Water level normal but stability of moraine dam of Thorthormi Lake‘likely to be more uncertain’: NCHM

AWLS on Thorthormi repaired and put in place

The latest update sent by the National Center for Hydrology and Meteorology (NCHM) on Thorthormi on 24th June 9 pm said that while there is no significant change in the water level, however, due to the breaching of the subsidiary lake two, the ‘stability of the moraine dam is likely to be more uncertain.’

It says, “Therefore people living along the the Phochu river and downstream are requested to be vigilant.”

The above statement from NCHM who has a four-man team of experts in the lake site apart from other staff already present there means that while there is no unusual activity in the lake, the danger from Bhutan’s most dangerous lake is far from over.

The NCHM team which had left for Thorthormi on 23th June by helicopter after an earlier attempt on 22rd June was not successful due to bad weather.

The team on 23nd June had conducted an aerial survey of Thorthormi and also the nearby Rahpstreng Tsho below it and the Lugge Tsho next to it.

The NCHM said that the damaged Automatic Water Level Station (AWLS) on Thorthormi lake which is the first Early Warning System (EWS) was located on subsidiary lake two and relocated to subsidiary lake 1.

The reinstallation works of AWLS on Thorthormi Lake was successfully completed on 24th June, 2019 evening by the NCHM team with support from the Principal and teachers of the Lhadi Primary school and BHU staff.

Photo Courtesy: NCHM
Photo Courtesy: NCHM

The AWLS has now started transmitting water level data automatically to Wangdi GLOF control and NCHM headquarters in Thimphu from 5.10 pm onwards.

This essentially means that if there is another flood from the same place this will be the first EWS to detect the high levels.

The NCHM is will also carry out repaid and maintenance of the Thanza AWLS which is a further down stream and another important EWS.

The manual monitoring of the Thorthormi lake will be called off from today with the AWLS now fixed but monitoring of lakes will continue from the Wangdue GLOF Control room and NCHM Thimphu. However, manual observation and monitoring of the water level at Thanza will be continued on a 24 hours by seven days’ basis to supplement the monitoring by the equipment systems.

The team will not be able to detailed measurements in absence of heavy equipment needed for it but it will do a visual assessment, check on the moraine dam and do other additional checks.

On the Command of His Majesty the King, 25 RBA personnel have been sent to Lunana to reconstruct the Zamchu Bridge at Thanza and repair the other bridge at Toenchoe which was partially damaged.

17 civil servants of Lunana and 3 engineers of Gasa Dzongkhag have been mobilized to assist the RBA personnel in the reconstruction effort.

This is because a lot of the able bodied people have gone in the mountains to collect cordyceps. Many have started to return after the incident.

Additionally, logistical arrangements have also been made by the Gasa Dzongkhag Administration for transportation of cement and other necessary materials to the site.

The Incident Command System (ICS) for disaster management remains on alert including the Dzongkhag disaster management committees.

Background

At around 6:55 pm on 20June, the remote monitoring station of the GLOF EWS on Thorthomi Lake detected a sudden rise in the water level from 6.55 meters to 8.04 meters, a rise of 1.49 meters, within approximately 25 minutes. The rise in the water level set off the GLOF EWS in Punakha-Wangdue valley into an alert status.

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 by 1.19 metes below 6.55 meterswhich was an indication of increased water leaving the lake.

There were large ice boulders in the channel between subsidiary lake 1 and 2 showing a surge had taken place.

The impact of the water leaving the lake had emptied out the subsidiary lake two below breaching it and it had also impacted the higher subsidiary lake 1.

The water washed away a bridge and damaged another one.

NCHM at the time found a major threat of outburst of the main Thorthormi Lake as a result of rapid increase and then fall in the water level and the breaching of the two subsidiary lakes.

Pema Singye from Department of Disaster Management said that most lakes in Lunana are considered as potentially dangerous lake. The Lugge Tsho outburst in 1994, which killed 21 people, is located in Lunana. The lake is situated adjacent to the Thorthormi Lake above it.

“The Thorthormi Lake was lowered by 5 meters from 2010 to 2012. While RaphstrengLake below Thorthormi is a fully formed lake, Thorthormi is a combination of water and ice and between these two lakes, there is only a thin layer of moraine dam (ice, rocks and mud), and it looks like Thorthormi is already draining out through two subsidiary lakes which breached on 20th June. Because of the disturbances created from the breaching of the small lakes, there is a risk that the main Thorthormi Lake might also breach. There will be accelerated erosion because of the gradient,”Pema Singye said at the time.

Being identified as a potentially dangerous lake, if the breaching impacts the moraine dam of ice and mud, then it would directly pour into Raphstreng Lake directly below it. This would impact the five dzongkhags;Gasa, Punakha, Wangduephodrang, Tsirang and Dagana and unleash a flood far worse than in 1994.

Earlier the bed and channel erosion of the moraine at the subsidiary lake 2 was reported to be very intensive due to higher flow of water and as a result, reports said there are are chances of destabilizing the main Thorthomi Lake. But as of now, NCHM is intensively monitoring the situation and there is no warning of flood. “The situation is stable as of now,” assured Pema Singye.

He said that as soon as they were informed about the incident, information was shared amongthe staff through WhatsApp group. “We perceived that if the incident happens, firstly Gasa will be affected followed by Punakha, Wangdue, Tsirang and Dagana,” he said.

At the time, he said they were prepared for the possibility of the main lake breachingso they couldminimize the destruction to both human and property.“And particularly for Lunana, places like Thanza and Tenchey are too close to the lake and there is hardly any time to evacuate the people if the lake breaches as by distance it is just less than 5 km between the lake and the two places,” he added.

He also stated, “After we received the information from NCHM we requested NCHM to inform the Gup and community leaders of Lunana to be alert should anything happen as NCHM has their officials in Thanza and as they can directly communicate with the people of Thanza,.”

He said the people were alarmed once the news was shared andstarted to evacuate. “The message from DDM was that people in areas which are close to lake should be on the mode of evacuation because there is hardly any time. We are not worried about people in downstream because they have time,” he added. He also said officials in Punakha, Wangdue, Tsirang and Dagana were alerted as well.

He further said, “Some hazard map was developed which tells us how much water will reach the river banks, and how much area will be submerged, as we have classified two types of zones such as red and yellow zone, of which red zone indicate high risk and yellow shows less risk.”

The officiatingDzongda of Gasa, Dorji Gyeltshen, said that they were closely monitoring the situation,as two places in the dzongkhag are located close by to the lake. “We had warned the people about the possible risks and the people have already moved to safer places. We have also sent our staff from the schools to help the people at the station in Thanza and if anything happens, we are prepared,” he said at the time.

Dzongdas from Wangdue, Tsirang and Dagana said that as there are disaster management committees in each of the dzongkhags they areextensively engaged in informing the people about the possible risks and all the people are well informed and alert.

The Wangdue Dzongda said, “We have a disaster incident command system, and we have alerted it. For those who are residing near the river banks, we have instructed them to be alert and vigilant. But at the same time, we are trying not to create panic distress because as soon as the information was shared, people seemed to panic.”

According to Dagana Dzongda, Phuntsho Choden, since the lake is located in Lunana which is under Gasa, people in Gasa will be affected the most if the main lake breaches, but at the same time, people in Daganaare being alerted and informed of the risks.

Minister for Economic Affairs, Lyonpo Loknath Sharma, at the time,said that as a result of two subsidiary lakes breaching, there was a threat that the main lake might breach too. He saidin case the Thorthormi Lakebreaches then it will damage the equipment and the works on the hydropower project.

He said, “We have informed all the contractors and staff of the project that there is a possibility of outburst of the main lake and we have instructed them to take immediate measure or action.”

Flood Analysis

From the analysis of the information available it appears that a surge of water, ice or other material entered the Thorthormi Lake around 6.55 pm on 20th June.

This lead to a sudden rise of the water level in the lake by 1.49 meters within the 25 minutes.

This in turn would would have created additional pressure on the exit point of Thorthormi where water flows out into two much smaller subsidiary lakes below it and then down into the Phochu.

So the sudden increased water level meant increased outflow from the Thorthormi exit point which lead to more erosion of the exit point.

This explains the large ice boulders visible outside Thorthormi.

This increased flow dumped so much water into the subsidiary lakes below it that one emptied out subsidiary lake two due to erosion while another subsidiary lake one partially emptied out.

With water rushing out of the lake the lake level fell back to the normal of 6.55 meter depth by 9.30 pm on 20th June.

However, since the earlier erosion was so much, more water went out from the lake mouth and so it fell below normal by 1.19 meters below 6.55 meters.

With measurements showing normal levels (so far) it appears that the situation has stabilized for now and there was no serious breach.

Though the NCHM did not share the information it seems they suspected that some water from Lugge Tsho may have entered Thorthormi but this theory was eliminated when they did an aerial survey on 23rd June where the inlet between the two was found to be in place.

As said by NCHM the breach of subsidiary lake two and the consequent erosion after that is likely to have weakened a section of the moraine dam which needs to be watched closely.

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