New Delhi: An article published by scientists from the National Remote Sensing Centre (NRSC) in a science journal, Current Science, in 2013 had flagged the ‘very high’ risk of a Glacial Lake Outburst Flood (GLOF) in the South Lhonak Lake in Sikkim Himalaya 10 years before the flash flood occurred in the state. More recently, a case study presented by the Sikkim State Disaster Management Authority at the Second Regional Conclave of SDMAs held in October 2022 had also sounded an alarm bell when it said about the Lhonak Lake: “this Lake is highly vulnerable for GLOF event” which will cause a “flash flood in downstream areas” and “important infrastructure like dams, power houses (will) likely be damaged.” The sudden discharge of large volumes of water and debris from glacial lakes is termed as GLOF.
A little after midnight on October 4, a sudden surge in water flow in the Teesta River washed away several bridges, parts of NH-10, the Chungthang dam and impacted villages towns and infrastructure projects in the upper reaches of the river valley. At least eight people are feared to have been killed in the flash floods, while 49 others, including 23 Army personnel, are missing, as per latest reports.
In a statement released late on Wednesday evening, the National Disaster Management Authority (NDMA) said, “While scientists are investigating the exact cause of the flash flood, the primary reason for the sudden surge appears to be a likely combination of excess rainfall and a GLOF event at South Lhonak lake in North Sikkim. The lake is at a height of 5,200 metres, with a towering ice-capped feature at about 6,800 metres to the north of and in close proximity to the lake.”
While noting that climate change in the 20th century had a significant impact on glaciers and glacial environments, leading to the enlargement of glacial lakes in many mountain ranges, the NRSC report had said that the South Lhonak Lake had expanded by 463 percent between 1977 and 2008. “The change detection study revealed that the glacier retreated 1.9 km during the 46-year period of 1962–2008. The 2008 satellite imagery shows that the lake is still attached to the snout, but is expanding laterally and increasing in areal extent and bounded by moraines and hence characterised as moraine-dammed lake. By considering the probability model of McKillop and Clague22 and incorporating the predictor parameters, the model yields a very high outburst probability of 42 percent (<6 percent being very low; 6–12 percent being low; 12–18 percent being medium; 18–24 percent being high and > 24 percent being very high). The very high outburst probability shows that, if the lake increases its extent in due course of time, it may cause a dangerous outburst flood,” the research article published in Volume 104, No 3, had said. According to the estimates prepared by the researchers then, a flash flood would discharge 586 m3 water per second.
In the presentation shared by the Sikkim State Disaster Management Authority at the regional conclave of SDMAs in October 2022, the authority had flagged that a flash flood due to a GLOF in the South Lhonak Lake would impact Chungthang , Dikchu, Singtam, Rangpo and hydropower projects downstream like, Teesta III Chungthang Dam, Teesta V NHPC Dikchu Dam, Teesta VI NHPC Singtam Dam and Teesta Low Dam III NHPC Rimbik. “GLOF at South Lhonak Lake can cause huge devastation downstream,” it had said.
The mitigation measures recommended by the state authority included siphoning of lake water as the most viable option and the installation of sensors to monitor water level. According to the presentation, the Sikkim SDMA had installed three pipelines of 130-140 metres for siphoning which had siphoned 150 litres of water per second.
However, action on these recommendations obviously seems to have been slow as the statement released by the NDMA on Wednesday said that the authority plans to “install early warning systems for real-time alerts at most of 56 at-risk glacial lakes in India.”
