The flash flood that tragically claimed the lives of numerous individuals, leaving hundreds missing, including 23 army personnel, in the Sikkim region, recently is not an isolated occurrence, considering the context of our ever-changing climate. Such events are likely to persist in the world's high-mountain glaciers.
In response to this catastrophic incident, India Today's OSINT (Open-Source Investigation) team has harnessed the power of open-source satellite data and social media videos to meticulously track the course of a vast 100-hectare glacial outburst. The objective is to identify the remote settlements and critical infrastructure that have been swept away, necessitating immediate and life-saving rescue operations.
To accomplish this, an exhaustive analysis of remote sensing data spanning from the South Lhonak Lake to the damaged Chungthang dam was conducted through a satellite examination. As a result, the geolocation of previously undiscovered settlements, ranging from small to large, was successfully mapped.
These settlements were identified based on factors such as approximately 10 houses located near streams, often connected by makeshift bridges, as well as others housing around 60 residences positioned within close proximity to the glacial outburst's flow path.
Moreover, the OSINT team cross-referenced social media videos that depicted the destruction of bridges and villages with Google Earth imagery. This comprehensive approach served to document the full extent of the devastation caused by this tragic disaster.
In addition to this, data from the Government of Sikkim website was employed to map out the relief camps that have been established at various locations, including Bihari Bhawan, Nepali Bhawan, Singtam Senior Secondary School, Food Preservation Factory, Sirwani Senior Secondary School, Sirwani Army Camp, Dikchu Senior Secondary School, and Sneha Kinetic Guest House. These relief camps play a vital role in providing support and assistance to those affected by the disaster.
It's crucial to recognize that the risks associated with glacial lake-induced flooding (GLOF) are on the rise, particularly in regions like India, where an estimated three million people are at risk of GLOFs, signifying the highest global exposure to such events.
This threat stems from the accelerated retreat of glaciers in the Himalayan mountain range, home to approximately 600 billion tons of ice. As these glaciers recede, they give rise to unstable glacial lakes, with their numbers increasing. While these lakes serve as sources of drinking and irrigation water for local communities, they can quickly turn into deadly hazards when the natural barriers that contain them, such as moraines, become compromised, releasing torrents of water downstream.
A critical point to consider is that certain glacial lakes are situated within bowl-shaped depressions, surrounded by moraines composed of often unstable rock debris left behind as glaciers recede. The collapse of these moraines, either due to the weight of the water, seismic activity, or avalanches from the mountains above, can lead to a sudden, catastrophic release of water. This deluge has the potential to devastate communities and infrastructure located in downstream valleys.
South Lhonak Lake, where this tragic incident occurred, is categorized as a glacial-moraine-dammed lake, situated in the far northwestern region of Sikkim. It is among the fastest-expanding lakes in the Sikkim Himalaya area and is one of the 14 lakes identified as potentially dangerous and susceptible to Glacial Lake Outburst Floods (GLOFs).
This particular lake had been a cause for concern, as it was rapidly expanding in size, primarily due to the melting of the associated South Lhonak glacier and the additional influx of meltwater from the adjacent North Lhonak and main Lhonak glaciers. In fact, studies have consistently highlighted the substantial increase in the size of South Lhonak Lake as a result of glacier melting, firmly establishing it as a potential hotspot for glacial lake outburst floods (GLOFs).
Sikkim is home to 733 glacial lakes, with 288 of them situated at altitudes exceeding 5,000 meters, as reported by the National Remote Sensing Centre (NRSC). While these lakes are predominantly located in remote and sparsely populated mountain valleys, the far-reaching impact of glacial lake outburst floods (GLOFs) can extend for tens of kilometers downstream, posing a significant threat to both lives and infrastructure.
Recognizing the gravity of this situation, it becomes imperative to assess GLOF hazards associated with current and potential future changes in glacier retreat.Â
GLOFs typically fall into two distinct categories. The first type occurs when moraine banks, acting as natural dams for glacial lakes, rupture due to ground disturbances, such as earthquakes. The second type can materialize when glacial lakes experience a surge in water volume caused by accelerated glacial melting attributed to climate change. In the case of South Lhonak Lake, it is vital to understand that both these triggering mechanisms were at play.
Situated at an elevation of 5,200 meters above sea level, South Lhonak Lake is the largest and fastest-expanding glacial lake in the region. Over a 46-year span, from 1962 to 2008, the glacier overseeing this lake retreated by nearly two kilometers—an alarming rate of retreat. In the subsequent years, from 2008 to 2019, further observations documented an additional retreat of 400 meters, exacerbating the potential hazard associated with this dynamic glacial environment.
During this period, the glacier's length dwindled from 6.4 kilometers to 5.1 kilometers, marking an overall reduction of approximately 0.96 square kilometers. Acknowledging the growing risks, the Central Water Commission initiated an advisory to assess the lake system's condition.
Several factors and events appear to have contributed to the glacial outburst in the South Lhonak region:
1. Nepal Earthquake: A series of earthquakes, with the
 most potent registering a magnitude of 6.2, struck Nepal in rapid succession, with tremors also being felt in parts of India. Scientists are exploring the possibility of a link between these seismic events and the glacial outburst. Both Nepal and Sikkim are situated in the young-fold Himalayas, which are still undergoing geological uplift, making them more susceptible to seismic activities.
2. Climate Crisis: The heightened risk of GLOFs in the Himalayan region is attributed to an increase in the glacial lake area and glacier retreat. These changes primarily result from climate change and rising global temperatures. In the Sikkim Himalayas, the area covered by lakes has expanded, accompanied by a rise in the number of new lakes. These trends serve as indicators of GLOF risk. The glaciers are receding due to an increase in Land Surface Temperature (LST), primarily driven by climate change and anthropogenic activities such as industrialization and deforestation, which contribute to global warming.
3. Avalanches: South Lhonak was particularly vulnerable to a GLOF due to its expansion towards steep slopes, which heightened the risk of potential landslides and avalanches. Remote sensing data analysis by ISRO indicated that approximately 105 hectares of land had been drained from the lake, which likely triggered a flash flood downstream.
The disastrous events in Sikkim also resulted in the breach of the Chungthang dam, the largest hydropower project in the state, forming part of the 1,200-megawatt Teesta Stage III Hydro Electric Project, in which the state government holds the majority stake.
Satellite imagery clearly demonstrated that the South Lhonak Lake's area had decreased by over 100 hectares compared to its extent on September 17. This reduction in lake size directly correlated with the lake's outburst, which initiated a flash flood in the Teesta River basin in North Sikkim. Regrettably, this catastrophe has left 98 individuals missing, including 22 army personnel, while impacting a total of 22,034 people, as reported by the Sikkim State Disaster Management Authority (SSDMA).
Over the years, monitoring the South Lhonak Lake area and glacier melt has been critical. This ongoing assessment, conducted from 1990 to 2020 at five-year intervals, has helped to observe the evolving trends in glacier and lake areas. Such data analysis through ArcGIS software allows researchers to estimate changes in the lake area and glacier retreat.
The findings indicate a clear pattern of glacier retreat and lake expansion over time, emphasizing the pressing need for continued research and intervention in the face of this evolving environmental challenge.
Â
