcher in Nuwakot district of Nepal on Thursday. (AP)

Experts at ICIMOD, which is a regional intergovernmental organisation that represents all eight Himalayan nations, said an ice-rock avalanche originated in a high-altitude area that likely triggered the devastating flash floods.

ICIMOD, in a statement released late Wednesday, called the disaster as an example of the increasing frequency of cryosphere hazards becoming more visible.

Cryosphere is the collective term for the Earth's frozen zones, including glaciers and permafrost. Permafrost is any part of Earth, including rocks and soil, that stays below 0°C for two or more consecutive years.

When rising global temperatures rapidly alter these frozen landscapes, these episodes are referred to as cryospheric hazards. As seems the case in Wednesday’s disaster, warmer Himalayas make its cryosphere behave like a structural failure in a decaying building.

The deep ice holding mountains together melts, causing massive sheets of glacial ice to lose their grip and violently snap off without warning.

“The increasing frequency of cryosphere hazards is becoming more visible than ever before. The pace of change is so rapid that current efforts are struggling to keep up,” said Mohd. Farooq Azam, Senior Cryosphere Specialist at ICIMOD.

The disaster in the Lhende Khola, he said, “underscores the need for stronger regional collaboration, with governments taking coordinated action to respond to cross-border hazards.”

A policeman with a sniffer dog looks at the damage in the aftermath of flash floods in Nuwakot district. (AP)

Daniel Shugar, an Associate Professor of geomorphology at the University of Calgary in Canada, said the flash floods were triggered by a glacier that rolled off a cliff at 5,200 meters. “Comparing satellite imagery from yesterday and today, the glaciers in the vicinity do seem to have lost a fair bit of snow in the last 24 hours,” Shugar was quoted as saying by the New Scientist magazine.

“My guess is that snow melt, and glacier ice melt as well, injected a lot of liquid water into the glacier, which percolated down to the bed and lubricated it. That was enough for it to collapse,” he said.

ICIMOD’s preliminary assessment based on video footage and information from affected areas suggest that the large volume of ice and rock debris entered the Lende Khola.

“Hydrological observations indicate the exceptional speed and magnitude of the flood wave. Water levels on the Trishuli River at Galchchi reportedly rose by as much as nine metres within 30 minutes, while levels at Malekhu increased by seven metres over a similar period,” it explained.

Damage in the aftermath of flash floods. (AP)

Experts are also assessing whether debris temporarily blocked the river at a narrow section, forming a landslide dam. Such blockage can create secondary hazards if impounded water is suddenly released.

“The same site devastated by last year’s Rasuwa flood is once again under threat,” said Qianggong Zhang, Head of Climate and Environmental Risks at ICIMOD.

“An ice avalanche in the Lhende Khola is the suspected trigger, but the cause remains unconfirmed. With a blockage still lodged upstream on the Nepal-China border, authorities warn a second flood is imminent and evacuation orders are now in place for Jilong port,” the ICIMOD said in its statement.

Damage in the aftermath of flash floods. (AP)

Basanta Raj Adhikari, Director, Centre for Disaster Studies, Tribhuvan University, Nepal, said multi-hazard risks are increasing in the Himalayas and the Lhenda Khola case is one such example.

“The exposure and risk profile of downstream communities is changing dramatically. With these communities at the forefront, real time risk information needs to reach local decision-makers and translate into life-saving decisions,” he said.

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