Guwahati: A preliminary study by Nepalese scientists has attributed the devastating floods that swept through the Bhotekoshi-Trishuli corridor on August 26 to a massive ice-rock avalanche in the high Himalayas, ruling out conventional rainfall as the primary trigger.
The report, commissioned by the Nepal Environment Society and released on Sunday, also found no evidence that the disaster was caused by a glacial lake outburst flood (GLOF).
The findings came as authorities continued recovering bodies from the disaster. The death toll from the flash floods had risen to 903 by Monday, with widespread destruction reported across northern and central Nepal. The floods also damaged critical power infrastructure, while a preliminary assessment estimated the economic losses at around Rs 3 trillion.
According to the study, a huge mass of ice, rock and debris broke away from an area around 5,200 metres above sea level in the Langtang-Lirung region of Rasuwa and plunged into the Lhende River.
The surge then travelled downstream through the Bhotekoshi River before entering the Trishuli, generating a powerful debris-laden flood that caused extensive destruction.
The researchers said the ice-rock mass began descending from the northern section of Langtang-Lirung at around 8.37 am on Wednesday. It moved rapidly down the steep slopes before reaching the Lhende River at an elevation of about 2,930 metres.
Analysis of satellite imagery showed significant geographical changes across nearly 10 square kilometres. The study also identified the loss of a glacier section covering approximately 0.56 square kilometres.
The research team examined satellite images alongside hydrological and meteorological data, terrain characteristics, elevation changes and river slopes. Their analysis found no evidence of a glacial lake outburst flood.
The study estimated that the ice-rock mass travelled around 22 kilometres to Rasuwagadhi in just over seven minutes, reaching an average speed of 46.3 metres per second, or about 167 kilometres per hour.
The flow subsequently slowed to approximately 73 kilometres per hour between Rasuwagadhi and Betrabati and to around 22 kilometres per hour further downstream. Despite the reduction in speed, the flood continued transporting large amounts of rock, sediment and soil.
Researchers also observed that river levels and flow rates in the Rasuwagadhi and Syafrubesi areas had declined several hours before the flood reached the locations.
At Rasuwagadhi, the average river level fell between 5 am and 8 am, while water flow at Syafrubesi began declining at around 7 am.
The study found no evidence of exceptionally heavy rainfall in the upper watershed on the day of the disaster. This, the researchers said, weakened the assumption that the floods were caused solely by extreme rainfall or a cloudburst.
However, the scientists cautioned that further field investigations are required to determine the precise geological mechanism behind the avalanche, assess the role of melting ice and establish whether a temporary upstream blockage of the river contributed to the disaster.