After Nepal disaster, Sikkim steps up measures to tackle glacial lake outburst flood risks

The devastating glacier-related flash flood in Nepal has brought renewed attention to glacial lake risks in the Himalayas, with Sikkim pointing to its ongoing efforts to assess and mitigate hazards from its high-altitude lakes.

Nepal's August 26 disaster, triggered by a collapse or avalanche of ice and rock near the Nepal-China border, has killed more than 1,000 people and left nearly 4,000 missing, according to the latest official figures. The disaster has raised concerns over the vulnerability of Himalayan settlements to glacial and mountain hazards.

Sikkim itself witnessed a catastrophic glacial lake outburst flood (GLOF) less than three years ago when South Lhonak Lake burst on the night of October 3-4, 2023, devastating large stretches of the Teesta valley.

Sikkim Forest Minister Pintso Namgyal Lepcha has described the state's glacial lake hazard study as the first such initiative in the country. Officials said the programme has evolved significantly since the 2023 disaster, with multiple agencies involved in assessing high-risk lakes and planning mitigation measures.

The National Disaster Management Authority (NDMA) has identified 189 high-vulnerability glacial lakes across the Indian Himalayas, including 40 in Sikkim. Of these, 16 are categorised as Category A, considered the highest risk, while two are Category B, nine Category C and 13 remain unclassified.

A High-Level Steering Committee headed by the Chief Secretary oversees policy decisions, while a Multidisciplinary Task Force handles field-level implementation. The Science and Technology Department is the nodal agency for the programme.

A Sikkim Commission on Glacier Hazard, headed by former Government of India adviser Akhilesh Gupta, is also in the final stages of preparing its report.

Since 2024, Sikkim has undertaken five major expeditions to assess its vulnerable glacial lakes. The latest, a 17-day multi-agency expedition in July-August this year, included the installation of automatic weather stations near Gurudongmar Lake and at Dolma Sampa.

Principal Secretary Sandeep Tambe said 13 of the 16 Category-A lakes are located in Mangan district and three in Gyalshing district. All are situated above 17,000 feet, making access difficult.

Teams are using unmanned ultrasound boats to determine the volume of water in the lakes, followed by Electrical Resistivity Tomography to assess the composition and stability of terminal moraine dams.

The surveys have been completed at 13 of the 16 high-risk lakes.

One mitigation plan has already emerged from the assessments at Shako Chho, where officials found a fragile moraine with no natural outlet and water seeping through the dam.

Officials said cutting the moraine to drain the lake would therefore be unsafe. Solar-powered pumping has been proposed as the most practical solution, considering the absence of electricity and the logistical difficulties of transporting fuel to the high-altitude site.

The Detailed Project Report has been submitted to the Sikkim State Disaster Management Authority (SSDMA), forwarded to the NDMA and is currently under review.

A second proposal covering three critical lakes in the Lhonak Valley is being developed, while a safety wall has been proposed at Gurudongmar to strengthen the moraine plug. The plan is being prepared in partnership with the Brahmaputra Board while taking into account the religious significance of the lake.

Early warning system remains a challenge

Despite the progress in hazard assessment, officials acknowledged that Sikkim and the country still lack a fully developed early warning system for glacial lake outburst floods.

Science and Technology Department Secretary and Principal Director Dhiren G Shrestha said the department had studied available sensor technologies but there was no readily available technology in the country that could currently provide a complete early warning system.

With support from Switzerland, two pressure probes have been installed at Lhonak Lake and Shako Chho to transmit water-level and pressure data to the SSDMA control room.

However, Shrestha said these should be considered monitoring units rather than full-fledged early warning sensors.

At the national level, the NDMA has designated C-DAC Pune as its technical partner for developing early warning sensors. Officials said creating an effective system would also require adequate communication and network infrastructure in remote high-altitude areas.

Only two Category-A lakes — South Lhonak and Shako Chho — are currently under continuous real-time monitoring through stations supported by the Swiss Development Corporation.

Shrestha said Sikkim's glacial water systems originate and remain within the state's territory, and there was no direct threat from a glacial lake on the Tibet side similar to the trigger seen in Nepal. Earthquakes, however, remain an unpredictable hazard that cannot be confined by borders.

Comparing the 2023 South Lhonak disaster with the recent Nepal catastrophe, Shrestha said South Lhonak released more than 30 million cubic metres of water.

He attributed Sikkim's comparatively lower loss of life not merely to chance but also to lower habitation in the flood path and community sensitisation and evacuation planning undertaken by the SSDMA.

Shrestha acknowledged that preliminary siphoning efforts at Shako Chho and South Lhonak before the 2023 disaster were undertaken without detailed studies and represented an early stage in the state's understanding of glacial lake behaviour.

"We were at the infancy level... but that was also first time in the country," he said.

South Lhonak is currently assessed as stable. Its length is around 2.70 km and its area 148 hectares, compared with 2.60 km and 155 hectares before the 2023 outburst, officials said.

Beyond the 40 high-vulnerability lakes identified by the NDMA, Sikkim has 84 glaciers and 677 wetlands, including 613 small lakes, 52 medium-sized lakes and 12 large lakes.

The scale of the inventory highlights the challenges involved in monitoring the state's rapidly changing high-altitude environment and developing effective warning and mitigation systems across the Himalayan region.