How high temperatures and a glacial surge caused the Thorthormi flood

Thorthomi Lake stabilized as of now but risk of GLOF still high

According to the team of experts from the National Center for Hydrology and Meteorology (NCHM) who went to Lunana to do a rapid assessment of the Thorthomi Lake, the excessive melting of Thorthomi glacier due to the increasing temperature from April to June 2019 has not only increased the water level of the lake, but has also resulted in glacier surge from Thorthomi Lake on 20 June.

Glacier surges are events where a glacier can advance substantially, moving at velocities up to 100 times faster than normal glacier.

As a result of glacier surge, the excessive water that was accumulated in the main Thorthomi Lake was drained into two subsidiary lakes of main Thorthomi Lake below it. It was found that there was an increasing pressure on the outlet of the lake from the excessive flow of water, resulting in constant erosion at the outlet that eventually led to the emptying of the lake.

This explains why the water level of the lake first increased drastically from 6.55 meters to 8.04 meters, a rise of 1.49 meters, within approximately 25 minutes and then dropped back to normal and further decreasing below 6.55 meters causing the breach on the 20 June.

Rapid Assessment by NCHM

According to NCHM officials, abnormal temperature was recorded in Bhutan in May and June 2019. Although the temperature has been increasing from 12 degree Celsius in 2011, however, the temperature rose significantly in May and June 2019 to more than 14 degree Celsius, which shows the temperature of Bhutan rose by about 2 degree Celsius over a period of 9 years.

“The first and the most obvious reason – being climate change, other causes could be heat waves happening currently in India, Pakistan and other Southeast Asian countries which could have contributed for rise in the temperature in the country over recent times,” an NCHM official said.

In addition to this, he said, “Bhutan usually receives monsoon at around 5 June to 10 June but our monsoon got delayed which also contributed to the rise in the temperature.”

On 20 June 2019, because of the breaching of the subsidiary lake, many movement or changes happened in the main Thorthomi Lake and subsidiary lake II, but nothing much happened in subsidiary lake I.

Because of the rise in the temperature in the last few days prior to 20 June incident, starting from May till 20 June, excessive melting has taken place from the Thorthomi glacier.

Excessive water from the main lake drained into the subsidiary lake I and then into subsidiary lake II, thereby eroding the outlet or channel of the subsidiary lake II. From the aerial picture that was taken by the NCHM expert team who went to Lunana, the outlet of the subsidiary lake II has been eroded by almost 9 meters and the lake is completely emptied.

The total volume of water drained out from subsidiary lake II on 20 June was 18,160 cubic meters and the height of the water drained out was 3.1 meters and since the volume was less, the impact was not felt.

According to the survey carried out by Sangay Tenzin from NCHM on the water level of two subsidiary lakes, the difference in water level of two subsidiary lakes is 6.1 meters.

Due to the 20 June event, many fresh movement of mass or landslide occurring towards Thorthomi has been recorded. The earlier studies also predicted that if Thorthomi and Rapstreng lakes combine due to failing of Moraine Wall, it would be catastrophic and this area is considered critical.

A glaciologist from NCHM, Karma Toeb, said, “A small glacier called Lugge II drains with Thorthomi Lake and when lots of water came into Thorthomi Lake on the evening of 20 June, we thought that this might be one of the sources where excessive water came in, but upon observation no disturbances was observed which means no new melt water from Lugge lake occurred.”

He said, “If we look at the total volume of water drained out from the main Thorthomi Lake, it comes around 2.73 billion cubic meters and the water level was lowered by 0 .8 meters on 23 June, but we should not be alarmed by this because this didn’t happen in a day, it took about three days for this much water to be drained out.”

This, according to him, is indeed good for us because in 2012 during the mitigation work which took as long as 4 years, the water level of Thorthomi Lake was lowered by 5 meters draining out about 17 million cubic meters of water and within three days about 2.7 billion of water was drained out naturally.

“And we don’t consider this as GLOF because GLOF is something that happens spontaneously or suddenly and finish off in a short span of time,” he said.

Current Situation

Karma Toeb said that glacier movement has subsided and Thorthomi Lake has stabilized as of now, but it is not risk or hazard free because other factors, like seismic activities is always a threat. Therefore, risk of GLOF from Thorthomi Lake is still high.

He said, “We cannot avoid this situation, but what we can do is adapt. And for us early warning system is the best. We also recommend enhancing of existing early warning system,” and further adding, “At present, our early warning system can only detect when main events happens. When we say enhancing, we mean in the sense that if our early warning system could forecast.” NCHM is planning to insert more sensors at Lunana.

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