Glacial Lake Outburst Flood Risk in Bhutan

A glacial lake outburst flood (GLOF) is the catastrophic release of water from a moraine-dammed or ice-dammed lake when its containing structure fails. Bhutan's territory contains around seven hundred glacial lakes in the high Himalayan north, of which a much smaller subset is classified as potentially dangerous. GLOFs have caused significant loss of life and damage to downstream infrastructure in Bhutan, most notably in 1994, and the country has built one of the more developed GLOF-mitigation and early-warning programmes in the Himalayan region.

GLOF risk is among the most prominent climate-change-related hazards in Bhutanese policy and is a recurrent feature of the country's National Adaptation Programme of Action and its Nationally Determined Contributions to the UN Framework Convention on Climate Change.

The 1994 Lugge Tsho GLOF

The most consequential historical GLOF in Bhutan occurred on 7 October 1994, when Lugge Tsho, a moraine-dammed lake in the Lunana region of Gasa Dzongkhag, partially burst. Approximately 18 million cubic metres of water were released, surging down the Pho Chhu valley and reaching Punakha, around 100 kilometres downstream, in just under five hours. Peak flow at the Wangdue Phodrang hydrological station was estimated at 1,800-2,500 cubic metres per second.^[1]

The flood killed at least 21 people, damaged around 90 houses, and seriously affected Punakha Dzong, the second-oldest dzong in Bhutan and at the time the dual-language seat of the Zhung Dratshang. Walls of the dzong were undermined and parts of the perimeter were destroyed; reconstruction work continued for several years afterwards. Agricultural land along the Mo and Pho Chhu valleys was inundated.^[2]

Inventory of dangerous lakes

Glacial lake inventories of Bhutan have been compiled by the International Centre for Integrated Mountain Development (ICIMOD) and by the National Center for Hydrology and Meteorology (NCHM) of Bhutan. Successive inventories have given different totals depending on classification thresholds:

• The 2001 ICIMOD inventory identified 24 lakes as potentially dangerous, later expanded to 25 in subsequent updates.
• The NCHM 2021 reassessment, applying tighter risk-based criteria, classified 17 lakes as potentially dangerous out of more than 500 surveyed.

The most-watched lakes lie in Lunana in the upper Pho Chhu basin, where Thorthormi Tsho, Raphstreng Tsho, Lugge Tsho and Bechung Tsho sit in close proximity. Modelling commissioned by the Department of Geology and Mines and ICIMOD suggests that an outburst from Thorthormi Tsho or a multi-lake cascade could put more than 16,000 people, two major hydropower projects and substantial agricultural land at risk along a 150-kilometre stretch downstream.^[3]

The Thorthormi Tsho mitigation project

From 2008 to 2012 the Royal Government, with finance from the Global Environment Facility's Least Developed Countries Fund and implementation support from UNDP Bhutan, undertook a manual lake-lowering programme at Thorthormi Tsho. Bhutanese labourers from the surrounding gewogs spent four summer field seasons cutting an outflow channel through the moraine wall and lowering the lake water level by approximately five metres. The work was conducted at altitudes above 4,400 metres, without heavy machinery, and is widely cited as a benchmark example of community-based GLOF risk reduction in the Himalaya.^[4]

Even after lake-lowering, Thorthormi remains classified as highly dangerous. Continued monitoring and a follow-on project funded by the Green Climate Fund have extended adaptation work to additional vulnerable basins.^[5]

Early warning and downstream infrastructure

Since 2002 the Department of Geology and Mines and later the NCHM have maintained manned and automated monitoring stations on the Pho and Kuri Chhu basins. Personnel stationed at Thanza, where the outflows of the four major Lunana lakes converge, relay water-level readings to monitoring stations in Wangdue Phodrang, Dobani and Thimphu several times daily. An automated GLOF early-warning system on the Pho and Mo Chhu basins, installed in the 2010s, is designed to give downstream communities and the operators of the Punatsangchhu-I and Punatsangchhu-II hydropower projects a window of one to several hours before peak flow arrives.^[4]

GLOF risk has also been a factor in hydropower siting decisions, including the geological problems at the Punatsangchhu-I dam site that have delayed commissioning since 2018; an outburst flood is among the design scenarios that engineers must protect against.

Climate-change context

Bhutan's glaciers are retreating in line with the broader Hindu Kush-Himalayan trend documented by ICIMOD's Hindu Kush Himalaya Assessment. Maurer and colleagues (2020), using historical satellite imagery, estimated that ice loss across the Himalaya doubled between 2000 and 2016 compared with the previous fifteen years. The expansion of pro-glacial lakes in the Bhutanese Himalaya is one of the most visible local manifestations of this trend, and GLOF risk-management is correspondingly central to Bhutan's climate-adaptation framework.^[6]

References

1. The 1994 Lugge Tsho Glacial Lake Outburst Flood, Bhutan Himalaya — ResearchGate
2. The Palace of Great Happiness and a GLOF — Through the Sandglass
3. Reassessment of Potentially Dangerous Glacial Lakes in Bhutan — NCHM (PDF)
4. Adapting to the Himalayan meltdown — Global Environment Facility
5. Protecting Bhutan's most vulnerable from GLOF risks — UN Glaciers Year
6. Maurer et al., "Acceleration of ice loss across the Himalayas," Science Advances 2020 (PDF)
7. Bhutan Glacial Lake Inventory 2021 — NCHM (PDF)