South Lhonak Lake disaster and climate crisis: Insights from Scientific Research

This tragedy could have been avoided. Why must people suffer when we know that we can tackle the problem

Dr A. Kulkarni,faculty member at Indian Institute of Science, Divecha Centre for climate change and ISRO scientist

“South Lhonak Lake was said to store around 90 million m3 of water (as of 2016), a wash of that volume and high speed down the slopes is alarming”

Smriti Basnett, Glaciologist from Sikkim

Sikkim: After the 2011 earthquake, Sikkim faces another disaster in the form of a flash flood. On October 04, due to a combination of excessive rainfall and a Glacier Lake Outburst Flood (GLOF) at a moraine-dammed South Lhonak Lake, towns and settlements on the teesta basin was hard hit by flash flood.

South Lhonak Lake is situated at 5,200m above sea level and is formed due to the melting of the Lhonak glacier, which expanded over 12.5 km². However, the actual cause of the outburst of the lake is still unknown.

Moraine-dammed Lake: is a type of glacial lake that forms when a glacier advances and then recedes, leaving behind a natural dam made of glacial debris called moraine. Moraines are composed of rocks, boulders, sand, and other sediment that the glacier picked up as it moved. When the glacier retreats, this moraine material is often deposited at the glacier’s terminus, creating a barrier that can block the flow of water.

The image of South Lhonak Lake taken by ISRO satellite on September 28, 2023 shows that, the lake is covered in approximately 167.4 hectares. Another image taken after the disaster on October 4, 2023, at 6:00 am shows that the size of lake was reduced to approximately 60.3 hectares, indicating a significant decrease in the lake’s size by 107.1 hectares.

ISRO Satellite Images

Many scientists, organizations, and agencies, including ISRO, have conducted research at South Lhonak Lake in the past.

In 2013, a research paper titled “Remote sensing-based hazard assessment of glacial lakes in Sikkim Himalaya” was published. According to the findings of the research paper, South Lhonak lake was recorded as the most vulnerable and had a 42% chance of an outburst.

“The temporal satellite data analysis from CORONA to LISS III shows the glacier receded 1.9 km from 1962 to 2008 and the formation of a moraine-dammed glacial lake at the snout of the South Lhonak glacier, Sikkim Himalaya. The lake’s outburst probability shows a very high value of 42%,” reads Remote sensing-based hazard assessment of glacial lakes in Sikkim Himalaya, 2021

Soon after the paper was published, the central government, at the special request of the Sikkim government, formed a committee to assess the future risk factors associated with the South Lhonak Lake. The committee was led by Dr A. Kulkarni, an ISRO scientist.

Speaking with Sikkim Chronicle, Dr A. Kulkarni expressed his disappointment over the disaster, saying, “Since morning, I have been so depressed. This tragedy could have been avoided. Why must people suffer when we know that we can tackle this problem”

Regarding the nature of South Lhonak lake, Dr Kulkarni stated, “As far as South Lhonak Lake is concerned, this is one of the most studied glacier lakes in India. The first report, from my early inventory work in 1990, identified the formation of this lake. In 2013, another ISRO scientist published a paper stating that this lake had been expanding since 1970 and required attention. After that paper was published, the Sikkim Government made a special request to the central government to study this lake. In 2013, a committee was formed by the Government of India to study the potential risk from South Lhonak lake, and I led that committee.”

“We went to the site, conducted an in-depth study of the lake, measuring its depth. Afterward, we conducted modeling to understand how the lake would expand in the future. We also conducted a resistivity survey to measure and understand the ice beneath the moraine of the lake. Along with that, we conducted modeling to understand which areas would experience flooding if the lake were to burst,” added Dr. Kulkarni.

Dr Kulkarni emphasized that the recommendations from their committee’s report were not taken seriously, stating, “We submitted recommendations in our committee report, suggesting urgent attention to South Lhonak lake as it was increasing in size. The report recommended two methods: the Siphoning Method and expanding the drainage outlet. Since the ice beneath the moraine was not very large, we could have expanded the depth and width of the outlet so that water could drain faster, minimizing the risk. Unfortunately, these recommendations were not taken very seriously. If the state government had addressed the issue at that time, we could have been in a much better position now and significantly reduced the risk.”

Siphoning system: typically refers to a method or infrastructure put in place to manage the water level in the lake by transferring water from the lake to a lower elevation. This is often done to mitigate the risk of a glacial lake outburst flood (GLOF) caused by the potential breach of the moraine dam or the rapid increase in water volume due to melting glaciers.

Dr. Kulkarni also mentioned that in 2016, the state government attempted a siphoning system, but it didn’t work, “The siphoning system of draining water didn’t work because there was not enough information about how much water was coming into the lake. To reduce the lake’s size through siphoning, the extraction rate should have been substantially higher than the incoming water. Additionally, the water from North Lhonak Lake flows into South Lhonak Lake, making South Lhonak larger, and this was the main reason the siphoning didn’t work. What would have worked is increasing the size of the lake’s outlet.”

Regarding the possible reasons for the South Lhonak Lake’s outburst, Dr. Kulkarni said, “Now, we have to critically evaluate what has happened in the lake that caused the disaster. There are two possibilities: either the dam holding the water collapsed, permanently reducing the lake’s size, or there was piping, meaning that the ice beneath the moraine melted, creating a pipe through which water drained. Over time, that pipe could seal, allowing a larger lake to form again. So, we don’t know the exact situation; both possibilities exist. We must conduct thorough research to permanently resolve this issue.”

Piping: refers to a phenomenon where water from a glacial lake begins to drain through or beneath the glacial ice or the moraine (a mix of rocks, debris, and sediment deposited by glaciers) that acts as a natural dam. This process involves the formation of channels or pipes within the glacier or moraine through which water can flow.

According to “Future glacial lake outburst flood (GLOF) hazard of the South Lhonak Lake, Sikkim Himalaya,” published at Zurich Open Repository and Archive, University of Zurich in 2021 states that, “The South Lhonak lake has been exhibiting significant growth over the years, expanding from 0.42 km² in 1990 to 1.35 km² in 2019.”

The study, conducted by Ashim Sattar and a team at South Lhonak Lake, In its conclusion states, “Our study evaluates the future GLOF hazard of the largest proglacial lake in Sikkim, South Lhonak Lake. The future volume of the lake, based on an ice-thickness approach, is calculated to be 114.8 × 10^6 m³. This enormous volume of water in a highly dynamic high-mountain environment makes this lake a priority for GLOF risk management. Our results show that the GLOF susceptibility will increase due to the expansion of the lake towards steep slopes, which are considered potential starting zones of avalanches. These avalanches can create an impulse wave when hitting the lake and are considered the most likely GLOF trigger for the South Lhonak Lake.”

“A number of GLOF scenarios were defined (both dam overtopping and breach), considering different avalanche scenarios (magnitudes). Modeling results conclude that dam overtopping would likely be attenuated several kilometers downstream from the lake due to a moderate overtopping volume and gently sloped topography directly downstream of the lake, where the flow energy dissipates. Whereas the main hazard for downstream areas is associated with the potential dam breach events, that would result in far-reaching floods.” Added the statement

Global warming and Melting Glaciers: Global warming, driven by the relentless increase in greenhouse gas emissions from human activities, is causing the alarming and accelerating melting of glaciers worldwide. As Earth’s average temperature rises due to the greenhouse effect, glaciers, which are sensitive indicators of climate change, are receding at an unprecedented pace. This phenomenon leads to several grave consequences, including rising sea levels, the loss of freshwater resources vital for communities, changes in ecosystems, and heightened risks of glacial lake outburst floods. The melting of glaciers serves as a stark reminder of the urgent need for global action to reduce greenhouse gas emissions and mitigate the far-reaching impacts of climate change on our planet.

The Bridge connecting Dzongu washed away

Chungthang Dam washed away

Sikkim comprises 0.5% of India’s landmass and boasts 84 glaciers, the largest number among any state or union territory. Currently, there are more than 300 glacial lakes in the Sikkim Himalayan region. Out of these, 10 have been identified as vulnerable to outburst floods, and one of them is the South Lhonak Lake. Past research has consistently indicated the risk factor associated with the South Lhonak Lake.

The exact cause of the outburst in the South Lhonak Lake remains a matter of research and investigation, but one undeniable truth is that climate change and natural disasters are exacerbated by alarming global warming.

As of the latest records from October 6, the tragic disaster that occurred on October 4 has claimed 26 lives, with 142 individuals still missing. Additionally, 1173 structures have been damaged or destroyed, and 2413 has been rescued.

Adarsh Gaon, Singtam

The affected areas include Lachen, Chungthang, Dzongu, Dikchu, Singtam, Rangpo, and Melli towns in Teesta basin have been the hardest hit by the flood.

 This tragedy could have been avoided. Why must people suffer when we know that we can tackle the problem Dr A. Kulkarni,faculty member at Indian Institute of Science, Divecha Centre for climate change and ISRO scientist “South Lhonak Lake was said to store around 90 million m3 of water (as of 2016), a wash ofRead More