Any hydroelectric project situated downstream of unstable lakes carries the risk of being washed away.
Photo: X (formerly Twitter) @ Prem Singh Tamang (Golay) The ongoing scientific investigations over Chungthang dam bursting are trying to unravel the exact cause of the abrupt eruption of South Lhonak Lake, situated in northern Sikkim. This catastrophic event led to the collapse of the Chungthang dam on October 4, 2023.
The dam’s collapse triggered a staggering surge in water levels, elevating them by an alarming 15-20 metres within the Teesta river. The resultant flash flood wreaked havoc across numerous districts. On October 9, the death toll was at 74, according to news agency Reuters.
Moreover, approximately 100 individuals, encompassing civilians and army personnel, are still reported as missing.
The Chungthang dam was a part of the 1,200-megawatt (MW) Teesta Stage III Hydro Electric Project. It was constructed along the Teesta River and became operational in 2017, primarily for hydroelectric power generation.
The dam stood at 817 metres and had a net head of 778 metres. This hydroelectric project was established to generate revenue for the Sikkim government.
The project was designed to generate electricity at a scale nearly twelve times greater than the state’s demand, surpassing the typical requirement of approximately 100 MW. The primary objective was to export surplus power to neighbouring states, including West Bengal.
Chungthang is situated at 1,700 metres above mean sea level and is the confluence point for the Lachen River, originating from South Lhonak Lake and the Lachung River, together forming the Teesta River.
South Lhonak Lake, located at an altitude of 5,200 metres above mean sea level, was created due to the melting of the Lhonak glacier. The accumulation of rock debris carried by glaciers or ice sheets is called moraine.
When these moraines accumulate at the glacier’s edge, marking its maximum extent, they create a terminal moraine. If a terminal moraine obstructs the outflow of meltwater from the valley, it forms a moraine-dammed lake, much like South Lhonak Lake.
Was there any prior warning of a potential collapse of this lake? Indeed, there was. A study titled Future Glacial Lake Outburst Flood (GLOF) Hazard of the South Lhonak Lake, Sikkim Himalaya published in the Geomorphology journal in 2021 by Ashim Sattar et al., clearly alerted to the possibility of South Lhonak Lake experiencing GLOF.
GLOF is a type of flood that occurs when a dam containing a glacial lake collapses. These lakes, formed by the embankment of unstable moraines, can burst due to various triggers, including heavy rainfall, avalanches, rockfalls, landslides and earthquakes.
South Lhonak Lake is one of the rapidly expanding lakes in the Sikkim Himalayan region, and its growth is inversely related to the accelerated melting of the Lhonak glacier resulting from global warming.
Over the past 29 years, this glacier has shrunk by approximately 0.96 square kilometres, with its submerged area expanding to 1.35 square km in 2019 from 0.42 square km in 1990. This rapid growth increases the lake’s vulnerability to collapse when subjected to heavy rainfall, earthquakes and other similar events, particularly in the Himalayan region where geological activity is more pronounced.
The satellite image above illustrates the glacier’s shrinking areas from 1962 to 2006. Source: AE Racoviteanu, Y Arnaud, MW Williams, WF Manley.
Reducing emissions is the only viable option. Hydroelectricity is unquestionably one of the cleanest energy sources available today.
Sikkim has significant potential for generating power by utilising the fast-flowing rivers in the Himalayan mountains. However, we cannot pursue this potential if it endangers human lives. Any hydroelectric project situated downstream of unstable lakes carries the risk of being washed away.
On one hand, global warming is causing glaciers to melt and increasing water pressure in glacial lakes. On the other hand, the growing energy demands are causing us to overlook the potential risks of hydroelectric power dams along rivers originating from these lakes.
Human activity is the root cause of global warming. The only way to address this issue is by reducing emissions. The commitments made by countries to reduce emissions are insufficient.
Achieving net-zero emissions requires all governments, especially the largest emitters, to significantly enhance their Nationally Determined Contributions and take immediate, bold steps to reduce emissions. Furthermore, we should stop the construction of large dams that could pose a potential hazard to human life.
Views expressed are the authors’ own and don’t necessarily reflect those of Down To Earth.
The authors work for Professional Assistance for Development Action (PRADAN).
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