Experts at the International Centre for Integrated Mountain Development (ICIMOD) said an ice-rock avalanche originating in a high-altitude area was likely behind the devastating flash floods that swept from Tibet into central Nepal on Wednesday.
Calling the disaster as an example of the increasing frequency of cryosphere hazards becoming more visible, ICIMOD also underscored the need for early warning systems and stronger regional collaboration, with governments taking coordinated action to respond to cross-border hazards.
Nearly 100 people died and over 500, including 133 Indians, were missing after the torrential flash flood swept through multiple districts, causing devastation in towns and villages, washing away bridges and damaging at least a dozen hydropower projects.
The flood surged through the Bhote Koshi River at around 9 am (local time) and the flood waters then spread to several other districts. The flash floods sent a powerful surge of water, sediment and boulders through the Bhote Koshi and Trishuli river systems.
Scientists from the Kathmandu-headquartered ICIMOD and partner institutions in Nepal and China are investigating whether the flood may have been triggered by an ice-rock avalanche originating from a high-altitude area, a statement from the ICIMOD said.
Preliminary assessment based on video footage and information from affected areas suggest that a large volume of ice and rock debris may have entered the Lende Khola, a tributary of the Bhote Koshi river, the statement said, adding, researchers said the event could have generated a sudden surge that carried water, sediment and large boulders downstream.
“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.
Several water monitoring stations along the Trishuli river were reportedly damaged or washed away during the event, although the Glachchi monitoring station remains operational, it observed further.
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. Authorities and scientists are continuing to evaluate the size, location and stability of any potential blockage, the ICIMOD, which is a regional intergovernmental organisation that represents all eight Himalayan nations, said.
“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 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.
“Today’s disaster in the Lhende Khola underscores the need for stronger regional collaboration, with governments taking coordinated action to respond to cross-border hazards,” he said.
“The Lhende Khola has flooded twice in 14 months, this time most probably triggered by an ice avalanche that blocked the river and released a sudden surge downstream. This still needs to be confirmed, as we receive further information. These are cascading hazards: a cryosphere event becomes a flood in a settlement within hours,” said Saswata Sanyal, Disaster Risk Reduction Specialist, ICIMOD.
Preliminary analysis of seismic records has identified unusual signals recorded during the period of the disaster. Data from a seismometer in Jilong, approximately 12 kilometres from the affected area showed anomalous signals, while a second station in Zhangmu also recorded unusual activity.
Scientists are examining whether these signals are connected to the suspected avalanche. At present, no causal link has been established between seismic activity and the flood.
Experts at ICIMOD are continuing to analyse hydrological, seismic, remote-sensing and field data to better understand what triggered the event and assess ongoing risks to downstream communities, the statement said.
Scientists cautioned that while climate change is altering glaciers, snow conditions, permafrost and mountain slopes across the Hindu Kush Himalaya, it is too early to determine what role climate change played in this specific event, it added.
“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,” said Basanta Raj Adhikari, Director, Centre for Disaster Studies, Tribhuvan University, Nepal.
(With inputs from PTI)
