Nepal Floods Blamed on Ice-Rock Avalanche, Not Just Rainfall

A scientific study has concluded that the devastating floods in Nepal's Bhotekoshi-Trishuli corridor were primarily caused by an ice-rock avalanche originating from the high Himalayas. This finding challenges previous assumptions attributing the disaster to heavy rainfall or glacial lake outburst floods (GLOFs). The report, commissioned by the Nepal Environment Society (NES), was published recently and reveals significant insights into the events leading to the floods.

According to the preliminary findings, a massive mass of ice, rock, and debris fell from an altitude of approximately 5,200 metres in the Langtang-Lirung region of Rasuwa. This event occurred on August 26, when floods devastated several towns and villages in northern and central Nepal. The resultant surge entered the Lhende River before flowing downstream into the Bhotekoshi and Trishuli rivers, creating a formidable debris-laden flood.

Authorities have reported a rising death toll from these flash floods, which has reached 903, with recovery operations still ongoing. The NES study indicates that the ice and rock debris began its descent at about 8:37 am on the day of the disaster, rapidly travelling approximately 22 kilometres to Rasuwagadhi in just over seven minutes, at an average speed of 46.3 metres per second, equating to about 167 kilometres per hour.

The research employed satellite imagery, as well as hydrological and meteorological data, to evaluate the geographical impact of the avalanche. It was noted that drastic changes occurred across a 10-square-kilometre area, and a section of glacier measuring roughly 0.56 square kilometres detached during the event. Nevertheless, experts have stated that further field investigations are required to fully understand the geological mechanisms involved, particularly the roles of melting ice and potential river blockage upstream.

Interestingly, the study found no evidence of a GLOF, which had been a key hypothesis prior to this analysis. It concluded that rainfall in the upper watershed was not exceptionally heavy on the day of the flood, thereby undermining earlier narratives attributing the disaster primarily to rainfall or cloudbursts.

The floods also caused severe damage to hydropower infrastructure, impacting 13 hydropower projects and one solar project, with an estimated loss of around 431 megawatts of generation capacity. Additionally, preliminary assessments have indicated economic losses of approximately Rs 3 trillion, though detailed evaluations across sectors are still forthcoming.

The events highlight growing concerns regarding infrastructure development in fragile Himalayan regions. Geological and environmental experts have called for more stringent regulations to mitigate risks arising from infrastructural expansion in such vulnerable areas.