The South Lhonak lake in the Himalayan state of Sikkim breached on the night time of October 3, ensuing in a glacial lake outburst flood (GLOF) that ravaged 4 districts. At least 42 individuals died as a consequence and 77 extra are nonetheless lacking. More than a month after the catastrophe, the lake stays a potential hazard, say scientists.
The South Lhonak lake is situated in North Sikkim, at an altitude of 5,200 m. According to scientists, the present interpretation of the flood – which depends closely on satellite tv for pc knowledge – means that on October 3 night time, a slope failure occurred alongside the lateral moraine (a mass of particles and rocks) on the lake’s left financial institution. Preliminary analysis discovered that some of the landslide materials scraped the glacier terminus; most of it hit the lake, creating an impulse wave that moved in the direction of the lake’s slim entrance finish.
It widened the lake’s outlet, ensuing in a partial breach, mentioned Ashim Sattar, a mountain hazard specialist who’s at present finding out the GLOF and has authored analysis articles predicting completely different GLOF eventualities for the South Lhonak lake. There was additionally a possible overtopping as water moved over the moraine alongside the lake’s outlet. The consequence was a GLOF that affected round 88,400 individuals in Sikkim’s Mangan, Gangtok, Pakyong, and Namchi districts.
The lake is one of the largest and quickest rising glacial lakes in Sikkim, and has been a potential hazard for a number of years now. Dan Shugar, a geomorphologist from the University of Calgary in Canada, who’s at present conducting analysis on the GLOF, famous that earlier than the flood, the lake’s space was 1.62 sq. km, and after water being drained out in the GLOF, the space dropped to 1.46 sq. km.
Dr. Sattar mentioned, “While the lake level was lowered due to the flood, it did not drain completely. A lot of water still remains in the lake, making it a potential hazard.”
Adding to the current hazard, weeks after the catastrophe, there have been steady landslides alongside the slope that first failed on October 3, Dr. Shugar added.
When the flood gushed via the valley, it triggered a landslide about 30 km downstream of the South Lhonak lake, which blocked the river, forming a landslide-dammed lake. After reviewing satellite tv for pc pictures, Dr. Sattar surmised that the lake had not drained even by November 1.
Considering the threat of sudden launch of water downstream, it requires monitoring.
What brought about the GLOF?
The Sikkim authorities has been claiming that the GLOF was triggered by a cloudburst. In reality, Sikkim State Disaster Management Authority’s every day reviews on deaths and damages proceed to name the flood “cloudburst induced”. The National Disaster Management Authority’s October 4 press launch additionally known as the occasion a “likely combination of excess rainfall and a GLOF”.
However, based on G.N. Raha, who heads the Meteorological CentreinGangtok, the chance of both a cloudburst or excessive rainfall triggering the GLOF may be very low. The 10 climate stations throughout North Sikkim recorded light-to-moderate rainfall for the interval of the GLOF, Dr. Raha mentioned.
On the morning of October 4, the station at Lachen, virtually 46 km from the South Lhonak lake, recorded solely 6.7 mm rainfall for twenty-four hours, he mentioned. However, scientists proceed to analyse meteorological knowledge for correct data concerning the position of rainfall in the GLOF.
Researchers are additionally at present checking whether or not earthquake tremors might have triggered the GLOF.
How did the catastrophe occur?
The catastrophe downstream of the lake was not brought about solely by the GLOF, however by a cascade of a number of occasions.
One of the most affected locations was Chungthang, a city round 62 km from the lake, the place the dam of Sikkim’s greatest hydropower mission – the 1,200-MW Teesta-III – broke. Water from the broken reservoir mixed with the GLOF, resulting in much more destruction downstream of the dam.
While the reservoir’s gross storage capability was 5.08 million cubic metres, the quantity of water saved in it at the time of the catastrophe is at present not identified. Further downstream of the Teesta-III dam, two extra hydropower tasks have been broken: the 510-MW Teesta-V and the 500-MW Teesta-VI that’s underneath building.
“In the Teesta Basin, where GLOFs are evident, placing mega-dams was a bad decision,” mentioned Mayalmit Lepcha, an activist related to the Affected Citizens of Teesta (ACT), a collective of Sikkim’s residents protesting towards dangerous dams on the Teesta river for greater than a decade and a half. The lack of early warning additionally proved to be detrimental, she mentioned.
Maximillian Van Wyk de Vries, an assistant professor of bodily geography at the University of Nottingham, U.Ok., mentioned that at the South Lhonak lake, a giant part of the lateral moraine was “moving for many years preceding the collapse, at a rate of several metres per year”.
Prior evaluation of a attainable landslide, which will be inferred utilizing satellite tv for pc imagery, would have guided glacial lake administration efforts and infrastructure planning in the Teesta Basin, he mentioned.
On October 13, the Indian Space Research Organisation’s (ISRO) National Remote Sensing Centre printed satellite tv for pc pictures exhibiting a giant deposit of sediments and a number of other landslides alongside the path of the flood, particularly in and round the Chungthang dam. The unfastened sediment could pose a risk to downstream areas in future, mentioned Kalachand Sain, Director of the Dehradun-based Wadia Institute of Himalayan Geology.
In view of the many hydropower-related dams in the Teesta Basin, the great amount of unfastened sediment that may simply be eroded by heavy water movement could suggest a shorter lifespan for dams and decrease effectivity for hydropower tasks, scientists mentioned.
What is the position of local weather change?
In the South Lhonak glacier, the indicators of local weather change emerged many years in the past and have become stronger as the price at which the glacier melted elevated, ensuing in a quickly rising lake that was sure to breach – as a number of analysis research acknowledged. In 1990, the South Lhonak glacier was 6.4 km lengthy. A analysis article printed in 2021 discovered that, by 2019, it had lowered by about 1.3 km and that its space had declined by about 0.96 sq. km.
In 1976, the South Lhonak lake space was a mere 0.20 sq. km, based on a analysis article printed in 2018. As the glacier shrank, the lake grew bigger. By 2019, the lake was protecting 1.35 sq. km, per the 2021 article.
South Lhonak lake has been quickly rising in measurement
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Research articles
GLOFs are pure, however the fast enhance in the lake’s measurement as a consequence of the glacier’s accelerated melting tied carefully to anthropogenic local weather warming, mentioned Dr. VWDV (as he prefers his final identify) of the University of Nottingham.
Anil V. Kulkarni, Distinguished Scientist at the Bengaluru-based Indian Institute of Science’s Divecha Centre for Climate Change, mentioned, “The lake’s rapid expansion might have weakened the permafrost that was holding the moraines, which would have been one of the important reasons behind slope failure on October 3.”
Kavita Upadhyay is an impartial journalist and researcher who writes on disasters in the Indian Himalayan Region.
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