Rasuwa flood likely a GLOF

The sudden and massive flood on the Bhote Kosi early Tuesday morning has washed away a strategic bridge on Nepal’s main border crossing with China for trade, tourists and pilgrims.

Slurry from the nearly 10m high debris flow left 18 people missing, 12 of them Nepalis and six Chinese nationals. Nine others, mostly security personnel deployed in the rescue operation were rescued.

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Four hydropower plants Rasuwagadi, Trisuli III, Trisuli and Benighat have sustained damage, taking out 200MW of Nepal’s power supply – approximately 8% of the total. Over 100 cargo trucks parked on the Nepal side of the border as well as 50 newly imported Chinese electric cars at the Timure Dry Port were washed away.

Nepal’s Department of Hydrology and Meteorology (DHM) noted that there was no major precipitation on the Nepal and China side in the last 24 hours, discarding the possibility of a cloudburst-induced flash flood.

The National Disaster Risk Reduction and Management Authority (NDRRMA) spokesperson Suresh Sunar said it was too early to tell, but the Bhote Kosi flood was most likely the result of a glacial lake outburst in Tibet. Bhotekosi begins on the north slope of the Ganesh Himal near the Nepal-China border, and its tributary Lende drains the catchment area north of Langtang.

DHM’s initial report states that the level of water at a self-monitoring station in Timure in Rasuwa increased by 3.5m at 3:10AM in the morning while 13km away in Syabrubesi, the level rose by 3.65m at 3:30AM. The same station recorded the level at 5.37m ten minutes later, after which it stopped giving a reading, possibly because the station was also washed away.

The floodwaters reached Betrawati by 5AM, and the department has since warned of a downstream impact with debris flow possibly reaching as far as the Budhigandaki confluence in Benighat which straddles the border of Dhading and Gorkha districts.

The Himalaya is melting twice as fast as the rest of the world due to a phenomenon called  ‘elevation dependent warming’, and receding glaciers have led to an increasing number of transboundary GLOFs. According to one ICIMOD study, of the 47 of the most dangerous lakes that threaten Nepal’s valleys 25 are on glaciers in Tibet.

Moreover, there is also an added risk of seismic activities that further destabilise the fragile Himalayan landscape, and add to the risk of GLOFs.

This underscores the urgent need for an early warning system in the high mountains and better cooperation between countries. Two weeks before Tuesday’s flood, Chinese officials are said to have warned the Rasuwa District administration in Nepal to be prepared for extreme monsoon rainfall causing possible floods. However, experts say there needs to be a monitoring of glacial lakes to warn downstream valleys in Nepal in case they burst.

And while much of the attention is on larger glacier lakes like Imja and Tso Rolpa, even the bursting of relatively small lakes in Tibet has caused major damage in Nepal in the past. Interestingly, the earliest recorded flash flood from a glacial lake burst in Tibet that caused destruction along the Bhote Kosi was in 1934.

Another recent case of smaller glacier lake bursts is that of Thame in August last year. While there were no casualties, half of the village in the Everest region was swept away. The overtopping of the South Lhonak Glacial Lake on the Indian side of Kangchenjunga because of a moraine collapse in October 2023 swept away the Chungtang Dam and other infrastructure in Sikkim and killed about 100 people.