Sikkim’s warning to Nepal

The devastating flood on Wednesday morning in the Indian state of Sikkim is another shocking proof of what can happen if rampant, unplanned and poorly engineered infrastructure are built along Himalayan rivers at risk from climate breakdown and weather extremes.

Heavy rains caused a cloudburst at the tri-junction of India Nepal and China in the Kangchenjunga region this week. The South Lhonak proglacial lake burst, unleashing a debris flow that destroyed the Chungthang Dam and causing a deadly flash flood on the Teesta River in Sikkim, West Bengal and as far downstream as Bangladesh.

Till press time, at least a dozen people are confirmed dead, with scores missing. Other hydropower projects in Sikkim were also damaged, while many bridges and highways were washed away. Indian satellite imagery confirmed that the flood was triggered by a sudden outflow from South Lhonak, which then led to the collapse of the Chungthang Dam, draining its reservoir, and unleashing the combined debris flow.

Location of the South Lhonak glacial lake near the ti-junction of the borders of Nepal-India-China, and the Chungthang Reservoir in India's Sikkim state.

The 4 October flood on the Teesta proves that there could be much more destructive glacial lake outburst floods (GLOF) in store in future as global warming melts the Himalayan icecap. These mountains are warming 0.7 Celsius faster than the global average, making glaciers recede and shrink, leaving large moraine-dammed lakes. 

Hundreds of new proglacial lakes have emerged in the past decades, mostly in eastern Nepal, India, Bhutan and southern Tibet. Glaciologists are tracking at least 40 of them which they say are fragile, either because of water accumulation, from seismic activity, or because of avalanches causing the lakes to overtop their moraine dams. There are so many new lakes that many do not even have names, but numbers. 

In an uncannily accurate forecast of Wednesday’s disaster, Indian glaciologist Ashim Sattar had co-authored a paper in the journal ScienceDirect in 2021 titled Future Glacial Lake Outburst Flood (GLOF) Hazard of the South Lhonak Lake, Sikkim Himalaya, warning of the danger of avalanches falling into the lake causing it to burst. The team measured the lake volume at 115 million cubic metres.

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The Chungthang Dam and its reservoir before Wednesday's  disaster.

‘This enormous volume of water in a highly dynamic high-mountain environment makes this lake a priority for GLOF risk management,’ the paper warned. ‘Our results show that the GLOF susceptibility will increase due to the expansion of the lake towards steep slopes, which are considered potential starting zones of avalanches.’ 

Wednesday’s GLOF event washed away the 1,200MW Chungthang Teesta III Project which had a 60m concrete and rockfill dam, tunnel and underground powerhouse built at a cost of $1.7 billion. Sikkim has invested heavily on hydropower projects, with more than 35 cascade schemes on the Teesta and its tributaries that drain Mt Kangchenjunga, the world’s third highest mountain.

There are no reports of damage to infrastructure on the Nepal side of the Kangchenjunga catchment, where there are numerous hydropower plants in operation and under construction on the Tamor, Arun, Kabeli and other rivers. An early monsoon cloudburst in Taplejung in June killed at least 40 people, and damaged several of those hydropower projects.  

The South Lhonak glacial lake had 115 million cubic metres of water, and glaciologists had warned it could burst.

A flood in July 2021 in Nepal similar to this week’s Sikkim disaster damaged a $800 million project to supply water from the glacier-fed Melamchi River through a 26.5km tunnel to Kathmandu. Although the tunnel was saved, the headworks were destroyed, and the project is still not back in full operation.

In 2015, a glacial lake burst in Tibet and brought down debris flow that damaged the 44MW Bhote Kosi project in Sindhupalchok district that had just been repaired after the Jure landslide and the 2015 earthquake. In 2022, heavy rains damaged a dozen under construction hydropower projects in central Nepal, causing huge losses.

Dangerous glacial lakes in Nepal, but there are other lakes in Tibet that are also at risk of bursting.

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There are glacial lakes in central and eastern Nepal like Thulagi in Himalchuli, Tso Rolpa in Rolwaling and Imja in the Khumbu that would be just as dangerous as South Lhonak because of hydropower, highways and settlements downstream. In addition, dozens of glacial lakes on the Chinese side in Tibet also drain into tributaries of Nepal’s transboundary rivers and pose great risk. 

The water levels of Tso Rolpa and Imja have been lowered by building weirs on the moraine, but such construction is expensive and there are just too many lakes. A new lake that has emerged on Langtang Glacier is being drained to lower its level as well as generate hydropower, the first in Nepal. 

Another view from the social media post of the breached Chungthang Dam.

As Nepal embarks on generating up to 30,000MW by 2035, the warning from Sikkim is clear: spread the risk and plan for the worst.