Dolma Sampa may become Sikkim’s GLOF shield

Retention structure proposed to slow future GLOFs from Lhonak watershed; DPR being prepared for submission to Jal Shakti Ministry

GANGTOK, : The Sikkim government is exploring the possibility of constructing a large-scale flood retention structure at Dolma Sampa, a naturally widened and relatively flat stretch of the Lhonak Valley, to slow down glacial lake outburst flood (GLOF) waters and reduce the impact of future disasters in North Sikkim.

The proposed site near Muguthang, located about 24 km downstream of South Lhonak Lake, has emerged as a potential flood mitigation location after scientific investigations found that the devastating October 2023 GLOF naturally slowed down at Dolma Sampa, where the otherwise narrow Lhonak Valley opens into a broad flat terrain. The Department of Science & Technology (DST) is pursuing the proposal in collaboration with the Central Water Commission (CWC) under the Ministry of Jal Shakti.

DST principal secretary Dr Sandeep Tambe, secretary-cum-principal director Dhiren Shrestha and other officials shared details of the proposed hazard mitigation measures for the Lhonak Lake cluster.

According to DST officials, field investigations after the October 2023 South Lhonak GLOF revealed that the floodwaters, expected to race downstream at high speed, lost momentum after reaching Dolma Sampa. The wider terrain allowed large quantities of boulders, sediment and debris to settle before the waters continued downstream.

“During our field survey we found that when the GLOF came, the flood actually slowed down. The GLOF should have come down at great speed, but surprisingly this GLOF slowed down at Dolma Sampa. It is a location in Lhonak Valley just near Muguthang, and is called Dolma Sampa, which is 24 km below Lhonak Lake,” Dr Tambe said.

The Lhonak Valley remains extremely narrow through most of its course. However, at Dolma Sampa, the valley expands to nearly one kilometre in width for almost three kilometres, creating a relatively flat area of around 300 hectares.

“When the GLOF reached Dolma Sampa with great speed, it spread across the flat area and slowed down, and the accompanying boulders and debris got deposited there,” he said.

The department is now examining whether these natural characteristics can be strengthened through structural intervention to create a large-scale retention structure capable of delaying and reducing peak floodwaters before they travel further downstream.

“So we are looking at this natural solution possibility given by nature and exploring what we can do to further slow down the GLOF and halt it at Dolma Sampa itself,” Dr Tambe said.

Officials stressed that the proposal remains at the conceptual and feasibility-study stage, and no final decision has been taken on constructing any structure.

Further studies, they said, will determine whether such an intervention is technically feasible and whether it could inadvertently worsen flood conditions instead of mitigating them.

The Central Water Commission accompanied the State government's scientific expedition to the Lhonak region last month and carried out detailed studies of the proposed site.

Some further studies are needed and the concept is under development phase, Dr Tambe said.

The CWC is preparing a Detailed Project Report (DPR) in collaboration with the DST, which will later be submitted to the Ministry of Jal Shakti for consideration.

According to the department, the proposed intervention could potentially provide protection against GLOF events originating from the major glacial lakes in the upper Lhonak watershed, including South Lhonak, North Lhonak and Changsang lakes.

DST officials said that if the proposal proves feasible and is successfully implemented, it could serve as a single-point mitigation mechanism for a large number of glacial lakes across the upper watershed.

“If this proposal becomes successful, we will be catering to above 100 lakes at a single time. We are trying to have a retention structure at the Dolma Sampa watershed area and not at the lake level,” officials said, adding that such an approach had not been attempted elsewhere and could become a pioneering initiative by Sikkim.

The proposal has emerged from a series of scientific investigations carried out by the DST with State departments and Central agencies after the October 2023 South Lhonak GLOF.

A multi-disciplinary team found significant differences in the way floodwaters behaved across the river valley. While narrow sections experienced severe erosion, wider sections witnessed substantial deposition of sediment, boulders and other debris.

The findings reinforced the importance of naturally widened stretches such as the three-km-long and one-km-wide Dolma Sampa area in slowing floodwaters and reducing their destructive force.

The proposed intervention forms part of the State government's wider effort to assess and reduce the growing risks posed by high-altitude glacial lakes in Sikkim.

The National Disaster Management Authority (NDMA) has identified 189 high-vulnerability glacial lakes across the Indian Himalayas, including 40 high-hazard glacial lakes in Sikkim. Of these 40 in Sikkim, 16 have been placed in the highest-risk category.

Since August 2024, the Department of Science & Technology, in coordination with State departments and Central agencies, has studied 13 of these 16 highly vulnerable lakes, most of them in North Sikkim.

All these lakes lie at elevations above 17,000 feet and are extremely difficult to access, with expeditions requiring extensive preparation comparable to mountaineering missions.

The latest multi-disciplinary expedition involving scientists, engineers and geologists was conducted from July 23 to August 8 this year.

Based on these field studies, the DST has prepared mitigation proposals for three major high-hazard glacial lake clusters in Sikkim.

The Lhonak Lake cluster, comprising South Lhonak, North Lhonak and Changsang lakes, has been identified as one of the State's three major high-hazard glacial lake complexes, alongside the Shako Chu and Gurudongmar complexes.

The focus on the Lhonak cluster follows the catastrophic October 2023 GLOF, which demonstrated the scale of destruction that a sudden release of water from a high-altitude glacial lake can inflict downstream.

South Lhonak Lake, situated above 17,000 feet in the extreme northern region of Sikkim, was the source of the 2023 GLOF. The resulting flood travelled through the Lhonak and Teesta river systems, causing widespread destruction downstream.

Reaching the lake remains a major challenge for scientists and engineers. With no road access, field teams must undertake a two-day trek, while the terrain has become considerably more fragile since the 2023 disaster, making future expeditions and mitigation work even more difficult.

Meanwhile, the DST has continued monitoring the present condition of South Lhonak Lake.

According to the department, the lake now measures about 2.70 km in length and covers approximately 148 hectares. Before the October 2023 GLOF, it measured around 2.60 km in length and covered about 155 hectares.

Despite changes in its shape and extent following the 2023 event, the department said South Lhonak Lake is presently considered stable.