2023 Sikkim flash floods

2023 Sikkim flash floods

Date	4 - 5 October 2023
Location	Sikkim districts Mangan, Gangtok, Pakyong, and Namchi West Bengal districts Cooch Behar, Jalpaiguri, Darjeeling, and Kalimpong
Cause	Outburst of the South Lhonak lake due to heavy rainfall
Deaths	At least 40
Property damage	15 bridges, one hydroelectric dam destroyed

On 4 October 2023, heavy rains caused the glacial South Lhonak lake in Sikkim, a state in northeastern India, to breach its banks, causing a glacial lake outburst flood. The flood reached the Teesta III Dam at Chungthang at midnight, before its gates could be opened, destroying the dam in minutes. Water levels downstream in the River Teesta rose by up to twenty feet, causing widespread damage.

Course of events

Background

The South Lhonak Lake is a moraine-dammed lake fed by the meltwater of the Lhonak glacier. It was first seen in CORONA satellite images from 1962 as a supraglacial lake. Landsat MSS images show that it became a separate lake by 1977, with a surface area of 17 hectares. In four decades, as the glacier retreated 1.9km, the lake swelled in size, covering nearly a hundred hectares by 2008. It was identified as potentially being at risk of causing glacial outburst floods, and in 2018 pipelines were carried up by yak and installed to pump water out of it.

Sentinel-1A images from 28 September 2023 showed the lake covering an area of 167.4 hectares.

The flood

In early October 2023, a cloudburst caused Sikkim to receive more than double its normal rainfall; between 3 and 4 October alone, the state received five times the usual precipitation. The South Lhonak burst its shores, causing a flash flood. Satellite images from the Indian Space Research Organisation's RISAT-1A show that the lake's surface area shrunk by more than a hundred hectares. Based on a warning from the Indo-Tibetan Border Police at midnight, the operators of the Teesta III Dam at Chungthang rushed to open the dam's gates, but were too late; the flood quickly destroyed the dam, as well as the bridge to its 1200MW hydroelectric powerhouse, which went underwater.

Water levels downstream in the River Teesta subsequently rose by fifteen to twenty feet, inundating many areas in Mangan, Gangtok, Pakyong, and Namchi districts in Sikkim, and Kalimpong, Cooch Behar, Jalpaiguri and Darjeeling districts in West Bengal. The flood also went onwards to Bangladesh, where it affected hundreds of villages along the Teesta River and Char areas.

Fifteen bridges across the state were washed away, and the north of the state, including the capital Gangtok, was cut off from the rest of India as parts of National Highway 10 collapsed. 3,000 tourists were stranded across the state.

Relief operations

The government of Sikkim declared the flood a disaster, and the Indian central government released ₹48 crore ($5.76 million) in disaster relief funds. The National Disaster Response Force and the Indian Army are involved in the ongoing relief operations. 2,400 people were evacuated from flood-hit areas, and 7,600 others were in relief camps. 10,000 more were evacuated in West Bengal.

Fourteen people were believed to be trapped inside the tunnels of the destroyed Teesta III Dam; a 60-strong team of the NDRF - including scuba divers - was assembled to rescue them.

Casualties

At least 40 people were killed and 22 injured, while 75 were reported missing as of 6 October. Of the missing were 23 Indian Army personnel, of whom seven were subsequently found dead and one rescued alive. One of the dead soldiers was found floating in the Teesta in Nilphamari, Bangladesh; the Border Guard Bangladesh handed over the body to the Indian officials through a flag meeting. One child was killed and six injured in the neighbouring state of West Bengal when a mortar shell carried downstream from Sikkim exploded after being picked up by locals.