Cause and Impact: The 2015 Lemthang Tsho GLOF in Bhutan

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Location of Lemthang Tsho GLOF and other glacial lakes at the adjoining valley.
Source: google map.

A scientific paper on the July 2015 glacial lake outburst flood (GLOF) in Lemthang Tsho in Bhutan recommends regular monitoring of critical glacial lakes as a starting point for good risk management. The paper specifically recommends revisiting critical glacial lakes and assessing risk while considering recent changes. The need to consider supraglacial lakes has been identified as a criteria for evaluating GLOF risk. 

The report is based on findings of a joint field assessment carried out by experts from the International Centre for Integrated Mountain (ICIMOD) and organizations under the Government of Bhutan. The field expedition was led by the Department of Hydro-Met Services (DHMS) in Bhutan (now the National Center for Hydrology and Meteorology) whose researchers visited the site in the aftermath of the GLOF. 

Experts from ICIMOD on request from NCHM joined the assessment team to scientifically document the cause, process, evolution, and effects of the lake outburst, and assess potential GLOF hazards from glacial lakes in the adjoining valley. 

The resultant paper, based on the findings of the assessment, has been published in Geoenvironmental Disasters (https://geoenvironmental-disasters.springeropen.com). It assesses the cause and impact of the Lemthang Tsho GLOF event using field and remote sensing data. 

The collapse of a near vertical wall of the supraglacial Lemthang Tsho Lake caused the GLOF. The collapse was triggered by two days of incessant rainfall, which opened up an englacial conduit. This resulted in the emptying of interconnected supraglacial lakes into Lemthang Tsho. Compared to previous events, the 2015 GLOF is significantly small, with the volume of water unleased estimated to be 0.37 million m3, peak discharge estimated within the 1,253 to 1,562 m3/s range, and velocity of travel estimated somewhere between 7.14 to 7.57 m/s. Its impact was minimal and confined to 30 km downstream from the lake. The flood took the lives of four horses, washed away four timber cantilever bridges, 148 pieces of timber, damaged one acre of land, and washed away one kilometre of trail. The incident was witnessed by a yak herder, and the communities downstream were alerted on time.  

Along with Lemthang Tsho, the research team monitored two other critical glacial lakes: Latshokarp and Langdo Latshokarp, both located in a valley adjacent to Lemthang Tsho. Considering the size of these lakes, their moraine stability, their surrounding geomorphology, and the glacier sources that feeds them, the research team found that the lakes do not pose immediate GLOF risks and hence downgraded the risk level. This brings the number of critical glacial lakes in Bhutan down to 22 from 25 previously identified critical glacial lakes.