The Bhote Koshi-Trishuli Flood: Unprecedented Damage Due to Extreme Speed, Flow, and Behavior
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A senior government researcher revealed that the intensity, level, and power of the flood in the Bhote Koshi and downstream Trishuli rivers originating from Tibet were beyond what is typically expected from a “once-in-a-millennium flood.”
During preliminary studies by the Water Resources and Hydrology Research Centre focusing on the Rasuwa-Bhote Koshi flood, the speed, flow, and dynamics of the event were evaluated.
“This was definitely not a typical flood. We identified three to four unique characteristics,” stated Sanjiv Baral, the centre’s Executive Director.
What Was the Nature and Magnitude of the Flood?
The centre’s research identified three key distinct features of the Bhote Koshi flood, with the ninth highlighted as its unprecedented magnitude.
“Normally, for constructing houses along riverbanks, flood statistics of at least 50 years are considered, and for bridges, data for 100 years is reviewed. However, this flood surpassed even the mildest floods of the last 1,000 years. Hence, it was much more devastating than any conceivable extreme flood,” Baral explained.
The second distinct feature revealed by the study was that the flood was unlike typical monsoon-induced floods.
“At the time, there was no rainfall in the catchment area. This was not a normal flood caused by rain but a flash flood generated by a landslide with exceptionally high velocity,” he added.
The landslide originated approximately 22 kilometers northeast of Rasuwa Gadhi, situated between Langtang Lirung and Changburi, at elevations ranging from 4,000 to 5,600 meters above sea level, descending steeply about 1,200 to 2,000 meters to the upper section of the Lhende Khola, according to Nepali researchers.
Large broken boulders, yellow chunks of ice, and debris combined with rapid momentum rapidly swept down the riverbed, causing extensive damage on the lower reaches within minutes.
At 8:37 a.m. Nepal time on Bhadra 10 (August 26), a magnitude 5.2 shaking was recorded during the event. Initially presumed to be caused by an earthquake triggering the landslide, geologists later confirmed it was ground shaking generated by the landslide itself, not an actual earthquake.
The third characteristic was that the flood was not composed solely of water. “Usually when we say flood, we think of water only. However, here, massive rocks and sediment flowed at tremendous speed,” Baral clarified.
Devastating Velocity
Research shows that this destructive flood reached the Nepal-China border at Rasuwa Gadhi within seven minutes.
The flood covered the 22-kilometer distance between the landslide site and the border at an astonishing speed of 52 meters per second (188 kilometers per hour).
Although its speed decreased somewhat downstream, it still maintained a velocity of 65 kilometers per hour upon reaching Bhetrawati, nearly 70 kilometers further down the river.
From Bhetrawati to Phurke Khola (Malekhu), the flood covered 53.3 kilometers at a rate of 29 kilometers per hour, and its speed reduced to approximately 26 kilometers per hour when it reached Devghat before merging with the Kaligandaki to form the Narayani River, according to the study.
“The velocity was so great that the water-level monitoring stations themselves were swept away,” Baral noted.
According to reports, the station at Rasuwa Gadhi was washed away at 8:40 a.m., while the Syafru Besi station recorded its final measurement at 8:50 a.m.
Syafru Besi’s Langtang Khola station, Bhetrawati station, Palkhu Khola station, and the Phurke Khola station at Malekhu were all destroyed by the flood.
Preliminary estimates suggest that reconstruction will require at least 7 billion Nepalese rupees due to the extent of the damage. The human toll has been extraordinarily high, with experts citing the flood’s immense velocity as a major contributor to the destruction.
Could This Flood Have Been Forecasted?
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Sunil Acharya, Associate Professor at Tribhuvan University’s Central Department of Hydrology and Meteorology, explains that forecasting such floods is complex and challenging.
“This type of flood cannot be easily forecasted. Continuous camera monitoring and satellite-based regulation in high-risk areas are necessary,” Acharya stated.
Experts agree that identifying high-risk areas throughout the entire Himalayan region is a daunting task and maintaining continuous monitoring remains highly challenging.
Sanjiv Baral from the Water Resources and Hydrology Research Centre also believes traditional forecasting and early warning systems are now insufficient.
“During this flood, more than 600,000 people in downstream areas received SMS alerts, but some messages were delivered only a day later. Traditional early warning systems are failing. Therefore, to inform people about extreme floods, use of multiple sensors, cameras, SMS alerts, public announcements, and other channels is essential,” he noted.
Following the Bhadra 10 flood event, Nepal implemented a CCTV-based real-time monitoring system for the first time, the study reports.
“On the seventh day after the Bhote Koshi flood in Rasuwa, the Water Resources and Hydrology Research Centre installed CCTV cameras near Timure, close to the Nepal-China border…”
While this system marks significant progress in Nepal’s direct river monitoring and flood forecasting capability, it remains primarily focused on continuous observation.
The camera is installed approximately 70 meters above the river surface, providing continuous 24-hour live surveillance.
