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Background The Hindu Kush Himalayan (HKH) region being seismically active and sensitive to climate change is prone to glacial lake outburst flood (GLOF). The Lemthang Tsho GLOF breached in the evening of 28 July 2015 innorth-western Bhutan is reminds of the looming threat, and stresses the need to have good risk management plan. The need to understand the physical processes in generating GLOF to is therefore imperative in order to effectively manage the associated risk. The paper therefore assesses the cause and impact of the Lemthang Tsho GLOF event using field and remote sensing data. Results The collapse of near vertical wall of supraglacial lake triggered by 2 days of incessant rainfall, opened up the englacial conduit resulting in emptying of interconnected supraglacial lakes into Lemthang Tsho. The5.1 magnitude earthquake epicentered 187 km to southeast in the Indian state of Assam in the morning (7:10 am Bhutan Standard Time) of the same day is unlikely to have played any role in triggering the event. The estimated volume of water unleased is 0.37 million m3, with peak discharge estimated to be ranging from 1253 to 1562 m3/s, and velocity of 7.14–7.57 m/s. The impact was minimal and confined up to 30 km downstream from the lake. The flood took lives of 4 horses, washed away 4 timber cantilever bridges, 148 pieces of timber, damaged 1 acre of land, and washed away 1 km of trail. The team also monitored 3 out of 25 identified critical glacial lakes and downgraded the risk of all 3 critical glacial lakes based on the finding. This brings the number of critical glacial lakes in Bhutan to 22. Conclusion The threat of GLOF still looms large in the Himalaya, particularly in view of impact of climate change and frequent seismic activities. There is a need for good risk management practices which starts fromidentification of critical glacial lakes, to prioritize in-depth investigation. As the present list of critical glacial lakes are largely based on inventory done over a decade based on topographic maps some of which datedback to 1960s, we need to revisit the critical glacial lakes and assess the risk considering recent changes. The new assessment needs to consider supraglacial lakes as one of the criteria in evaluating the GLOF risk, as highlighted by the Lemthang Tsho GLOF.

Glacial Lake Outburst Flood (GLOF) is an inherent threat in countries in the Hindu Kush Himalayan (HKH) region and is projected to increase in the face of climate change. One of the recent GLOF events in the region was the outburst of Lemthang Tsho (lake) in Bhutan on 28 July 2015. This paper discusses the cause and impact of the event based on the analysis of multi-temporal satellite images and field observation. The event, which was small to medium in volume, was triggered by the emptying of two supraglacial ponds located upstream of Lemthang Tsho. The area had experienced heavy rainfall till the morning of 28 July. On the same day, at 7:10 am local Bhutan time, a 5.1 magnitude earthquake struck about 177 km southeast of the lake. In the absence of firsthand information from the field, it cannot be confirmed that these two events triggered the lake outburst that took place around 3 pm. However, the possibility cannot be ruled out, either. The rainfall and earthquake are likely to have had a role in triggering the outburst.