GLOF: Investigation team has found that Memari Tsho, also known as Lemthang Tsho, that burst on June 28 was triggered by the sudden drainage of supraglacial ponds above the lake.
Supraglacial pond is one atop a glacier. Two such ponds had interconnected.
Director of Department of Hydro-Met Services (DHMS), Karma Tshering, said: “Impact of outburst of supraglacial ponds were visible in erosion and remnants found at the lake.”
Outburst of supraglacial lakes increased the discharge of lake outlet from 3pm. He added that erosion increased at the end moraine channel and that the stability of moraine dam was depended on the boulders at the channel.
The big boulders of the moraine channel had shifted with the increasing discharge, causing the channel to erode with the displacement of the big boulders that resulted in the glacial lake outburst flood. The gushing water widened the lake’s outlet to an opening of 30 metres wide.
The river discharge at end of July was 38.76 centimetres. The river had swollen to 250 centimetres during the flood.
Karma Tshering said that the flood did not occur at one go. “Eyewitnesses said that it took several hours for the lake to outburst after the supraglacial ponds burst.”
The incident was reported from Lemthang Tsho on 28 June 2015 around 5pm.
A team comprising of 10 officials from DHMS, DGM, Department of Disaster Management, Gasa Dzongkhag and scientists from International Centre for Integrated Mountain Development (ICIMOD) conducted scientific investigation on its causes, process, evolution and effects from July 22 until August 7.
The expert team ruled out the possibility of the 5.5 magnitude earthquake in Assam as the cause that led to the GLOF. Locals reported rain that week but there were no records to validate it.
Karma Tshering said that reports from US Geological Survey show that the lakes experienced no major vibrations to cause the flood.
The flood washed away four cantilever bridges, four horses, affected an acre of land and caused three major landslides downstream, damaging the horse trails between Gasa and Laya.
“As the river keeps cutting at the toe of the slides, the tracks can be blocked from repeated landslides for sometime,” Karma Tshering said.
The team also assessed potential GLOF hazards from glacial lakes in the adjoining valley.
Four lakes downgraded
Bhutan has now only 20 potentially dangerous lakes. The investigation discovered that four lakes, including Memari Tsho at the source of Mochu did not pose any flood risk.
According to ICIMOD and DGM inventory of 2001, there were 677 glaciers and 2,674 glacial lakes, out of which 24 were considered potentially dangerous.
“We assessed any indication of trigger possibility such as slide or rock fall and avalanche close by, seepage and outlet channel condition, among others. There is no short-term threat,” Karma Tshering said.
Latshokarp and Langdo Latshokarp are smaller in size with shallow depth, far from glacier, no supraglacial lakes and no expected slides from the adjacent area.
“Hence both of these lakes are not dangerous,” the team said.
Lhatshokarp (bigger of the two lakes) has 0.35 million cubic metre of water and is located 828 metres from the nearest glacier.
Another lake, which is believed to be Tshojokha, is a smaller remnant of cirque type lake without glaciers nearby.
“The studied lakes’ GLOF hazards potentials are low. However, regular monitoring and assessment of the lakes are necessary to ascertain the risk,” Karma Tshering said.
GLOF simulation will be carried out to reconstruct Lemthang GLOF. The findings will then be used to predict flood propagation and inundation scenarios of potential GLOF events.
There have been 12 GLOF events since 1966.
By Tshering Palden