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