KATHMANDU, Aug 27: A powerful flash flood in Rasuwa on Wednesday morning caused extensive loss of life and property.
Experts have urged people in downstream areas to remain alert, warning that the powerful flood carrying water, debris and large rocks through the Bhotekoshi and Trishuli river systems could cause further damage.
The water level in the Bhotekoshi basin began rising rapidly at around 9 a.m. Wednesday. The flood caused extensive damage in Rasuwa, Nuwakot and Dhading.
Scientists from the International Centre for Integrated Mountain Development (ICIMOD), along with partner institutions in Nepal and China, are investigating whether the flood may have been triggered by an ice rock avalanche originating in the high mountain region.
A preliminary assessment based on video footage and information from affected areas suggests that a large volume of snow and rock debris may have fallen into Lhende Khola, a tributary of the Bhotekoshi River. Researchers said the event may have generated a sudden surge of water, carrying debris and large boulders downstream.
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Hydrological observations indicate that the speed and volume of the flood were exceptional. The water level of the Trishuli River at Galchhi reportedly rose by as much as nine metres within 30 minutes, while the level at Malekhu increased by seven metres over the same period.
Several water monitoring stations along the Trishuli River were damaged or swept away during the flood. However, ICIMOD said the monitoring station at Galchhi remains operational.
Experts are also assessing whether debris temporarily blocked a narrow section of the river, forming a landslide dam. A sudden release of water accumulated behind such a blockage could create a second wave of risk. Authorities and scientists are continuously assessing the possible size, location and stability of any such obstruction.
A preliminary analysis of seismic data recorded during the disaster has also detected unusual signals. A seismic station in Jilung, about 12 kilometres north of the affected area, recorded an unusual signal, while another station in Zhangmu also recorded abnormal activity.
Scientists are investigating whether these signals were linked to a possible avalanche. So far, no causal connection has been established between the seismic activity and the flood.
Syangong Zhang, head of climate and environmental risks at ICIMOD, said the same area that was devastated by last year’s Rasuwa flood had once again come under threat. He said an avalanche could have been the possible cause of the flooding in Lhende Khola.
“Authorities have warned of a possible second flood because an obstruction remains in the upper reaches of the river near the Nepal China border. An order has already been issued to move the Jilung port to a safer location,” he said.
The flood caused extensive damage to infrastructure in the affected areas, including hydrological monitoring stations. Several hydropower projects are located along the Bhotekoshi River and its connected river systems.
Regional cooperation needed to address cross border disaster risks
Mohammad Farooq Azam, a senior cryosphere specialist at ICIMOD who studies glaciers, snow conditions, climate change impacts and risks such as glacial lake outburst floods, said the increasing frequency of cryosphere related hazards had become increasingly evident.
“The pace of change is so rapid that existing efforts are struggling to keep up. Wednesday’s disaster in Lhende Khola has shown the need for stronger regional cooperation to address cross border risks. Governments must work together in a coordinated manner to respond to such risks,” he said.
ICIMOD experts said they were continuing to analyse hydrological, seismic, remote sensing and field data to better understand the cause of the event and the continuing risks facing downstream communities. ICIMOD said it was maintaining close contact and coordination with the Government of Nepal, scientific institutions and partners in China as the assessment continues.
ICIMOD Director General Pema Gyamtsho expressed grief over the sudden and unexpected disaster and solidarity with the people and government of Nepal during the difficult time. He also extended condolences to the victims, their families and all those affected, and pledged to provide whatever assistance possible.
Scientists said climate change is altering glaciers, snow conditions, permafrost and mountain slopes across the Hindu Kush Himalaya. However, they cautioned that it would be premature to determine what role climate change played in this particular event.
