English | Dzongkha Sunday, February 23, 2020

GLOF threat from Lunana looms large

Sherub Dorji, Lunana
Sep 28, 2018

Tenchoe village in Lunana with the Table Mountain in the backdrop

Lunana, one of Bhutan’s northern-most regions, is pristine. It remains largely untouched by humans. But amidst its hypnotic beauty lurks danger. The danger of glacial lake outbursts flood (GLOF). And the threat is looming large.

Lunana houses four major lakes-Thorthomi, Raphstreng, Luggye and Baytsho.

Thorthomi and Raphstreng are two of the 25 glacial lakes in Bhutan that poses serious GLOF risks. Bhutan has close to 3000 glacial lakes altogether.

Thorthormi is not a full lake yet. It is in its forming stage. Small ponds of melted glaciers are slowly converging into a large lake.

Just below Thorthormi, divided by a large moraine wall, lies the Raphstreng Lake.

Raphstreng Lake with the unstable moraine dam on the right

The moraine wall, which divides Thorthormi and Raphstreng, has a huge ice body at 80 meters depth. Scientists have raised concerns over the structural strength and stability of the moraine dam.

The landmass is said to be shrinking by the year. Recent satellite images show the unsteady moraine dam is receding at an alarming 30 to 35 meters every year.

Thorthormi Lake in its forming stage

Officials at the National Centre for Hydrology and Meteorology (NCHM) say the moraine ridge will not hold forever and that Thorthormi will spill over into Raphstreng Lake one day.

What the convergence of the two lakes means is it could trigger a major GLOF.

“If worst case scenario happens, that is, Thorthormi Lake overtopping into Raphstreng and then creating a GLOF, the flood volume is estimated to be around 53 million cubic meters,” Karma, a Specialist with NCHM’s Cryosphere Service Division, says.

The last time Bhutan experienced a glacial lake outburst flood was in October 1994 when a partial burst of the Luggye Lake wreaked havoc downstream along the Punatsangchhu basin.

Should the imminent convergence of Thorthormi and Raphstreng happen, Punakha will experience a more severe flood, nearly three times more severe than the 1994 GLOF.

“The combination of Thorthomi and Raphstreng is the worst-case scenario from Lunana and the flood volume of about 53 million cubic metres that is likely to be caused is almost three times more in magnitude from the 1994 GLOF. And I am talking just about the pure water.”

Several efforts have been undertaken to mitigate GLOF risks from Lunana. A three-year project to lower the Raphstreng lake water level by four meters was completed in 1998.

A similar project was carried from 2008 and 2011 at the Thorthormi that saw 17 million cubic meters of water drained out from the lake.

Early warning sirens have been set up and safety evacuation zones identified along the Punatshangchu and Pho Chhu basin.

Tshering Dorji, a technician with the Flood Warning System in Lunana, is a key part GLOF risk mitigation efforts.

Every day, he undertakes a three-hour journey to one of the four lakes in Lunana to inspect the water level. It’s undoubtedly a tough job, but Tshering doesn’t mind. It’s part of his service to the country.

“It is difficult to work here. It’s high up in the mountains. The route to the lakes itself poses risks to our life. But this is my job, my responsibility, so I have to do it,” he said.

While GLOF mitigation and risk reduction works are continuing, NCHM officials say it’s important to stay prepared for the worst.

“Especially the glacier lake outburst flood, you never know when it will happen and people tend to forget. They tend to neglect,” Sangay Tenzin, the Chief of the NCHM’s Hydrology & Water Resources Services Division, said.

“So, what I wanted to say is the GLOF is real, it’s going to happen and nobody can say when but the only thing we know is it’s going to happen. It’s not like nothing happened in the past five years and it won’t happen. It will happen and people have to keep that in mind.”

Traces of the 1994 GLOF can still be seen in Lunana

The World Wildlife Fund (WWF) says if worst comes to worst, half of the fertile land in Punakha and Wangdue valley will be submerged under water. Punakha Dzong and much of Khuruthang will be washed away.

Punatsangchhu hydropower projects will be in grave danger and biodiversity in the region would be affected.

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