Nepal Floods Triggered by Himalayan Ice-Rock Avalanche, Preliminary Report Finds

A preliminary scientific report has identified a massive ice-rock avalanche in the Himalayas as the likely trigger behind Nepal’s devastating August 26 floods, challenging earlier assumptions that extreme rainfall or a glacial lake outburst caused the disaster.

The study, commissioned by the Nepal Environment Society, said a huge mass of ice, rock and debris fell from the Langtang-Lirung area of Rasuwa and entered the Lhende River. The surge then travelled downstream through the Bhotekoshi and Trishuli river systems, producing a powerful debris-filled flood.

Ice-rock avalanche triggered massive surge

According to the researchers, the avalanche began at an altitude of around 5,200 metres at about 8:37 am on August 26.

Satellite imagery indicated major geographical changes across approximately 10 square kilometres, including the collapse of a glacier section covering around 0.56 square kilometres.

The study estimated that the ice-rock mass travelled around 22 kilometres to Rasuwagadhi in just over seven minutes, reaching an average speed of approximately 46.3 metres per second, or 167 kilometres per hour.

As the flow moved downstream, it slowed but continued transporting huge quantities of rock, sediment and soil.

Rainfall and glacial lake outburst ruled out as main causes

Researchers found no evidence of exceptionally heavy rainfall in the upper watershed on the day of the disaster.

River levels and flow rates had also declined several hours before the flood reached some downstream locations, further weakening the theory that an extreme rainfall event or cloudburst was the primary trigger.

The study also found no evidence of a glacial lake outburst flood (GLOF).

However, scientists stressed that the findings are preliminary. Further field investigations are required to determine the precise geological mechanism, the role of melting ice and whether a temporary blockage of the river occurred upstream.

Death toll rises to 903

The disaster has caused widespread destruction across northern and central Nepal. Authorities said the death toll had risen to 903, while infrastructure, homes, roads and bridges were severely damaged.

The floods also caused major losses to Nepal’s power infrastructure. Preliminary Nepal Electricity Authority data cited in the study indicated that 13 hydropower projects and one solar project were affected, removing around 431 MW of generation capacity from the system.

A preliminary government assessment has put the overall economic losses at around Rs 3 trillion.

The findings underline the growing risks posed by sudden geological and climatic hazards in the Himalayan region, where rapidly changing terrain, glaciers and extreme environmental conditions can trigger destructive events with little warning.