Explained | Is Sikkim flash flood not caused by cloud burst but glacial lake outburst?

The flash floods triggered by glacial lake burst has left a trail of destruction in India’s northeastern Sikkim state. More than 25,000 people are affected while 14 people are dead, with at least 100 people, including 22 army men, still missing.

The Lhonak Lake in North Sikkim — a glacial lake situated in the state’s northwest at 17,000 feet — burst, unleashing a flood of water that eventually rose the water levels of Teesta River to alarming levels.

The rise of water levels in river flooded at least four districts, including Mangan, Gangtok, on Wednesday (October 4), the Sikkim State Disaster Management Authority (SSDMA) said.

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“The lake outburst in portions of Lhonak Lake in North Sikkim caused a rise in water levels with very high velocities near about 15m/sec, crossed theCWC(Centre Water Commission) Melli site measuring 227 m, near about 3m above Danger Level, at 0600 am,” it added.

The lake outburst also resulted in the breach of the Chungthang dam, which is the largest hydropower project in Sikkim.

The National Remote Sensing Centre, one of the ISRO’s arms which conducted a satellite-based study on the lake outburst, said, “It is observed that Lake is Burst and about 105 hectares area has been drained out (28 September 2023 image).”

The National Disaster Management Authority believes that the possible cause of the flash flood in Sikkim could be a combination of excess rainfall and a Glacial Lake Outburst Flood (GLOF) event at South Lhonak lake in North Sikkim.

Though an official statement from the Sikkim government is awaited over the cause of the lake outburst, the latest episode has turned the attention towards the GLOF incident in Uttarakhand’s Kedarnath in 2013 where a sudden burst of floods killed thousands of people. The GLOF was caused by the Chorabari Tal glacial lake.

What is GLOF?

Glacial Lake Outburst Flood (GLOF) is a type of catastrophic flood that happens when the dam containing a glacial lake fails, releasing a large volume of water.

This type of flood istypically caused by rapid melting of glaciers or the buildup of waterin the lake due to heavy rainfall or the inflow of meltwater.

As the volume of the glacial lake grows, so does the pressure on the dam containing it. Failure of the dam can also occur due to internal processes, such as water seeping through the structure.

Occurrences of glacial lake outburst floods (GLOFs) have become increasingly frequent and caused a lot of havoc for the people living in the Himalayan foothills.

The formation of these lakes, which vary in size from 5 to 10 hectares, has been primarily attributed to global warming, climate change and local factors.

Was Sikkim flooda result of GLOF?

Sikkim, which lies on the foothills of the Himalayas, is house to an estimated 300 glacial lakes.

One of those lakes is the South Lhonak. Indian Express newspaper, which cited a report by the Sikkim Forest and Environment Department, stated that the glacier lake has been under the observation of the officials for the past five decades.

A report published by the Sikkim Forest and Environment Department found that the lake’s area had significantly increased in the past five decades, the newspaper reported.

Did the state govt take any action?

The Indian Express reported that the Sikkim State Disaster Management Authority and Sikkim’s Department of Science and Technology and Climate Change, among others, immediately took note and tried to siphon off the lake water.

The authorities installed three eight-inch wide and 130-140 metres long High Density Polyethylene (HDPE) pipes in the lake to take out the water. They siphoned off 150 litres of water per sec, the Sikkim SDMA said.

While it is difficult to pinpoint the Sikkim flash floods to the GLOF incident, the state government has maintained that it was caused by cloud burst.

What happened in Uttarakhand’s Kedarnath?

In the early hours of June 17, 2013, a flash flood came bursting through the overflowing banks of the Chorabari lake in Uttarakhand.

The floods happened due to an unusual amount of rainfall June 13 and 17, which resulted in the melting of the Chorabari glacier, and the Mandakini river erupted due to excess water.

The floods rocked large parts of Uttarakhand, Himachal Pradesh and Western Nepal.

According to figures provided by the Government of Uttarakhand, more than 5,700 people were "presumed dead." The death toll was later revised to 6,054. Most of the dead were pilgrims.

The heavy rainfall caused massive flash floods and landslides resulting in the death of residents and tourists as well as extensive damage to property. Reportedly the worst hit was the Kedarnath valley popular for the 8th century temple dedicated to Lord Shiva.

(With inputs from agencies)

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