What 190 years of glacial lake outburst flood data tells us about the Hindu Kush Himalayas

Pulaha Roy, Down To Earth, 15 December 2023: An analysis of the International Centre for Integrated Mountain Development’s (ICIMOD) latest data on glacial lake outburst floods (GLOF) since 1833 has suggested that over 70 per cent of the 700 GLOF outbreaks have happened in the past 50 years. 

The year 1980 saw the highest number of GLOF events, 15, followed by 2015 (13). The years 1973, 1974, 2002 and 2010 all experienced 10 GLOF events. 

But, according to ICIMOD, “the overall increase in GLOFs is mainly due to better observations and reporting, but the overall damage to infrastructure from these events has likely increased in the recent past”. 

A GLOF outburst, according to ICIMOD, “is sudden release of water from a lake fed by glacier melt that has formed at the side, in front, within, beneath, or on the surface of a glacier”. 

The report, Glacial lake outburst floods in High Mountain Asia documented in regional effort, also quantified the number of lives lost — over 7,000 have died due to GLOF outbursts since 1833 across the Hindu Kush Himalayan region. 

Over half of the GLOF outbursts (54 per cent) were triggered due to mass movements like avalanches, rock falls and landslides while another 18 per cent were due to extreme rainfall events, it highlighted. 

“This points to a need for better forecasting of precipitation at altitude as well as mapping of an increasingly unstable cryosphere,” the report said.

Cryosphere, according to the National Oceanic and Atmospheric Administration, is the frozen water or ice water of the earth system.

GLOFs also tend to have a domino impact. According to the analysis, while eight GLOF events occurred in China, their impacts were felt in neighbouring countries like Nepal. 

The report also cautioned how “without a major step up in both monitoring, mitigation and early warning for all, it is likely we will see more major examples of GLOFs such as South Lhonak.”

According to ICIMOD, the GLOF outburst in the South Lhonak lake in Sikkim on the intervening night of October 3-4, 2023 was responsible for 102 deaths.

Earlier, Down to Earth reported how researchers had earlier warned about the outburst in the South Lhonak lake. 

According to the researchers, glacial retreat over the years has led to significant growth in the area around the South Lhonak lake. This increase had led to heightened chances of a GLOF in the region.

Sikkim: Threat of South Lhonak lake bursting was forewarned by researchers two years ago

Twenty-one other glacial lakes in the Teesta river basin show potential threat Himanshu Nitnaware

Himanshu Nitnaware, 04 October 2023:  Scientists had warned in 2021 that the South Lhonak lake in Sikkim would burst. The disaster finally took place on the intervening night of October 3-4, 2023.

The detailed study, Future Glacial Lake Outburst Flood (GLOF) hazard of the South Lhonak Lake, Sikkim Himalaya, highlighted that South Lhonak Lake had witnessed a significant increase in lake growth in the past decades due to glacial retreat, thereby increasing its chances of glacial lake outburst floods (GLOF).

The lake, at a height of 5,200 meters above sea level, became the largest and fastest-growing glacial lake in the state, with a glacial retreat of almost two kilometres in 46 years between 1962 and 2008. Scientists recorded another 400-metre retreat of the glacier between 2008 and 2019, increasing the hazard potential.

The length of the glacier has reduced from 6.4 km to 5.1 km in 29 years. Overall, the glacier shrunk by about 0.96 sq km. The Central Water Commission of the central government had also initiated an advisory to evaluate the condition of the lake system.

According to the census of 2011, the Chungthang village downstream houses over 10,000 residents, who are now cut off from the world.

The study also noted that:

The width of the terminal moraine that dams the South Lhonak Lake is not uniform and is thinning towards the ends. Hummocky surface of the dam indicates that it likely contains buried ice and be subject to future degradation.

It further stated that the moraine’s stability would alter over time and compromise the structural integrity due to melting of buried ice and impact of waves that could lead to overtopping flows. Extreme meteorological conditions such as cloudbursts would result in quick overfilling of the lake, initiating progressive erosion.

Scientists also point out that climate change is causing an impact on Himalayan glaciers and glacial lakes. In a chapter — Global Climate Change and Melting of Himalayan Glaciers — published in the book, Melting Glaciers and Rising sea levels: Impacts and implications, researchers claim the glaciers to be retreating at 10 to 60 metres per year.

Another study — Remote sensing-based inventory of glacial lakes in Sikkim Himalaya: semi-automated approach using satellite data — published in 2012 states that the number of glacial lakes in the Sikkim Himalayas are increasing and prone to GLOFs.

A study done by the International Centre for Integrated Mountain Development (ICIMOD) in 2003 using satellite data observed that of the 266 glacial lakes located in the Sikkim Himalayas along the Teesta basin within an area of 20.20 sq km, 14 were potentially hazardous lakes.

The number of lakes increased to 320 in 2010. Of the 14 dangerous lakes, three showed evidence of past GLOFs. Another two lakes indicated reduction in area in 2010 which may be attributed to discharge or an outburst event.

Of the 14 dangerous lakes, 12 were identified to have shown an increase in area between 2003 and 2010. The reason for the change is climate change which caused glacial retreat.

A study, Glacial Lake Evolution (1962–2018) and Outburst Susceptibility of Gurudongmar Lake Complex in the Tista Basin, Sikkim Himalaya (India), published in Multidisciplinary Digital Publishing Institute highlights the potential risk of the Gurudongmar lake complex (GLC) causing an outburst.

Between 1962 and 2018, the overall lake area has increased by 74 sq km due to climate-driven factors. The study noted, “This outburst susceptibility assessment revealed that the entire complex consisting of three lakes has high risks of potential hazard in terms of outbursts or overflow.”

The researchers pointed out that by 2028, if global temperatures follow the same graph of increase in the Gurudongmar semi-arid region, the crevasses that have formed in the steep terrain near the icefall area of the GLC which comprises of three glacial lakes — GL-1, GL-2, GL-3 — will shrink.

The area will further shrink with accelerated melting and calving process, resulting in the creation of outward cryostatic pressure exerted by ice and inward hydrostatic pressure, which is exerted by water near the terminus of lake GL- 3.

Sikkim floods: Why wasn’t early warning system set up at glacial lake known to be highly vulnerable? Experts ask

South Lhonak Lake that burst was one of the fastest expanding lakes in the region & its dangers were not unknown 

Jayanta Basu, 04 October 2023: Within a month of a team of disaster management experts assessing its vulnerability, a glacial lake in northern Sikkim has burst on the intervening night of October 3-4, 2023. Coupled with extremely heavy rainfall, the outburst triggered a flash flood in Sikkim and swept away more than 20 army personnel as the water level in Teesta river rose alarmingly. 

Adjoining West Bengal and Bangladesh are also at risk of being flooded.

Weather experts claimed that the lake under scanner, South Lhonak Lake, has been extremely vulnerable for more than a decade, being one of the fastest expanding glacial lakes in the region.

The glacial lake collapse triggered a flash flood of water, mud, stones that damaged the Chungthang dam. This has created a flood scare in downstream Teesta in West Bengal and Bangladesh, specially with rain refusing to relent.   

The India Meteorological Department (IMD) predicted extremely high rainfall in Sikkim and West Bengal. “Exceptionally heavy rainfall has been recorded over Gangetic West Bengal: (with) a low pressure area lies over western parts of Gangetic West Bengal & adjoining Jharkhand.”

“Isolated extremely heavy rainfall is very likely over West Bengal and Sikkim and Assam and Meghalaya during next two days and exceptionally heavy rainfall over sub-Himalayan West Bengal and Sikkim on October 5,” stated the latest IMD communique.

Experts cry negligence

“As reported and noticed through midnight of October 3, 2023, a lake outburst in the portions of Lhonak Lake in North Sikkim caused rise of water levels at very high velocities near about 15 metres per second,” according to a Central Water Commission (CWC) update released October 4, morning.

The water level crossed the danger mark by about 3 metres at the CWC Melli FF site measuring 227 metres at 6am, it added. “Simultaneous Flood Forecasts for both India and Bangladesh made at 0600 hrs for 1400hrs.”

While Teesta was flowing below warning levels at CWC Domohani FF Station, according to CWC, with water level at 84.83 metres at 6am, it was expected to cross the Warning Level within six hours. 

Low-lying areas in the Teesta catchment such as Gazoldoba, Domohani, Mekhaliganj,Ghish, Bangladesh area may be affected,” stated director-general of IMD, Mrutyunjay Mohapatra.

There may be extremely heavy rainfall in Sikkim, adjoining northern West Bengal and also Bangladesh on Wednesday and Thursday, he told Down To Earth. 

Could have been 

At least 30 people, including 23 army personnel, have been reported missing and six bridges were washed away in northern Sikkim late on Tuesday night after the water level in Teesta river rose suddenly, possibly after the glacial lake burst, according to reports. National Highway 10, the lifeline of Sikkim, was badly damaged.

Statements from both the Indian army and Sikkim disaster management authority identified the cause to be a cloudburst.

“Due to a sudden cloudburst over Lhonak Lake in North Sikkim, a flash flood occurred in the Teesta river in Lachen valley. Some army establishments along the valley have been affected and efforts are on to confirm details. Release of water from the Chungthang dam led to a sudden increase in water level up to 15-20 feet high downstream,” a statement from the Indian army pointed out.

“This has led to Army vehicles parked at Bardang near Singtam getting affected. 23 personnel have been reported missing and some vehicles are reported submerged under the slush. Search operations are underway,” it added.

A spokesperson of Sikkim state disaster management also said that a cloudburst was possibly behind the sudden rise of Teesta’s water level on Tuesday night. The official said that soon after the information of sudden rise in water level of the Teesta was received, an alert was sounded and evacuation was started in low-lying areas along the river.

A senior IMD official, however, dismissed the cloudburst theory. “In case of a cloudburst you get 100 millimetres of rainfall within an hour. In this case, a maximum of 39 mm rainfall was recorded in 24 hours in one of the six station’s data that we have.”

Since then, Sikkim’s information and public relations department has also identified “the Glacial Lake outburst Flood in portions of Lhonak lake, Mangan district” as the cause.

A source pointed out that the South Lhonak lake has been vulnerable for more than a decade; and might have given away suddenly being aided by enhanced rainfall. Some locals have also reported a low-grade tremor, but could not confirm the claim, they added.

“South Lhonak lake was a ticking bomb for a long time. In the second week of September, a joint team of the national disaster management authority, Sikkim disaster management authority and some consultants visited it to assess the vulnerability of the lake,” added the official.

The lake expanded nearly five times in less than six decades, data showed.

“The disaster is partially human-made; why has the early warning system yet not been installed in this lake when even Wikipedia underlines its vulnerability,” questioned an environmentalist from North Bengal.

According to the local sources, while Teesta had swelled just before noon on Wednesday threatening to trigger floods in the stretches beyond Gajaldoba barrage in West Bengal where thousands live, the water level has gone down since noon.