The devastating floods in Nepal that swept through the Bhotekoshi–Trishuli corridor were triggered by a massive ice-rock avalanche in the high Himalayas, not by conventional rainfall, according to a preliminary report by Nepalese scientists.
The report, commissioned by the Nepal Environment Society (NES), was released on Sunday.
Death toll rises to 903
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The study found that a huge mass of ice, rock and debris fell from around 5,200 metres in the Langtang–Lirung area of Rasuwa and entered the Lhende River. The surge then travelled downstream through the Bhotekoshi and into the Trishuli, generating a powerful debris-laden flood, My Republica reported.
The disaster devastated towns and villages in northern and central Nepal on August 26.
Avalanche, not glacial lake outburst
The study concluded that the disaster was not caused by a glacial lake outburst flood (GLOF).
The researchers analysed satellite imagery, hydrological and meteorological data, terrain characteristics and changes in elevation and slope along the river system. No evidence of a GLOF, the report added.
According to the study, the ice-rock mass began falling from the northern part of Langtang–Lirung at around 8:37 am Wednesday. It descended steep terrain from about 5,200 metre before reaching the Lhende River at around 2,930 metre.
Satellite images showed major geographical changes across about 10 square kilometre, while a section of glacier covering approximately 0.56 square kilometre had broken away.
Avalanche travelled at 167 kmph
The study found that the ice-rock mass travelled about 22 kilometre to Rasuwagadhi in just over seven minutes, reaching an estimated average speed of 46.3 metre per second, or 167 kilometre per hour.
Its speed fell to around 73 kilometre per hour between Rasuwagadhi and Betrabati and to about 22 kilometre per hour further downstream. Despite slowing, the flow continued carrying huge quantities of rock, sediment and soil, the report added.
Rainfall theory weakened
The study found no evidence of exceptionally heavy rainfall in the upper watershed that day, weakening the assumption that the disaster was caused solely by extreme rainfall or a cloudburst.
It also found that river levels and flow in the Rasuwagadhi and Syafrubesi areas had declined several hours before the flood arrived.
However, experts said further field investigation is needed to establish the precise geological mechanism, the contribution of melting ice and whether the river was temporarily blocked upstream.
Hydropower projects hit
The disaster caused extensive damage to hydropower infrastructure. Citing preliminary Nepal Electricity Authority data, the study said 13 hydropower projects and one solar project were affected, taking around 431 MW of generation capacity out of the system.
Citing a preliminary government assessment, the study estimated economic losses at around Rs 3 trillion, although detailed sector-wise assessments are still underway.