NEW DELHI: Parts of West Bengal and Assam were placed on high alert after a glacial lake outburst in Bhutan triggered a flash flood, raising fears of a sudden rise in water levels along the Sankosh river. More than 10,000 people have been moved to relief shelters and safer locations in West Bengal, while authorities in Assam's Dhubri district have stepped up preparedness to deal with possible downstream effects.

The outburst sent large volumes of water downstream through the Mo Chhu river towards the Sankosh river system, prompting authorities to activate disaster response mechanisms in vulnerable areas.

The flood washed away infrastructure at the Gasa Tshachu natural hot springs in Bhutan's Gasa district. No injuries or loss of life have been reported, state broadcaster Bhutan Broadcasting Service (BBS) said on Friday.

The full extent of the damage from the late-night flood remained unclear. BBS said the assessment was hampered by “darkness and the strong flow” of floodwaters. Two Bailey bridges, constructed using portable, prefabricated sections, may also have been washed away.

According to Bhutan's national newspaper Kuensel, parts of the road leading to Laya village, where the hot springs are located, were washed away, making the route impassable.

The road has been closed to traffic indefinitely. The area was evacuated before the floodwaters reached the hot springs.

West Bengal shifts thousands to safety

Authorities in West Bengal have alerted residents living along the Sankosh river and its tributaries following reports of the glacial lake outburst flood in Bhutan.

Residents in the Darjeeling hills, Terai and Dooars regions have also been advised to remain alert and move to safer locations if they live in vulnerable areas.

BJP MP from Darjeeling Raju Bista issued an alert through a post on X on Thursday, urging people living in the catchment areas of the Sankosh river and its connected tributaries to move to safer locations.

"A glacial lake outburst flood (GLOF) has been reported at Lemthang in Bhutan. The overflowing water from the lake is flowing downwards through the Mofu River towards the Sunkosh River," the post read.

"Therefore, everyone living in the catchment areas of the Sunkosh River and all its connected tributaries have been informed to move to a safe distance immediately to protect their lives. Everyone in our Darjeeling hills, Terai and Dooars region are requested to stay alert," he added.

State minister Nisith Pramanik said the government had received reports of the glacial lake outburst and that the overflowing water was moving towards the Sankosh river through the Mo Chhu river.

"Reports have just come in regarding a glacial lake outburst in Bhutan. Water from the lake is overflowing and flowing downstream into the Mo Chhu River, heading toward the Sankosh River. Residents living along the Sankosh River and its tributaries have been alerted; they are advised to move to a safe distance immediately to ensure their safety," Pramanik said.

He said the water was expected to reach the plains within five to six hours, raising concerns about possible flooding.

"To manage the situation, the State Disaster Management Team has been activated, and the NDRF has also been notified. Disaster management volunteers have been instructed to be present on-site. The government is taking proactive measures and closely monitoring the situation to prevent any loss of life or property," he added.

Alipurduar Superintendent of Police Amit Kumar Shaw appealed to residents not to panic, saying the administration was coordinating with Bhutanese authorities and disaster response agencies.

"First of all, I would like to tell the residents of the district that there is absolutely no need to panic. The situation is currently under control. We are in constant touch with the authorities in Bhutan. Our police team, the District Magistrate (DM) and the Block Development Officer (BDO) with their respective teams, along with all relevant agencies and stakeholders, are working together in close coordination," Shaw told ANI.

He said the State Disaster Response Force (SDRF) had been deployed in the area, while a National Disaster Response Force (NDRF) team was also on its way. Civil Defence personnel had additionally been mobilised to assist with preparedness measures.

"We have already moved approximately 4,000 to 5,000 people into shelters, utilising school buildings as relief centres. Additionally, more than 6,000 people have been relocated to safe areas," he said.

Shaw said the intensity of the water flow had decreased slightly, according to the latest information received, but the administration remained on high alert.

"The police, the DM's office, and the BDO's office have made arrangements for dry food and drinking water. We have also ensured that other necessities, such as toilet facilities, are provided. We will wait to see what the situation looks like in two hours. We are fully prepared and ready to handle any emergency that may arise," he said.

He added that a red alert was in force for the Sankosh river system and warned that the approaching floodwaters could carry dead fish and debris, including large pieces of wood and concrete.

"We expect to see water flowing through the Sankosh River, likely accompanied by dead fish, as well as large pieces of wood and concrete debris," Shaw said.

Assam monitors possible impact in Dhubri

Meanwhile, the Assam government said it was closely monitoring the situation following the glacial lake outburst in Bhutan, with Dhubri district placed on alert over possible downstream effects.

The Sankosh river flows through Dhubri district, and authorities have stepped up vigilance amid the possibility of a sudden rise in water levels.

"The situation arising out of the Glacial Lake Outburst Flood in Bhutan is being closely monitored by HCM Dr @himantabiswa," the Chief Minister's Office said in a social media post.

"Team @dcofficedhubri is on ground and prepared to effectively deal with the downstream effects," it added.

An Assam State Disaster Management Authority spokesperson said the water level of the Sankosh river was steady as of 6.30 am on Friday. The district administration and disaster response teams were on alert to tackle any eventuality.

A special advisory issued by the ministry of Jal Shakti on Thursday for Assam and West Bengal said the Indian Embassy had reported a GLOF in Shintaphu Lake, located in Bhutan's Punatsangchhu river basin.

Following reports of flash floods in the river's upper reaches, water levels in the Sankosh river could rise suddenly in Assam's Dhubri and West Bengal's Alipurduar districts, the advisory said.

Local authorities in both districts were advised to remain alert, closely monitor the situation and take necessary precautionary measures.

What is a glacial lake outburst flood?

A glacial lake outburst flood (GLOF), also known as a jokulhlaup, occurs when the natural dam holding back a glacial lake fails or is breached, releasing large volumes of water downstream.

Such floods can cause sudden destruction of roads, bridges, settlements and other infrastructure, particularly in mountainous regions.

Shintaphu Lake, located below Wakila Mountain in Lungo, Laya Gewog, in Bhutan's Gasa district, is among 17 glacial lakes in the country identified as potentially dangerous, according to local reports.

Gasa Tshachu, a popular natural hot spring in northern Bhutan, was also hit by a flash flood in 2021 when the Mochhu river changed course and swept through the area.

The latest incident has renewed concerns over glacial lake outburst floods in the Himalayan region. In August, a flash flood near Nepal's border with China killed more than 1,500 people and left over 5,000 missing, according to the supplied report.