On the night of October 3, 2023, around 11 pm, South Lhonak Lake, situated at an altitude of 17,100 feet in the state’s northwest, experienced a catastrophic burst caused by a cloud burst-induced Glacial Lake Outburst Flood (GLOF), as was reported on October 4, 2023.
This event resulted in the sudden release of water downstream, leading to a substantial rise in water levels in the Teesta River. Consequently, the Teesta River banks in four districts—Mangan, Gangtok, Pakyong, and Namchi—were severely impacted on the early morning of Wednesday, October 4, as reported by the Sikkim State Disaster Management Authority (SSDMA).
After much speculation regarding the catastrophic event, the Indian Space Research Organization (ISRO) has provided an explanation for the cause of the Glacial Lake Outburst Flood (GLOF) that occurred on the night of October 3, 2023, as detailed in the accompanying picture above.
According to the latest imagery shared by the ISRO, posted by Dr. Smriti Basnett (Glaciologist), it is observed that debris from the lateral moraine cascaded into Lhonak Lake located at 5300 m elevation. This event triggered a substantial ripple effect, leading to a widening of the lake’s outlet as the water surged outward.
The report indicates that not all the water has been discharged; substantial ice sheets are observed floating within the lake, and the lake’s water level has notably receded. It is important to note that this incident is still under investigation, with a scientific team actively working on Digital Elevation Models (DEMs) to gain further insights.
Based on the provided imagery, it appears that a landslide likely occurred on October 3, 2023, leading to a significant amount of debris falling into the lake. This event resulted in the breaking of ice and exerted immense pressure on the lake water.
As of September 17, 2023, the total water-covered area in Lhonak Lake was approximately 163 hectares, which had expanded to 167 hectares by September 28, 2023. However, the most recent image from October 4, 2023, reveals a reduction in the water-covered area, which is now approximately 63 hectares.
According to a 2021 research report by Sattar et al. titled “Future Glacial Lake Outburst Flood (GLOF) Hazard of South Lhonak Lake, Sikkim Himalaya,” published in Geomorphology journal, the Teesta region in the Sikkim Himalaya has several high-altitude glacierized lakes, including the fast-growing South Lhonak Lake.
Despite their remote location, these lakes can trigger dangerous glacial lake outburst floods (GLOFs) downstream. The researchers assessed the future GLOF risk of South Lhonak Lake using glacier and hydrodynamic modelling, considering increased avalanche risk as the lake grows.
The report indicated potential scenarios of avalanches causing dam overtopping and examined three lake-breach scenarios, estimating peak discharges from 4311 m³/s to 12,487 m³/s. The report had stated that the worst-case scenario, high water depths and fast flows near Chungthang, close to a major hydropower station, pose risks to settlements and assets in the Teesta valley, especially in Chungthang.
This underscores the need for an environmental impact assessment and GLOF risk mitigation measures.
Similarly, a significant number of researchers and environmentalists have cautioned about the potential disaster since 2006. Nevertheless, the report’s influence in the contemporary world remained unheard.
Sikkim Urja Limited, formerly known as Teesta Urja Limited, was established to actively contribute to India’s hydro power development efforts. The company secured the 1200 MW Teesta Stage III Hydro Power Project on a BOOT (Build, Own, Operate, Transfer) basis for a 35-year duration from the Government of Sikkim.
Located in the North Sikkim district, this project operates as a run-of-the-river scheme, utilizing an available gross head of 800 meters. All generating units were successfully commissioned in February 2017.
Regarding the company’s current ownership structure, Sikkim Urja Limited, was operating as a state government enterprise, with the Government of Sikkim holding a majority stake of 60% through its investment arm, the Sikkim Power Investment Corporation Limited (https://sikkimurjalimited.in/ accessed on 7 October 2023).
Amidst the tragedy of Sikkim’s largest hydel project being washed away, the Teesta-III (Teesta Urja or Sikkim Urja) dam’s compromised history is brought into sharp focus, particularly concerning its capacity to withstand Glacial Lake Outburst Floods (GLOFs).
With a spillway capacity of 7,000 cubic meters per second (cumec), the dam was ill-equipped to survive a GLOF event that discharged as high as 15,000 cumec.
Early warnings were sounded long before the Teesta-III dam was commissioned in 2017. The hydropower project was developed at a cost of Rs 13,965 crore and operated by Sikkim Urja Limited, the project was flagged for its susceptibility to GLOFs.
The Environment Ministry’s steering committee in 2005 noted the potential hazard posed by glacial moraines and their temporary formation of glacial lakes in North Sikkim.
The draft Carrying Capacity report, used to approve Teesta-III in August 2006, also cautioned about the region’s vulnerability to disastrous hazards due to GLOFs. The expanding South Lhonak glacial lake had been under observation for decades by researcher and various agencies including ISRO.
The rapid growth of the infrastructure is seen after the establishment of the hydropower setup (Asim Sattar and others published in Geomorphology Journal in 2021).
During public hearings and legal challenges pertaining to the project, the developers offered assurances of having implemented sufficient mitigation measures to address any potential flood event, emphasizing the spillway’s capacity to manage a Probable Maximum Flood of 7,000 cubic meters per second (cumec). Tragically, this assertion proved unsubstantiated, as the dam was washed away during the night of October 3, 2023, resulting in the inundation of not only the entire Chungthang town, situated at an elevation of 1500 m elevations but also the entire basin, extending to the flood plains of Jalpagudi district in West Bengal.
However, experts pointed out that this capacity was inadequate for a GLOF, which demands much higher spillway capacity. Alarming developments occurred when attempts were made to reduce the spillway capacity from 7,000 to 3,000 cumec, alarming the Central Electricity Authority (CEA) in 2008, which warned of the dam’s safety being compromised.
In April 2009, the Environment Ministry’s Expert Appraisal Committee inspected the site, emphasizing the catastrophic risk a rockfill dam with inadequate spillway capacity faced.
Teesta-III eventually reverted to the approved spillway capacity of 7,000 cumec, but this proved to be significantly inadequate during this week’s GLOF event. Shri Sunil Saraogi, the Executive Chairman of Sikkim Urja Limited, stated that there was no operational lapse but acknowledged the need for precautionary measures based on science.
The disaster underscores the urgency of addressing climate change and strengthening dam safety governance. A credible accountability framework for lapses and a functional early warning system are essential, as highlighted by Himanshu Thakkar, coordinator of the South Asia Network on Dams, Rivers, and People.
None of the authorities, including the former leader of the Sikkim Democratic Front (SDF) government, have taken responsibility for this tragic and regrettable situation, which could have been avoided.
The Hon’ble Chief Minister of Sikkim Shri Prem Singh Tamang (Goley) stated, “We are fully aware of the recent natural disaster that has impacted our state. Emergency response teams have been deployed to the affected areas, and I have personally visited Singtam to assess the extent of the damage and interact with the local community. I earnestly appeal to all our citizens to stay cautious and avoid unnecessary travel during this crucial period. It is crucial that we remain composed and look forward to a swift return to normalcy in our region.”
Under the steadfast leadership of Hon’ble Chief Minister Shri Goley, Sikkim has witnessed resolute efforts. He has personally visited every affected location, connecting with and supporting distressed individuals.
The entire state machinery, including dedicated Ministers and MLAs, has tirelessly engaged in rescue, relief, and rehabilitation efforts along the Teesta River Basin, from Chungthang to Rangphu and Melli town.
These endeavours have received invaluable support from numerous NGOs, civil society organizations, community-based groups, and compassionate individuals. They have all worked tirelessly, collaborating with the state government in flood-affected areas, collecting essential relief materials for the victims.
Under the guidance of the Sikkim Krantikari Morcha, the Government of Sikkim has displayed continuous commitment, leaving no stone unturned in providing wholehearted support to every individual affected by the catastrophic Teesta flooding.
Furthermore, the Indian Army and the National Disaster Response Force (NDRF) have initiated a coordinated effort that involves multiple agencies engaged in search and rescue operations within the affected areas.
Moreover, the Indian Air Force stands ready for potential deployment to offer assistance as needed. Visual documentation shared by individuals on social media and “Sikkim Chronicle” led by Shri Nirmal Manger and Ms Kabita Sharma powerfully portrays the scale of devastation within the Teesta River Basin.
The verified death toll until 7 October 2023 evening resulting from the flash floods in Sikkim has now reached 30 individuals, while 81 individuals remain unaccounted.
A total of 1,320 houses have sustained severe damage, and 2,563 people have been successfully rescued thus far. Additionally, 26 individuals with severe injuries are currently hospitalized. Additionally, a total of 14 bridges situated along the Teesta River Basin have either been submerged or washed away, severely impeding communication and isolating Gangtok, Namchi, Pakyong, and Mangan Districts in Sikkim.
This disruption has not only resulted in a scarcity of essential commodities but has also impeded the distribution of vital relief and rehabilitation materials to the affected regions and their inhabitants.
The primary road link from Gangtok to Siliguri, passing through Melli and Teesta Bazar, including the Coronation Bridge, has suffered severe damage at multiple locations, rendering it impassable.
The Dzongu Lepcha Reserve has been completely cut off from the rest of the state, prompting the indigenous inhabitants to construct a makeshift suspension bridge using locally available materials to transport essential relief materials across the river.
Additionally, they have ingeniously assembled ropeways across the Teesta River using locally available resources and have initiated the transportation of goods to these remote areas since October 7, 2023.
Furthermore, more than 3,000 tourists from various parts of the country were reported to be stranded in Lachen, North Sikkim, according to Shri VB Pathak, the Chief Secretary of Sikkim.
Fortunately, all of them are safe and have received necessary assistance from the Lachen Dzumsa and the Government of Sikkim.
Furthermore, it has been reported that approximately seven workers employed at the Sikkim Urja Stage III dam in Chungthang were initially trapped within the dam’s tunnels, as stated by the Chief Secretary of Sikkim. As of noon on October 7, 2023, they are still awaiting rescue.
It should also be noted that the widespread flooding has inflicted significant harm to the road infrastructure, resulting in the collapse of 14 bridges. Among these, nine were under the jurisdiction of the Border Roads Organization (BRO), while five were the responsibility of the state government.
After conducting a thorough assessment of the conditions in Chungthang, Naga, Passingthang, Toong, Richu,Dickchu, Singtam, Majhitar, and Ramphu, the Hon’ble Chief Minister of Sikkim issued a definitive statement on Friday, October 5, 2023. He emphasized that the Sikkim Urja project had been completed during the tenure of the Sikkim Democratic Front.
It was determined that the dam had been constructed with substandard workmanship and outdated technology, making it incapable of withstanding floodwaters. Consequently, the dam was washed away, resulting in its catastrophic failure and extensive damage along the Teesta River Basin. He further clarified that the Dickchu Teesta Stage V dam, built by the National Hydroelectric Power Corporation, successfully managed the unanticipated flash flooding and remains secure.
To support these claims, it is essential to highlight that the planning for Hydropower Projects in Sikkim primarily relied on the Environmental Impact Assessment conducted by government authorities who signed Memorandums of Understanding with the power developers.
Unfortunately, crucial assessments like Geomorphological Impact Assessment, Hydrogeological Impact Assessment, Geo-Lithological Impact Assessment, Social Impact Assessment, and Cultural Impact Assessment were not adequately considered.
The absence of these impact assessments has created a significant gap, preventing the establishment of a scientifically robust foundation for evaluating the feasibility of high-capacity hydropower projects in the mountainous regions of the Sikkim Himalaya.
Consequently, thousands of innocent people, as well as the aquatic and terrestrial biodiversity along the riverbanks, had to bear the brunt of the flooding.
Union Minister of State for Home Affairs, Shri Ajay Kumar Mishra, conducted an assessment of the damages caused by the flash floods in Sikkim on Saturday, October 7, 2023. Shri Mishra arrived in Gangtok on Friday night and led a meeting with the Chief Secretary, relevant department heads of the state government, and senior officers from the Army, ITBP, BRO, NHIDCL, and NHPC on Saturday morning.
During the meeting, he conveyed that the Government of India has formed an Inter-Ministerial Central Team, comprising senior officers from five central ministries, namely Agriculture, Road Transport & Highways, Jal Shakti, Energy, and Finance, to address the situation.
The Hon’ble Chief Minister of Sikkim has announced that the state government has unveiled a dedicated relief package for the four severely impacted districts, supplementing the previously announced assistance.
Chief Minister stated, “In partnership with Sikkim Urja Limited, the state government has devised a special financial relief package. We have earmarked Rs. 25 crore for Mangan District and Rs. 15 crore each for the Gangtok, Pakyong, and Namchi Districts. This allocation is in addition to the relief assistance already extended by both the state and central governments.”
From an aquatic biodiversity perspective, the GLOF-induced flooding has led to a significant depletion of freshwater biodiversity, casting doubt on the prospects for the revival of these species in the near future. Studies by Biswas and Ghose in 1982 and 2014, Choudhury in 2013, and Khatiwara in 2020 have reported the presence of Eurasian otter (Lutralutra),
Small-clawed otter (Aonyx cinereus), and Smooth-coated otter (Lutrogaleperspicillata) in Sikkim. However, these species have received limited attention in Sikkim due to its remote and rugged terrain. In 1993, Dr. Pushpa Tamang documented the existence of approximately 48 fish species in the Teesta River of Sikkim.
Nevertheless, the recent GLOF-induced flooding event is likely to have had a detrimental impact on the freshwater biodiversity of the region, including fish species. This has raised significant concerns about the conservation and restoration efforts required to mitigate these effects.
According to the Energy and Power Sector-Vision 2015 report by the Energy and Power Department of the Government of Sikkim, a total of 28 power projects were planned between 2000 and 2018.
While some of these projects have been completed and are operational, others are still in various stages of development. Additionally, Dahal (2020) has pointed out the significant impact of the North East Industrial and Investment Promotion Policy (NEIIP), which was introduced in Sikkim in 2007.
This policy has led to the establishment of approximately 48 pharmaceutical companies in the state, many of which are located along the river basins that have been affected by the recent flash floods.
In a comprehensive research study conducted by Pandit and Grumble (2012), using a species–area relation model projected until 2025, it was estimated that almost 90% of the valleys in the Indian Himalayas would be impacted by dam construction.
Furthermore, it was projected that 27% of these dams would contribute to the extinction of 22 angiosperm and 7 vertebrate taxa. This would also result in a reduction in tree species richness by 35%, tree density by 42%, and tree basal cover by 30% in dense forests.
Moreover, approximately 54,117 hectares of forests would be submerged, and 114,361 hectares would be damaged due to dam-related activities.
However, despite these alarming projections, the precise and severe impact of hydropower projects on the pristine and globally significant biodiversity of the Sikkim Himalayan region remains largely unknown due to a lack of empirical research.
From a layperson’s perspective, it is evident that dam construction along the Teesta River and its tributaries is disproportionately concentrated in the species-rich subtropical and temperate zones.
However, due to limited studies and uncertainty about the completion of all proposed projects, it is challenging to accurately quantify the full extent of geomorphological and ecological changes that may result from the planned dam construction in the region.
Dr. Ghanashyam Sharma is the Program Manager & Head at The Mountain Institute India, Gangtok, Sikkim
