At least seven dead as glacial lake bursts in Sikkim

Subject :Geography

Section: Physical geography

Context:

The flash floods are believed to have been triggered after a lake, formed from the gradual melting of a Himalayan glacier, suddenly overflowed and inundated the Teesta river basin.

Details:

Cause of flash flood: The South Lhonak lake in northern Sikkim burst and led to the collapse of Chungthang dam.
Areas impacted: Mangan, Gangtok, Pakyong, and Namchi.
The Himalayas are estimated to have about 7,500 lakes, of which nearly 10% are in Sikkim.
The Teesta 3 hydropower project is operated by Sikkim Urja, which is 60% owned by the Sikkim government.
The Lhonak lake overflow is reminiscent of the 2021 deluge that resulted from an avalanche involving the Nanda Devi glacier in the Himalayas that flooded the Rishiganga river in Uttarakhand and washed away at least two hydroelectric power projects — the 13.2 MW Rishiganga hydroelectric power project, and the Tapovan project on the Dhauliganga river, a tributary of the Alakananda.

Lhonak lake:

South Lhonak Lake is a glacial-moraine-dammed lake, located in Sikkim’s far northwestern region.
It is one of the fastest expanding lakes in the Sikkim Himalaya region, and one of the 14 potentially dangerous lakes susceptible to Glacial lake outburst flood (GLOFs).

Scientists’ warnings:

Scientists at the Indian Institute of Science (IISc) in Bengaluru and the Indian Space Research Organisation (ISRO), through multiple studies in 2013 and 2019, have warned that the South Lhonak lake was “potentially hazardous” and cautioned the authorities to keep an eye on it as it was expanding “dangerously.”
Lhonak has grown nearly 1.5 times and South Lhonak nearly 2.5 times their initial size in 1989.
An earthquake of magnitude 4.9 on Sept 21, 1991 (as reported by United States Geological Survey) near the parent glacier feeding the South Lhonak Lake and the recent earthquake (magnitude 6.9) of Sept 18, 2011 approximately 70 km from the lakes and future earthquakes may trigger the GLOF events.

Glocal lake outburst flow (GLOF):

A glacial lake outburst flood (GLOF) is a type of outburst flood caused by the failure of a dam containing a glacial lake. An event similar to a GLOF, where a body of water contained by a glacier melts or overflows the glacier, is called a jökulhlaup.
The dam can consist of glacier ice or a terminal moraine. Failure can happen due to erosion, a buildup of water pressure, an avalanche of rock or heavy snow, an earthquake or cryoseism, volcanic eruptions under the ice, or massive displacement of water in a glacial lake when a large portion of an adjacent glacier collapses into it.
Example: GLOF at Chorabari Tal glacial lake that led to kedarnath flooding in 2013.

Causes of GLOF:

Increasing glacial melting because of climate change, alongside other environmental effects of climate change (i.e permafrost melting) mean that regions with glaciers are likely to see increased flooding risks from GLOFs. This is especially true in the Himalayas where geologies are more active.
GLOF can be triggered by several reasons, including earthquakes, extremely heavy rains and ice avalanches.

Regions vulnerable to GLOF:

A 2023 study found 15 million people at risk from this hazard, mostly in China, India, Pakistan, and Peru.
Glacial lake outburst floods (GLOF), as such events are called, are a major concern in Bhutan, Tibet, India, Nepal, and Pakistan.
The IISc study had recommended measures like early warning systems and mitigation measures to be put in place in potential GLOF areas.

Teesta river:

Teesta River is a 414 km (257 mi) long river that rises in the Pauhunri Mountain of eastern Himalayas, flows through the Indian states of Sikkim and West Bengal and subsequently enters Bangladesh through Rangpur division.
In Bangladesh, it falls into Brahmaputra River which after meeting some other major rivers of Bengal delta finally falls into the Bay of Bengal.
In India, it flows through Mangan District, Gangtok District, Pakyong District, Kalimpong district, Darjeeling District, Jalpaiguri District, Cooch Behar districts and the cities of Rangpo, Jalpaiguri and Mekhliganj.
It joins the Brahmaputra River at Phulchhari Upazila in Bangladesh.
305 km (190 mi) of the river lies in India and 109 km (68 mi) in Bangladesh.
Teesta is the largest river of Sikkim and second largest river of West Bengal after the Ganges.