The breaching of subsidiary lake II of Thorthormi lake on June 20 helped drain out about 2.73 million cubic metres (mcm) of water from the main lake, avoiding a disaster.
Officials from the national centre for hydrology and meteorology (NCHM) say the breaching was in a way like the artificial mitigating measure conducted in 2009-2012, but through a natural process. Between 2009-2012, lake Throthormi’s level was lowered in what was called Operation Thorthomi to reduce risk of glacial lake outburst flood (GLOF).
Officials said the 2.73mcm of waters did not drain out at once, which is why there was not much impact downstream, but it happened in the course of three days and helped in lowering the water level in the lake by 0.80ms.
Experts, who went to Lunana to assess the lake, say consistent rise in temperature in the months of April, May and June this year led to the melting of ice in the main Thorthormi lake, which caused a glacial surge.
Glaciologist Karma Toeb, who has been to the lakes more than 22 times said the presence of water accelerated glacier basil sliding, causing the rise in water level in the main lake on June 20.
Showing the ice blocks displaced about a metre away up from the shore of the main lake and at the subsidiary lakes, Karma Toeb explained that the displacements occur only when there is a rise in water level.
“The rise in water level was caused by excess water and the source of the excess water was from the main lake itself.”
He said they initially suspected the rise in water in the main Thorthormi to have come from Lugge I glacier, but when they visited the sites, they found nothing happened at Lugge Tsho and the dams between the two lakes and water outlets were intact.
Karma Toeb said the temperature sensor in Thanza station showed that there was abnormal rise in temperature this year.
He said the hourly temperature record of the last seven years showed that the highest temperature was on June 14, 15 and 16, which was above 16 to 17 degree Celsius.
NCHM’s weather and climate division analysed the temperature trend from Thanza station and it showed that in the last nine years, the temperature of June this year was the highest.
“The monthly average temperature for April, May and June of last nine years shows that the three months this year recorded the highest temperature. The temperature recorded for the months this year is above normal,” Karma Toeb said. “Satellite images also show lots of melting on June 1, 3, 13 and 15. This correlates with the temperature trend we observed.”
What caused the breach?
Officials say the morphosis of Thorthormi lake is changing every day because of the rise in temperature and satellite images show lots of movements have taken place in Thorthormi lake and the displacement of glacier is more than 0.4 metres a day, which is considered high.
Karma Toeb said that because of the rise in temperature for the days prior to June 20, lots of melting took place inside Thorthormi lake and that meltwater within the glaciers caused basil sliding.”
He said that when basil sliding rate is high, it causes glacier surge. “When the surge occurred, it pushed the excess water, which overtopped and caused the new breach.”
He said the new breach is the weakest moraine. “When the excess water overflowed to subsidiary lake I, which had strong moraine, it spilled to subsidiary lake II, which breached and drained out completely.”
Karma Toeb said 18,160 cubic metres of water was drained out from subsidiary lake II on June 20, which is why there wasn’t much impact downstream.
NCHM’s chief for weather and climate service division, Singay Dorji, said they conducted a lot of study after the breach of the lake and found out it was more to do with the heat wave this year and delayed monsoon.
Karma Toeb said Thorthormi looks stabilised for now, but it would depend on factors such as weather parameters and other external triggers like seismic activity.
He, however, said glacial lake outburst flood (GLOF) hazards from lakes in Lunana like Thorthormi, Rapstreng and Lugge still remain high. “We are not concluding that Thorthormi is risk free.”
Restoration of EWS and AWLS
There are 18 sirens, 15 along the Phochhu, three along the Mochhu and 10 remote automatic water level stations (AWLS) to warn vulnerable communities along the river valley downstream.
A team of experts, NCHM officials in Lunana, teachers and caretaker of Lhedi Primary School, health assistant and livestock officer reinstalled the AWLS of subsidiary lake II to subsidiary lake I.
Karma Toeb explained that they initially planned to relocate the AWLS to the main lake, but since the main lake now has two outlets, they had to shift it to subsidiary lake I to capture all water outlet.
In Thanza, officials rectified the damaged terminal assemblage by replacing the sensor’s damaged bubble pipe.
NCHM officials say that besides the usual monitoring of the water level through sensors, staffs manually monitors the water level thrice a week on alternative days.
NCHM officials say there is no way they could reduce the risk of GLOF but people could adapt through the GLOF early warning systems (EWS).
Karma Toeb said the EWS should be enhanced. “Right now our EWS is for detection but it could be enhanced to forecast and install seismic sensors.”
He said the temperature monitoring system should be reality based.
NCHM’s director, Karma Dupchu, said present EWS were installed in 2011 and it needs replacement. “There are also more hazards increasing by building infrastructure along the hazard zones.”