“Very High Probability…”: Sikkim Flash Flood Warnings in 2013, 2021

New Delhi:

The South Lhonak Lake in Sikkim that burst its boundaries early Wednesday – killing 14 people and destroying a hydroelectric project worth thousands of crores – displayed a “very high probability of sudden outburst” that could trigger flash floods downstream, scientists warned back in 2013.

In fact, there were warnings even as far back as 2001; the Sikkim Human Development Report spoke of the “catastrophic” impact of glacial lake outburst floods, or GLOF, and highlighted the state’s Onglokthang, Rathong Chu and Zemu glaciers, which had even then retreated by several kilometres.

A GLOF occurs when lakes formed by melting glaciers suddenly burst open due to various reasons, including excessive water accumulation (from cloudbursts) or triggers like earthquakes.

As it bursts, it releases an enormous volume of water all at once, causing flash floods downstream.

Back to the (relative) present, the Sikkim government’s Department of Science and Technology issued a warning in 2016 and, in 2021, a study published in the journal Geomorphology red-flagged “accelerated growth” of glacial lakes in the state and, in particular, the South Lhonak glacier.

Retreating Lhonak Glaciers

The South Lhonak glacier receded 1.9 – 2 km between 1962 and 2008 and 400m more in the following 11 years, the scientists behind the 2013 and 2021 studies said. What this basically means is the glacier melted – due to climate change and global warming – and increased the amount of water in the lake.

READ | ISRO’s Satellite Images Show How Sikkim’s Lhonak Lake Burst, Caused Floods

Situated 17,100 feet above sea level, the South Lhonak Lake was also fed by glacial run-off from the retreating North Lhonak glacier and the main Lhonak glacier. This increased the lake’s surface area by 500 metres and average depth by 50 metres, Dr SN Remya, the lead scientist of the 2013 paper, said.

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“We predicted (in 2013) the probability of the South Lhonak glacier breaking is 42 per cent. Regarding the lake burst in Sikkim, we predicted it would result in the release of up to 19 million cubic meters of water. We had said this lake is in danger… recommended early warning systems.”

How Much Water In South Lhonak Lake?

Satellite images released Wednesday by space agency ISRO showed the lake held nearly 170 hectares of water less than a week before a cloudburst broke, so to speak, tipped the scales.

On Wednesday morning – hours after the lake burst – there are only 60.3 hectares of water.

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Dr Remya’s study had predicted a peak discharge of nearly 600 cubic metres of water per second.

The 2021 study predicted a peak discharge seven times worse – around 4,300 cubic metres per second for a breach depth of 20 metres – and a terrifying 12,500 cubic metres for 50 metres.

“In Sikkim, lake-terminating glaciers have shown accelerated growth… South Lhonak glacier is no different. It is one of the fastest-retreating glaciers and the associated proglacial South Lhonak Lake has become the largest and fastest-growing in the state… this has raised concerns about hazard potentials as downstream (areas) are heavily populated…” the study highlighted.

Flash Floods Wreak Havoc In Sikkim

The resulting flash flood not only washed the Chungthang power station, killed 14 people and left (as of late Thursday night) over a 100 missing, and destroyed key infrastructure in the remote state.

READ |Parts Of Teesta River Dam Washed Away After Cloudburst In Sikkim

Reports indicate the Sikkim government did take some preventive measures, including siphoning some of the excess water from the South Lhonak Lake after an expedition in 2016. And, just last month, government officials and scientists planned to install early warning systems at the lake.

READ |How Sikkim Flash Flood Happened: Explained In Graphics

Before they could, though, came that fateful cloudburst – part of unseasonal rainfall that saw northern Sikkim (where the lake is) get 50 per cent more rain than normal.

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Sikkim has 300 glacial lakes in high altitudes areas, of which at least 10 are believed to be vulnerable to GLOF events. Early warning systems were supposed to be in place at some of these lakes.

The question now is – were these installed (wasn’t at South Lhonak Lake) and are they working?

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