Kathmandu, Aug 27: A suspected ice-and-rock avalanche in Lende Khola may have triggered the devastating flash flood that struck Nepal’s Rasuwa district, with scientists now investigating whether an upstream blockage could unleash another potentially catastrophic surge.
The disaster on August 26 has claimed at least 157 lives, while more than 750 people are reported missing, including 133 Indians.
The extraordinary speed of the flooding has emerged as a key clue. Hydrological observations showed that water levels in the Trishuli River at Galchchi rose by as much as nine metres in just 30 minutes, while Malekhu recorded a rise of around seven metres over a similar period.
Scientists say such a rapid increase is more consistent with the sudden release of a huge volume of water than with a conventional monsoon flood.
Preliminary assessments by the International Centre for Integrated Mountain Development (ICIMOD) and partner institutions in Nepal and China suggest that a large mass of ice and rock debris may have crashed into Lende Khola, a tributary of the Bhote Koshi River.
The impact may have displaced water and generated a powerful debris-laden surge carrying sediment and massive boulders downstream.
Water levels in the Bhote Koshi basin reportedly began rising rapidly around 9 am local time before the flood breached riverbanks and surged into the Trishuli River, threatening communities in Nuwakot and Dhading districts.
“This tragic disaster struck so suddenly and unexpectedly,” ICIMOD Director General Pema Gyamtsho said.
Scientists are also examining a potentially more dangerous scenario — that the avalanche debris may have temporarily blocked the river, forming a landslide dam.
If water continues accumulating behind such a blockage and the obstruction suddenly gives way, it could trigger another powerful flood downstream.
“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.
Zhang said an ice avalanche in the Lende Khola was the suspected trigger, though the exact cause had not yet been confirmed.
Authorities have warned that a blockage could still remain upstream near the Nepal-China border, raising fears of a secondary flood. Evacuation orders have reportedly been issued around Jilong port as scientists assess the stability of the suspected obstruction.
Seismic data is also being examined for clues. Seismometers at Jilong, around 12 km from the affected area, and at Zhangmu detected unusual signals during the disaster. Scientists are investigating whether the signals were generated by the suspected avalanche, although no direct causal link has been established.
The disaster has renewed concerns over the growing threat of cryosphere-related hazards in the Himalayas, including melting glaciers, unstable slopes, expanding glacial lakes and thawing ice.
ICIMOD’s Mohd. Farooq Azam said the increasing frequency of such hazards was becoming increasingly visible and that the pace of environmental change was testing existing monitoring systems.
Scientists are continuing to analyse satellite imagery, seismic records and hydrological data to determine what triggered the disaster and assess the possibility of another flood.
The suspected avalanche and the rapid water-level rise underline how an event beginning on an isolated Himalayan slope can transform into a destructive flood hundreds of kilometres downstream within a matter of minutes.
