Gangtok: Sikkim has proposed three major engineering interventions to reduce the threat from Glacial Lake Outburst Floods (GLOFs): a solar-powered pumping system to lower the water level of Shako Chho Lake, a flood-retention structure at Dolma Sampa to slow a potential GLOF from the Lhonak lake complex, and a moraine-plug safety wall at the outlet of the Gurudongmar lake complex.
The three proposals have emerged from detailed scientific assessments of Sikkim’s high-risk glacial lakes following the devastating South Lhonak GLOF of October 2023. The interventions are being taken up through different central agencies, with the state seeking to address the risk according to the physical characteristics of each lake rather than applying one solution to all.
Principal Secretary of the Science and Technology Department Sandeep Tambe said the focus is now firmly on reducing the hazard posed by the lakes.
“Studying glaciers is one thing, but we are mostly working towards glacial hazard mitigation,” Tambe said.
At Shako Chho, located around 12 km from Thangu on the route to Gurudongmar Lake, the proposed solution is to pump water out of the lake using solar power.
Shako Chho contains an estimated 27 million cubic metres of water and is considered particularly hazardous because it has no natural outlet. Its terminal moraine — the natural barrier holding back the lake — is high but permeable and considered too fragile for direct engineering intervention.
The proposal is to lower the lake level by around 20 to 40 metres and initially remove approximately eight million cubic metres of water using eight solar-powered pumps. The aim is to create additional freeboard — the empty space between the water level and the top of the lake barrier — and thereby reduce the volume of water available to form a destructive flood wave.
The Detailed Project Report (DPR) has been submitted to the National Disaster Management Authority (NDMA), with funding being sought under the National GLOF Risk Mitigation Project (NGRMP).
Science and Technology Department Director-cum-Secretary Dhiren Shrestha said the location makes conventional pumping difficult.
“There is no electricity supply at that altitude, while taking fuel there regularly would be extremely difficult. Solar power is therefore being considered as the most practical option,” Shrestha said.
The second proposal concerns the Lhonak Complex, comprising South Lhonak, North Lhonak and Changsang lakes.
Instead of attempting to modify the highly vulnerable lakes themselves, the state has identified Dolma Sampa near Muguthang, around 24 km downstream, as a possible location for a flood-retention structure.
The area naturally helped slow the 2023 South Lhonak GLOF because the valley widens sharply there, allowing water, boulders and debris to spread over roughly 300 hectares.
The government is examining whether this natural buffer can be strengthened through a dam-like stone masonry or retention structure. The objective would be to slow the flood wave and allow the water to spread and discharge more gradually.
A Preliminary Project Report has been submitted to the Ministry of Jal Shakti, while the Central Water Commission (CWC) is preparing the DPR in collaboration with the Science and Technology Department, Sikkim.
Tambe said the proposed intervention deliberately avoids disturbing the lake itself.
“We will not do something at the lake level. We will do it at a location called Dolma Sampa where the valley suddenly broadens out and the flood actually slows down there,” he said.
The third proposal is for the Gurudongmar Lake complex, comprising Lakes A, B and C.
Here, the government is examining a moraine-plug safety wall or stone masonry structure at the lake outlet to regulate the discharge of water and reduce the force of a potential flood.
The Gurudongmar complex has expanded substantially over the past three decades. The lake accessible to tourists, civilians and pilgrims is only one part of the system, with two other lakes in the complex being significantly larger.
The proposal is being developed by the Science and Technology Department in collaboration with the Brahmaputra Board, with the dual objective of reducing downstream GLOF risk while avoiding major alteration of the culturally and religiously significant lake.
Shrestha said the timing of an intervention is critical because a flood becomes increasingly difficult to control once it moves downstream.
“Once it starts rolling down the valley, it gains momentum and picks up boulders and debris. Then it is almost impossible to do anything about it,” he said.
Chief Engineer Ashim Basnett said the engineering approach is to create more safety space and reduce the force of a possible flood.
“The best way to stop it is at the lake itself,” Basnett said, referring to the need to reduce the volume and depth of water before an avalanche, landslide or other event can generate a major flood wave.
How Sikkim identified the vulnerable lakes
The three proposals are the result of a series of high-altitude expeditions and scientific investigations carried out since 2024.
The NDMA has identified 188 high-vulnerability glacial lakes across the Indian Himalaya, including 40 high-hazard lakes in Sikkim. Of these, 16 are classified as Category-A/high-priority lakes.
The state has completed detailed assessments of 13 of the 16 priority lakes.
The first task was to determine how much water the lakes actually contain and how stable the natural barriers holding them back are.
Bathymetric surveys have been used to measure lake depth and volume, while Electrical Resistivity Tomography (ERT) has helped scientists understand what lies beneath the surface of the moraine — including rock, soil, ice and other material.
Teams have also examined the possibility of avalanches, landslides and rockfalls entering the lakes and suddenly displacing water.
Tambe said reaching the lakes remains one of the biggest challenges.
“Most of these lakes are at around 17,000 feet and above. It is very complex and very difficult to reach these lakes and do these studies. But we have still organised five expeditions,” he said.
The major expeditions have included:
- August 31-September 14, 2024: 15-day multidisciplinary assessment covering six high-risk glacial lakes.
- November 27-December 5, 2024: Winter GLOF mitigation expedition to Shako Chho and the Lhonak Valley, covering water-level and discharge studies, ERT, DGPS surveys, flood modelling and mitigation feasibility.
- June 19-July 1, 2025: Expedition to Tikip Lake, Bhalay Pokhari and Doodh Pokhari in West Sikkim for ERT, bathymetry and hazard assessment.
- August 20-September 6, 2025: 18-day comprehensive GLOF hazard assessment and mitigation planning expedition.
- July 23-August 8, 2026: 17-day multidisciplinary expedition involving the Science and Technology Department, Sikkim State Disaster Management Authority, Department of Mines and Geology, CWC, Sikkim University, C-DAC and the Brahmaputra Board.
The latest expedition also strengthened monitoring infrastructure, with Automatic Weather Stations installed at Kerang near Gurudongmar and Dolma Sampa.
The investigations have covered lakes including Tikip, Bhalay Pokhari, Doodh Pokhari, Changsang, North Lhonak, South Lhonak, Lachen Khangse, Lachung Khangse, La Chho, Shako Chho, Yulhe Khangse, the Gurudongmar/Sora Funnel complex, Khangchung Chho and Tenchungkha Lake.
South Lhonak after the 2023 GLOF
South Lhonak remains central to Sikkim’s GLOF preparedness because it was the source of the October 2023 disaster.
The latest assessment puts the lake at approximately 2.70 km in length and 148 hectares in area. Before the 2023 GLOF, it measured around 2.60 km and covered approximately 155 hectares.
The lake has expanded in length by around 360 metres since the disaster, although the latest assessment describes it as presently stable.
During the 2023 GLOF, South Lhonak released nearly 30 million cubic metres of water, lowering the lake level by approximately 20-25 metres.
The incident also demonstrated that the threat is not determined simply by whether the water level is rising. A sudden avalanche, landslide or rockfall can enter a lake and displace a large volume of water.
Monitoring and early-warning systems
Alongside structural measures, Sikkim is strengthening its ability to detect changes in lakes and weather conditions.
Automatic Weather Stations have been installed at Shako Chho, Kerang and Dolma Sampa. At Shako Chho, the system provides photographs twice a day and records weather parameters, while a pressure probe measures the lake’s water level.
The information is transmitted to the Sikkim State Disaster Management Authority, allowing authorities to monitor changes without having to physically reach the lake each time.
The Swiss Development Corporation has also installed monitoring stations at South Lhonak and Shako Chho, where water levels, weather information and photographs are collected daily.
Shrestha said monitoring a lake means watching for unusual changes in both directions.
“A receding lake level can also be a warning sign, as it could indicate blockage or drainage through an unstable route,” he said.
The department is also exploring satellite-based monitoring through INSAT. However, officials noted that cloud cover can limit satellite observations, particularly during periods of heavy weather when information may be most needed.
Doppler radar among technologies being explored
Sikkim is also examining the possibility of installing Doppler weather radar to strengthen weather monitoring and improve understanding of intense rainfall and other weather events that can contribute to landslides and related hazards.
Scientific Officer Dr NP Sharma said possible sites are being examined in coordination with the Ministry of Earth Sciences and India Meteorological Department (IMD).
“Possible locations include Namthang in South Sikkim, Sukrabarey in West Sikkim, and the 17th Mile and 3rd Mile areas in East Sikkim,” Sharma said.
He said a radar site requires adequate power and communication facilities and, importantly, a location with a clear 360-degree view.
Officials, however, said technology has its limits. Cloudbursts cannot currently be predicted with sufficient precision, while earthquakes cannot be forecast in advance. Both can potentially trigger landslides or other events capable of disturbing unstable glacial lakes.
From assessment to mitigation
Sikkim has also created an institutional mechanism to oversee the growing GLOF preparedness programme.
A High-Level Steering Committee, headed by the Chief Secretary, provides policy-level direction, while a Multidisciplinary Task Force handles field-level implementation. The Science and Technology Department is the nodal department for GLOF hazard assessment and mitigation.
A Sikkim Commission on Glacier Hazard, chaired by former Government of India Advisor Dr Akhilesh Gupta, has also been constituted, with its report at the final stage. A proposal for a dedicated Glacier Studying Centre is under consideration.
The state has identified 84 glaciers covering more than one square kilometre and 677 wetlands, excluding supraglacial lakes smaller than one hectare.
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The broader programme now seeks to answer four basic questions: how much water a lake holds, how stable the barrier is, what could trigger a sudden release, and where the resulting flood can be slowed or controlled.
The three proposals are the clearest outcome of that approach — lowering the water level at Shako Chho, slowing a potential Lhonak flood at Dolma Sampa, and controlling discharge from the Gurudongmar complex.
For Sikkim, the objective is no longer only to understand where the danger lies, but to identify where engineering, monitoring and early-warning technology can reduce the consequences before a GLOF gathers unstoppable force.
Also Read | Two disasters, one Himalayan warning: Sikkim’s GLOF lessons for Nepal