Yangyel Lhaden

Following a government directive, the National Centre for Hydrology and Meteorology (NCHM) is exploring measures to bring down the water level at Thorthomi glacial lake in Lunana.

This decision is prompted by the potential threat of a glacial lake outburst flood (GLOF) from Thorthomi, which is considered the most dangerous among Bhutan’s 17 potentially hazardous glacial lakes.

Experts often refer to Thorthomi as a ‘ticking time bomb’ due to its elevated risk levels for potential outbursts.

The NHCM is exploring the siphon method to reduce the lake’s water level and reduce the risk of a potential GLOF. This technique uses gravity to move liquid from a higher elevation – the lake – to a lower elevation – the outlet, through pipes, operating on the same principle as drawing fuel from a vehicle using pipes.

The Chief of the Cryosphere Services Division, Karma, said that Thorthomi lake is rapidly expanding due to melting amid increasing temperature. “The situation is critical due to climate change. If the current climatic conditions persist and the moraine dam collapses, the water pressure from Thorthomi could surge into Rapstreng, potentially triggering a massive GLOF,” he said.

Thorthomi is now a fully formed glacial lake, with only a few chunks of ice floating.

The lake is separated from Rapstreng by a moraine dam. The size of the narrowest crest of the moraine dam has decreased from about 45 metres in 2008 to about 33 metres in 2019.

In a worst-case scenario, if the moraine dam collapses, Thorthomi lake would pour into Rapstreng Lake, triggering a combined GLOF with an estimated flood volume of 53 million cubic meters of water, according to an NCHM study.

The most recent GLOF scare from Thorthomi occurred on October 30 last year, when increased water levels led to an outburst that flooded Pho chhu. Fortunately, the water levels subsided by the following day.

A similar incident occurred in June 2019, when Thorthomi’s subsidiary lake II was completely drained.

During both GLOF events at Thorthomi, NCHM’s rapid assessment team identified increased temperature as one of the key factors contributing to the GLOFs.

In 2019, temperature data from the Thanza Automatic Weather Station (AWS) revealed that April, May, and June were the warmest months compared to the previous eight years.  A similar trend was observed in July, September, and October 2023. In both GLOF events, the AWS was completely damaged.

Karma said that the siphon method is the most cost-effective and practical option to reduce water level to mitigate GLOF compared to deploying people to physically draw out water, as was done in earlier mitigation measures. “We could use pumps as well but we would need thousands of them as Thorthomi glacier is huge, which would be expensive.”

Currently, Thorthormi glacial lake measures 4.33 kilometres (km) in length and 1.3 km in width, with a surface area of 4.33 square km – equivalent to 800 football fields.

However, the siphon method is only a temporary solution since it does not lower the water volume from the outlet. “This process needs to be continuous to draw out water from the lake as the glacier lake will be fed with water from other sources but we can maintain the water level as we can do siphon continuously as the lake does not freeze in winter now,” Karma said.

The first mitigation project to artificially lower the water level of Thorthomi glacial lake was carried out between 2009 and 2012, involving over 300 people who physically drained the water from the lake. This project successfully reduced the water level by five metres. Water level reduction in Rapstreng lake was carried out much earlier, between 1996 and 1998.

No machinery was used during the earlier mitigation projects to avert the risk of triggering an artificial GLOF due to potential disturbance. That time, much of Thorthomi was still covered in ice.

“Now that the ice has melted, other options could be considered,” Karma said. “While the earlier mitigation approach did effectively lower the water level, it was challenging and not always practical to have personnel working under such harsh conditions.”

Meanwhile, two communities of Thangza and Toenchoe chiwogs in Lunana, located below Thorthomi lake, have relocated to safer locations in Damthangkha, Bayza-Gangjuk, and Dungbiteng, a few kilometers above their chiwogs, due to GLOF fears.