Bhotekoshi-Trishuli Floods: Why Are Hydroelectric Project Tunnels at Risk?
Image source, Nepal Army
More than 900 staff members from 11 hydroelectric projects affected by the Bhotekoshi-Trishuli floods last Wednesday have gone out of contact, raising concerns among officials.
However, the exact number of people trapped inside the tunnels remains unknown.
Nepal Army spokesperson Rajaram Basnet told the BBC that as of Monday evening, rescue operations were underway at Rasuwagadhi, Langtang, Chilime, Upper Trishuli 3A, and Upper Trishuli 3B projects.
With assistance from experts from India and China, rescue efforts remain focused on these tunnels. Officials and specialists have discussed with the BBC the complexities involved in hydroelectric tunnels.
The Necessity and Risks of Tunnels
Tunnels in hydroelectric projects are used primarily to transport water from riverside locations to the power generation plants, which are typically located on the opposite side of a hill.
Rock and Tunnel engineer Biplav Ghimire explained to the BBC that unlike ‘metro tunnels’ used to shorten transportation distances, ‘hydraulic tunnels’ run close to rivers, making them inherently vulnerable to flood risks like those seen at Rasuwagadhi.
“The tunnel is bored through the hillside from the riverside, with both entrance and exit points located at the riverbank,” Ghimire said.
“The current main issue is blockage caused by sediment intrusion, which complicates rescue operations.”
Mechanical engineer Kshitij Koirala, involved in rescue efforts at Upper Trishuli 3A, said that although the actual conditions inside the tunnel remain unclear, based on the structural characteristics of hydroelectric tunnels, hope has not been lost.
Current Rescue Operations Overview
Image source, Nepal Army
Army spokesperson Rajaram Basnet informed the BBC that at the Rasuwagadhi hydropower project, the tunnel was found blocked about 250 meters in, though no people were found at that point.
“Inside Langtang tunnel, large boulders were discovered within 65 meters,” Basnet said.
“Foreign teams are working alongside the army in Chilime and Upper Trishuli forest areas, but so far no results have been achieved. These tunnels remain fully blocked, with ongoing efforts to clear them.”
At Upper Trishuli 3B, efforts are underway to locate the tunnel entrance, while at 3A, where individuals are believed trapped, water is being pumped out as part of the rescue.
Though the Electricity Authority is delivering necessary supplies by helicopter, officials report significant challenges working in the flood-affected, already difficult terrain.
“There are areas inside the tunnel suitable for residing, such as the powerhouse’s cab rooms and office chambers. Walkie-talkies are usually available for communication, but the specific arrangements made at each project may vary,” tunnel engineer Ghimire said.
“Additionally, provisions for food and water, fire safety, and extra oxygen are essential.”
Upper Trishuli 3A: A Particularly Challenging Project
The main tunnel at Upper Trishuli 3A lies approximately 15 meters below the surface.
Access to the powerhouse requires traveling through an underground tunnel 170 meters in length.
Early in the rescue operation, the closest reachable point was identified as a small ‘cable tunnel’ within the project.
This segment of the tunnel lies about 10-11 meters below the ground surface, now filled with stones and mud.
“Excavation was difficult, so we hired local laborers as helicopters cannot transport excavators. Helicopters can carry only up to three tons,” mechanical engineer Kshitij Koirala, from the project’s central office, explained.
“Thankfully, three excavators were found nearby and brought to the tunnel entrance. We constructed a bend to avoid a tunnel that would be too long with a straight path. We also created ventilation holes to supply oxygen.”
Koirala noted that although explosives were used to blast the ‘crown’ — the upper part of the tunnel — concrete beams and reinforcing rods remain inside, posing further challenges.
“Water accumulation has also been observed inside and needs to be pumped out,” Koirala added, expressing the belief that 35 to 40 people are trapped within.
“Repairs were underway, so we hope all individuals are on the lower floors. We believe the mud and sediment have not yet reached the tunnel entrance but have stopped somewhere inside, and efforts are ongoing based on this assumption.”
Tunnels in Nepal
Image source, Nagdhunga Tunnel Construction Project
In Nepal, tunnels are constructed using the drill and blast technique involving explosives. Experts note that hydroelectric projects nationwide have tunnels ranging from under 100 meters to nearly nine kilometers in length.
Additionally, the Sun Koshi-Marin irrigation project has a 13-kilometer diversion tunnel, while the Melamchi drinking water project’s tunnel stretches 26 kilometers.
The Nagdhunga tunnel, connected to the highway and Kathmandu, is 2.69 kilometers long; the under-construction Siddhababa tunnel is approximately 1.1 kilometers long and is nearly penetrated from both ends.
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