Why the Sikkim flash flood was a catastrophe waiting to happen

A cloudburst over the South Lhonak Lake in northwest Sikkim on Wednesday, October 4, caused a sudden gush of water in the Lhonak River, which in turn triggered a massive flood in the Teesta valley, resulting in the disappearance of 23 Indian Army personnel.

The flash flood wreaked havoc, not only in the next on the course, Lachen Valley, but also in areas in its lower course in North Bengal. So far, 3 bodies have been fished out of the Teesta, and the flash flood also resulted in landslides in the adjoining areas.

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According to DownToEarth, the same Lohnak Lake, which has two depressions, namely, north and south, is one of the fast-growing lakes out of 14 potentially dangerous lakes vulnerable to Glacial Lake Outburst Floods (GLOFs) in the Sikkim Himalayan region.

The impending danger has, for some time now, been on the watch of experts. It was a ticking time bomb.

Lhonak Lake and the threats

• In Sikkim's northwest, close to the India-China border, the glacial-moraine-dammed South Lhonak Lake is situated at an altitude of 5,200 metres above sea level, on the windward side of the Eastern Himalayas.

As the name suggests, a glacial moraine-dammed lake forms when glacial moraine (debris) deposits create a natural dam, blocking water flow into a valley. Water collects behind the natural dam, resulting in a distinct, small ribbon-shaped lake. These lakes are common in glacier-heavy regions at high altitudes.

• The lake area, which in 2008 was recorded to be around 1.2 square kilometres, grew steadily, and the lake's surface area expanded by almost 0.8 square kilometres since it was first imaged in 1962 by the CIA's CORONA program.
• The CIA reconnaissance program also revealed the lake was born as a supraglacial lake, which means a waterbody, in this case, a lake floating upon a glacier.
• These lakes are said to be vulnerable to fractures, as the ice beneath might not support the mass of the waterbody it has been carrying.

• The lake's formation was the result of continuous meltdown of the South Lhonak glacier.
• Hence, the lake's rapid expansion became a ticking time bomb, warned by glaciologists from the region, time and again.

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Race against nature

• Recognising the adverse potentials of climate change-induced glacial lake melting in the Northern Sikkim, a region densely dotted with glacial lakes, disaster managers and scientists in Sikkim initiated an experiment, back in 2016.
• The goal was clear: to siphon off excess water from the South Lhonak Lake, to defuse the looming disaster and to study the future implications.

• Using High-Density Polyethene (HDPE) pipes, each with an eight-inch diameter, around 50 litres of water per second was discharged, according to the India Science Wire.
• A following expedition in 2018 noted that a similar amount of water was also flowing out of the North Lhonak Lake.

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The factors aggravating cloudburst disasters like these in high altitudes include:

• features of these lakes,
• climate change,
• seismic activities,
• rugged topography,
• geological features,
• altitude, 
• unexpected natural calamities.

They, in turn, wreak havoc in lower-lying areas, which themselves are caught up with obstructed and squeezed river flow and non-scientific urbanisation.

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