Gangtok: 10 days after a Glacial Lake Outburst Flood from South Lhonak Lake ravaged the entire Teesta basin in Sikkim and North Bengal, the Sikkim Government has clarified the chain of events of October 3.
Addressing a press conference on October 13, Sikkim government’s Science and Technology Department’s Principal Director Dhiren G. Shrestha, who has led multiple expeditions to South Lhonak Lake since 2016, shared, “The size of Lhonak Lake was 168 hectares, with 96.11 million cubic metres of water. 35 million cubic metres of water has been discharged, which is almost a 30% decrease in water level. There is now a 24-metre drop in the water level of the lake.”
When asked what caused the lake to discharge so much water, he said, “On the North West side of the lake, there was an ice wall or moraine measuring 600 metres long, 225 feet high, 150 feet wide which fell into the lake. Apart from it, a 50-metre by 600-metre ice block also fell into the lake. The lake burst because of the pressure from these two collapses. Rainfall did happen that night, the altitude is so high that a cloudburst could have happened. For the wall to collapse, something has to have triggered. But even the IMD has countered that it was not a cloudburst.”
The Sikkim government official admitted of further threat looming large in South Lhonak Lake. Shrestha stated, “On October 10, when there was a warning issued of an increase in water level along Teesta, we learned of an additional wall collapsed near the previous ice wall collapse. But the threat upon the lake still looms large.”
However, the Science and Technology Department’s Principal Director denied GLOF as the reason for the devastation across the Teesta Basin. He contended, “The moraine that was protecting the lake, had that burst it would have been GLOF. But the moraines are almost half a kilometer long. The images show that the moraine is still intact. However, due to the fresh discharge, the natural outlet of the lake, which was smaller earlier, has grown 10-15 metres in area. The natural outlet of the lake has also increased and gone back by 257 feet. This was known after comparing the October 3 and October 4 satellite images. ISRO has been requested to send data, we are analyzing, we are consulting with scientists in Bangalore,” shared Shrestha.
On the fragility of the moraines now in South Lhonak Lake, the state government official admitted, “The moraines of the North side of Lake are still fragile, it will continue to fall. Had the moraine fallen on the outlet side, the water would not have flushed as the pressure would have gone back towards the lake. But when the collapse happened in the deepest part of the lake, which is estimated to be almost 400 feet deep, the wall collapsed, triggering the water level to increase and flow downstream.”
On the lack of an early warning system in Sikkim, Shrestha shared, “There was no early warning system installed in Sikkim when the South Lhonak Lake disaster happened. There was a need to map where the early warning systems would be laid. We invited Swiss scientists involved in similar efforts in their country, to survey locations for early warning systems to be laid. The experts wanted to conduct a field visit. Under the guidance of NDMA from September 9-19, the Swiss team surveyed the locations for EWS. The expedition was for Sakho Chu and South Lhonak Lake, with two teams going to one lake each. There were 4 Swiss experts, who laid Automatic Weather System with cameras. The cameras and weather system were functioning initially, however, the cameras from South Lhonak Lake stopped emitting the data. The Automatic Weather System worked for 2-3 days, only the one in Lhonak Lake stopped giving information. But the one in Sakho Lake is still functioning.”
The expedition in September was supported by NDMA, along with Disaster Protection Officers from Mountain states like Arunachal Pradesh, and Uttarakhand, along with NCPOR Goa, GSI Faridabad, and National Remote Sensing Hyderabad officials. “We had debriefed on September 20 to the Sikkim Government, two days later NDMA was briefed in Delhi about the status of the 2 lakes,” shared the state official.
Sikkim Government admitted that siphoning of water from the lake could not have stopped South Lhonak Lake from bursting. Shrestha shared, “There were also incidents of pipe damages in 2018 with additional pipes also being laid. If the water was released more, dead ice would be exposed which could again melt, so the natural dam of dead ice would also melt the ice and open the lake further. With the outlet increasing after the disaster, naturally, the flow of water will increase.”
In 2012, the Centre for Development of Advanced Computing from Pune collaborated with the Sikkim Science and Technology Department to develop a Glacial Lake Information System. The first visit to South Lhonak took place the same year. In 2013, a scientist based with National Remote Sensing in Hyderabad disclosed to the media and also published a journal stating that South Lhonak Lake has increased and poses a threat. The same was published in Sikkim local media. “Our scientists had already reached the site by then, to avoid a panic situation over minimal research until then, we concluded there was no immediate threat,” said Shrestha.
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The State Science and Technology Department wrote their national counterparts and formed a working group of Snow and Avalanche Study Establishment (SASE), the Geological Survey of India, the Indian Army, CDAC, and an All-India level expert committee group. They suggested then, “No doubt, the lake has expanded, needing a bathymetric survey to estimate the volume of Lake. For the Moraine that dams the lake, we wanted to conduct an electric residue survey of it, which was suggested. In 2014, we conducted the two surveys,” said Shreshtha.
The moraine comprises dead ice which from the surface looks like rocks. “The dead ice was strengthening the moraine. The dead ice is as deep as 8 meters in some locations. Following the survey, the committee suggested two solutions, one was short term which was siphoning the water to decrease the volume of the lake. The lake does have an outlet too. The long-term solution would be widening the lake or constructing a check dam,” said the Sikkim government.
Apart from South Lhonak Lake, which was expanding continuously, Sakho Lake, another lake under threat of GLOF, doesn’t have an outlet, but the growth is less. “We made the two lakes a priority by giving our data to NDMA, thereafter they laid the Glacial Lake Outburst Flood Management Plan in 2020. NDMA proposed to lay an early warning system in 50 vulnerable lakes across the country. The first pilot project would be in Sakho Chu (lake) and South Lhonak Lake, it was decided,” said Shrestha.
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