The Bhutanese Leading the way.
But another attempt to be made again
One of Bhutan’s biggest looming natural disasters, building up with every summer season, is the huge Thorthormi Glacial Lake in Lunana one day breaking the shrinking moraine dam that separates it from the huge Raphstreng Tsho below.
If it breaks, waters from Thorthormi and the lower Raphstreng Tsho could surge down like a deadly mountain tsunami, devastating everything in their path.
There was, therefore, big hopes placed in a 2025 siphoning project by the National Center for Hydrology and Meteorology (NCHM) funded by the World Bank.
The basic aim was to use the siphon technology to siphon water from the Thorthormi lake using pipes, and thus reduce its level and with that its danger and pressure on the moraine dam.
A lot of preparation was done with a team sent up and equipment placed at the site by helicopters and ponies, but ultimately due to a host of technical factors, the siphoning could not happen.
However, NCHM is not giving up hope yet, and plans to make another attempt this year by learning from the issues in 2025 and working to rectify them. In the meantime, it also released a report on the issue.
The main problem was that the distance between the lake and the siphon crest or the highest point to which water has to be sucked up to before it can go down to the other side was 9.771 meters.
The literature shows that at an elevation of 4,300 meters, the atmospheric pressure is only strong enough to allow siphon at 6 to 6.4 meters. The team relocated the pipes to 8 meters and even 6.44 meters but siphoning did not happen, but the water level climbed up high to three fourths of the level at 6.44 showing a link between height and suction power.
Another issue was that an air pocket developed at the very head between the submerged part of the head of the pipe and a ball valve designed to stop the flow of the water out into the lake, as the pipe was being filled with water by a water pump to create a suction force.
Added to this was that around 80 meters of the pipe was in an almost flat section on the other side which restricted suction power.
The backflow during the test also highlighted the need for proper laying of pipes avoiding kinks and bends and integration of air vent valves at the apex.
The lack of correlation with valve timing and inconsistency between repeated tests suggest that use of submersible non return valve be used instead of ball valve at the intake. In simple English, what this means is that when the pipe is loaded with water, both the valves on the intake and outlet part should open at the same time.
This cannot happen when it has to be done manually on both ends. So, the solution is that at least on the intake side, the ball valve be taken out and a one way valve be used which will not allow the water to spill back into the lake but allow lake water to come out once the outlet is opened by opening up when suction power is strong enough.
One of the contributors to the system’s hydraulic inefficiency was the increased frictional loss introduced by the internal bead, or fusion ring, at each butt-weld joint. During the field welding process, the heated ends of the HDPE pipe are pressed together, forming a raised internal ridge where the material fuses. While this creates a strong mechanical joint, it also introduces a localized reduction in the effective internal diameter of the pipe. For a siphon system operating with a marginal driving head, the cumulative effect of these periodic restrictions along a 550-meter conduit with numerous welds is substantial.
The primary constraint was identified as the fundamental physical limit imposed by low atmospheric pressure at altitude.
NCHM said that despite the pilot test, while not achieving operational success, provided invaluable lessons and data to validate the fundamental knowledge on natural siphon designing and construction methodology at remote high-altitude location to mitigate the GLOF risk.
The recommendation is not to abandon the developed system, but to strategically relocate and reconfigure it at a suitable site where the due consideration on fundamental physical constraints, construction methodology is re-examined and incorporated.
It said that re-initiating the siphon system can be done with minimal cost as the substantial upfront investments in equipment, materials, and, most critically, lesson learnt and knowledge have already been made and are preserved on-site.
One the lessons learnt and the way forward the NCHM team said to overcome the siphon height, it is recommended to relocate the setup to an alternative site.
An alternative location for deploying the system has been already identified close by the existing lake outlet. Though it has its inherent limitation and challenges, such as unstable moraine at the falling limb, and underneath barrier between the main lake and the new proposed intake pond or subsidiary lake I, the profiling survey data indicates that the vertical height from lake water level to the highest point is less than 6.00 m that would be within the operational head from intake water surface to siphon apex.
A detailed bathymetry survey is to be conducted at the outlet subsidiary lake to ascertain the depth of the barrier between the main lake and the subsidiary lake from where the inlet of the siphon shall be positioned.
Another solution is to replace the ball valve with a submersible non-return (check) valve directly at intake assembly to eliminate the trapped air pocket and improve the timing sequence between valve openings. With this set up the single action of opening the outlet valve only would be required to initiate the siphon.
NCHM is currently engaged with Swiss Polar Institute (SPI) under the project “Reducing Imminent Risk due to GLOF in Punatsang Chhu Basin”. It is recommended to leverage the project’s existing partnership with SPI and engage an expert in the field of multi hazard management to conduct a field visit to the project site.
The expert would conduct evaluation, review and assess the installed system to provide further recommendation. This recommendation to be incorporated into the final redesign.
The current proposed SPI project has included the activities to enhance the existing GLOF Early Warning System in Punatsangchhu basin by installing high-resolution CCTV camera and sensors for real time monitoring on lakes with star- link Satellite communication.
A Specialist at the Cryosphere Service Division, Karma Toeb, said, “There has been a lot of learning from this project as there is no siphon expert in Bhutan or even the region, and the only other country to do it successfully is Peru in the Andes Mountains which has similarities with Bhutan of glacial lakes and moraine dams.”
Meanwhile, climate change is having its impact on Thorthormi. Karma said that Thorthormi was once a glacier which has now transformed into a lake within 23 years, and during the recent survey the lake depth at certain points was well below 300 meters to the extent that the equipment was not adequate to get the real depth in the upper parts.
He said the moraine dam at the thinnest part, which is the crest on top, is 33 meters and every summer season it shrinks by a few centimeters. This is not even considering what a major earthquake could do to the natural dam separating two huge glacial lakes.
There is a plan to send up a team in March again to measure the depth at the subsidiary lake near the outlet of Thorthormi and see if the siphoning can happen from there.
