The outburst flood created a wall of water that swept down the valley of the Teesta River, overwhelming the Teesta 111 HEP dam, which collapsed, releasing a catastrophic flood downstream, resulting in:

• 42 confirmed deaths and 150 missing individuals.
• Severe damage to settlements located along the valley.
• 2,400 people evacuated and 7,600 displaced in relief camps.
• 15 bridges washed away or submerged, along with dozens of roads, including the only highway connecting the state to the rest of India.

South Lhonak Lake is a moraine-dammed lake that sits over 5500 m above sea level. Just before the outburst flood, it measured 1.97 km in length with a maximum width of 0.45 km. The lake had been even larger, but emergency drainage works had partially reduced its volume.

It is fed by meltwater from the South Lhonak glacier, which has been retreating rapidly since the 1990s (over 2.25 km) as a result of global climate change.

• The lake drains into the Teesta River through a narrow channel cut into a natural moraine dam.
• The moraine is unconsolidated and therefore unstable. It is estimated that within the moraine, there are large bodies of ice which could potentially melt, further destabilizing the natural dam.
• The rapid retreat of the glacier has led to an equally rapid growth in the lake. Since 1990, the lake has almost doubled in size, from 0.45 km² it had to 0.76 km² by 2019.
• Just prior to the outburst flood, it measured 1.97 km in length with a maximum width of 0.45 km.
• The glacier feeding the lake is continuing to recede, adding to the volume of the lake.
• The retreat of the glacier has exposed a significantly over-deepened valley with very steep slopes and unstable lateral moraines, which are liable to collapse into the lake, particularly after heavy rain.

The most likely scenario suggested by researchers is a significant avalanche of ice and rock from the lateral moraine into the lake, which created a seiche (a standing wave in an enclosed or partially enclosed body of water) that overwhelmed the moraine dam. This probably led to rapid erosion of the exit channel from the lake and a sudden release of a large volume of water into the valley below.

An idea of how much water was released can be gained from the fact that area of the lake was reduced from 167 ha to just 60 ha. That equates to over 5 million cubic metres of water! According to eye witnesses, it generated a “sudden wave” of around 5 m in height, overwhelming settlements by the side of the river.

The diagram below, from a research article, clearly shows the area of the lake increased as the South Llonak Glacier retreated. It also shows how the retreat of the glacier is leading to over deepening. The dotted lines show how much the lake levels would rise above the current lake level under different avalanche scenarios.

A GLOF is a naturally occurring event which only became a disaster because of its impact on the communities living in the Teesta Valley. It was a result of a combination of factors.

Physical Factors:

1. The rapid growth of South Lhonak Lake and the unstable nature of both the valley sides and the moraine dam.
2. The narrow valley of the Teesta river, which would not allow flood water to spread out and, so, lose energy. Researchers estimate that flow depths during a GLOF could reach beyond 20 m, creating a significant flood hazard for local communities.
3. Continuous heavy rain over an extended period, which would have contributed to the waterlogging and destabilization of the lateral moraines above the lake.

Human Factors:

1. The building of the HEP station at Chungthang in the middle of the valley has made electricity more widely available and at the same time created employment for local people.
2. This has attracted increasing numbers of people to the area not just in the town itself but also in smaller settlements throughout the valley, many on slopes closer to the river valley, thus increasing the risk.
3. The increasing infrastructure in the region has resulted in the loss of vegetated river banks, reducing this natural buffer to flooding.
4. Poorly regulated expansion of housing, roads, tourism, and other related infrastructure has increased the area’s vulnerability to a GLOF.

Warnings go unheeded:

1. The potential for a GLOF originating from the South Lhonak lake has been well documented in research papers, which predicted 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.” (Sattar et al.)
2. As far back as 2014, such as in an article by Tom Clement for the Pulitzer Center, concerns of many observers were echoed that the construction of the dam at Chungthang left it vulnerable to a GLOF. “In Chungthang, a sudden influx of glacial debris is likely to compromise the flush gates and the discharge capability of the dam.”
3. There had been recent attempts to reduce the volume of the lake by siphoning off water via high-pressure pipes. However, as hindsight shows it was too little too late.

There are over 300 lakes in this region similar to South Lhonak Lake. As global temperatures continue to rise and the valley glaciers retreat, the potential for more GLOFs will also increase.

• Many of these lakes are dammed by unconsolidated glacial deposits forming unstable moraines, which are prone to collapse, especially if ice trapped within the moraines melts.
• As temperatures rise, there is an increasing possibility of precipitation falling as rain, which also increases the potential for avalanches to occur.
• The Himalayas are seismically active; another possible cause for destabilizing the moraine dams which could cause them to collapse.

The high valleys have enormous potential for the generation of HEP, and with it the potential for rapid urbanization. However, there is a clear need for:

• The design and construction of dams that can withstand sudden increases in water volume and debris flow so that they are not overwhelmed, as was the case at Chungthang.
• Detailed risk assessments and flood hazard mapping in valleys below potentially hazardous glacial lakes.
• Land use zoning to keep settlements away from hazardous areas.
• The development of early warning systems to allow people to evacuate in time.

• Geography Factsheet 470. Glacial Outburst Floods: The Hazard Explained
• YouTube: Devastating glacial lake outburst causes flash floods in Sikkim
• YouTube: Scientific American GLOF
• Guardian Report, October 5^th 2023
• A ‘Ticking Bomb’: Understanding the 2023 Glacial Lake Outburst Flood (GLOF) in Sikkim Himalaya
• Future Glacial Lake Outburst Flood (GLOF) hazard of the South Lhonak Lake
• Chungthang, Sikkim: A New Dam's Potential Impact