Thorthormi Lake: the danger is far from over

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Although the risk has been reduced for now, the potential threat remains

Despite investing US$ 4mn to reduce the water level of Bhutan’s most dangerous glacial lake, Thorthormi, with mitigation works spanning over five years (2008-12), the lake still remains a threat with the potential of bursting its banks causing Glacial Lake Outburst Flood (GLOF). 

The efforts to lower the lake’s water level have not totally eliminated the risk of a disaster, according to the National Center for Hydrology and Metrology (NCHM).

“The risk of GLOF can be reduced to zero but will come at a huge cost,” said Karma, a Specialist in Glaciology with NCHM.

Besides, lowering Thorthormi’s water level may not be enough to reduce the risk, as GLOF is not caused by glacier retreat alone. Other external factors such as seismic activity can also trigger GLOFs. Reducing the risk to zero would also affect the natural water reservoir.

When the mitigation work was carried out from 2008 to 2012, more than 300 laborers were employed with no heavy machines. All works were done manually with the help of tools such as spade, pixel and crowbars among others.

“Using heavy machineries would mean inviting another danger like creating artificial GLOF by destabilizing the moraine walls surrounding the Lakes,” said Karma.

Budget constraint continues to be the major hurdle followed by mobilization of human resources.

“Workers have to work on a difficult terrain in freezing cold waters and most of the time in knee-deep water. The rations had to be transported to the glacial site and finding logistic was the hiccup for the mission and challenging too,” said Karma.

There were risks to the lives of the people and a few lives were lost in the past due to the harsh climatic conditions at high altitude, according to Karma.

During the last phase of the project, due to lack of human resources back in 2012, 126 army personnel were mobilized.

“The mitigation work in Thorthormi Lake is a fight against Mother Nature and we do not want to disturb more as it might retaliate now or in future,” said Karma.

However, as of now with the outlet clear, it is unlikely that the water level of Thorthormi Lake would reach the danger level, which would also prevent build up of hydrostatic pressure exerted on the moraine walls surrounding the lake.

Higher hydrostatic pressure resulting from increasing water level in the lake will result in failure of the moraine dam surrounding the lake giving way to GLOF.

“If there is any blockage, we will do the study and take up the issue with the government for further intervention,” he said.

The Center has prepared hazard zonation maps for GLOF in downstream region in three sub basins of Punatsangchhu, Mangdechhu and Chamkharchhu to study the impact of flood, if it occurs.

Safe evacuation zones in these areas have been also identified in an event of an unexpected flood. In addition the Center has also installed early warning system to warn the people in case of GLOF in the river basins.

Two personnel from the Center are stationed in Lunana to monitor the lake and warn people living downstream in case the early warning system fails. “They report on the situation on the lake on a daily basis. They work round the clock,” said Karma.

Thorthormi Lake is one of the largest glacial lakes in Bhutan, with a size of 3.42 sq.km and with the depth of approximately 130 meters. The lake is situated at a height of more than 4,400 meters.

There are 25 glacial lakes identified as potentially dangerous in Bhutan.

Chencho Dema from Thimphu