It has been reported that the moraine dam is stable as of now and a breach into Rapstreng Tsho is unlikely
The people living along the Pho Chhu and downstream in Punakha are requested to be vigilant by the National Center for Hydrology and Meteorology (NCHM) after it detected a sudden rise and drop in water level of the Thorthormi Lake, located at around 4,428m above sea level in the remote Lunana area of northern Bhutan, Thursday evening.
According to the NCHM, the remote monitoring station of the Glacial Lake Outburst Flood (GLOF) EWS on Thorthormi Lake detected water rising by around 1.49m within approximately 25 minutes, up from 6.55m to 8.04m, at around 7 pm on June 20. This rise of water level accordingly asserted the GLOF Early Warning System in Punakha –Wangdue into ALERT condition.
However, the water level started to recede from 7:20 pm and was back to normal by 9:30 pm. The water level further receded from normal, which was an indication of emptying of the lake as per GLOF EWS system logic, according to the NCHM.
Further, the rise of water level was also detected by GLOF EWS monitoring station at Thanza, located approximately 4.6km along river channel downstream of Thorthormi Lake.
Following the detection of the sudden rise and fall in water level, field observers stationed at Thanza (Lunana) were deputed for field verification to the lake, which is located at about 2.5 hours walk from the base station.
The field observers reported that the subsidiary lakes (Subsidiary Lake II of Thorthormi were fully emptied through breach of moraine. It was also reported that no seepages were observed and that the bed and channel erosion of the moraine at Subsidiary Lake II was very intense.
Because of the breaching of these two subsidiary lakes, according to NCHM, the stability of the moraine dam of the main Thorthormi Lake is likely to be more uncertain.
The NCHM has announced that they as of now are intensively monitoring the situation and that any drastic changes at the lake would be communicated through existing Standard of Procedure.
It has also been reported that the moraine dam is stable as of now and a breach into Rapstreng Tsho is unlikely.
The impact if Thorthormi Lake bursts?
The main danger from Thorthormi Lake is not only itself, but it is located adjacent to the Rapstreng Tsho. With Thorthormi at a higher elevation, experts say in the worst case, it could “overtop” into Rapstreng.
It has been calculated that should there be a breach at the dam, around 53mn cubic meters of water could be unleashed from that particular area- from both Thorthormi and Rapstreng. This would be almost around three times what gushed out in 1994 during the GLOF outbreak of Lugge Tsho.
Meanwhile, the 1994 Glacial Lake Outburst Flood from Lugge Tsho took more than 20 lives and devastated villages and properties downstream in Punakha.
The Thorthormi Lake is, therefore, considered Bhutan’s likeliest climate-induced disaster. Studies by the Austrian consultants earlier projected that there could be a GLOF from Thorthormi Lake as early as 2010.
Accordingly to reduce the possibility of a GLOF, the government started works in 2008 to lower the lake’s water level at Thorthormi Lake to eliminate risk of a disaster, which experts say may sweep away communities living in the upper catchment of the Pho Chhu.
This was Bhutan’s first proposal to artificially lower the Thorthormi Lake accepted under the National Adaptation programmes of Action (NAPA) in 2006. Supported by the United Nations Development Programme, the project aimed to lower the water level of the lake by 5m or 17mn cubic meters of water in four years.
Tshering from Thimphu