Hongsa Power Plant 100% completed, Xayaboury Hydropower 70% Completed

Experts unveil Xayaburi fish passage technology


Xayaburi hydropower project plan shows how sediment flow is facilitated by cutting edge technology. 

(KPL) Mekong River fish of all sizes will be able to pass the Xayaburi Hydropower Project being built in northern Laos, according to expert consultants.

The 820-m long Xayaburi barrage will be the first to straddle the Lower Mekong in a part of the river that is wholly within Lao territory. At a technical workshop held on 15 July in Vientiane Capital, Dr Tobias Coe of the UK firm Fishtek Consulting explained the science behind the design of the project’s multiple fish migrating systems.

“The fish population is very diverse and there are multiple species from many families,” said Dr Coe. “At the Xayaburi site, fish scientists sampled 120 species from 26 families. The fish range in size from 30 mm to 3,000 mm. They migrate in different seasons and swim at different speeds. There is a very large amount of biomass migrating through the area, especially in May.

Before arriving at the design for the fish passage technology, scientists collected data for three years, investigating the timing of fish biomass and migration, the diversity of species caught by gill netters, and the swimming ability of the most commonly found species. In addition they conducted 10 field investigations over a period of one year, in both near-shore and mid-river locations. Monitoring included the use of a sonar-acoustic camera.”

“Little was known about the swimming ability of Mekong species,” said Dr Coe. “Consultants built a flume on the site and tested the five main species in the river with turbulent flow conditions. We found a wide range of velocities. We knew we couldn’t design the system for Usain Bolt,” a reference to the Jamaican runner who holds multiple world records.

“In light of research findings, engineers determined that a single fish passage over the length and 30-m height of the barrage would not work,” he went on. “We realised we would have to use man-power rather than fish energy to move the bulk over the height of the barrage. The system has to be flexible and adaptable.”

Fishtek’s investigations helped engineers determine that Xayaburi would require multiple systems for both upstream and downstream passages.

The main system for upstream migration is being built on the right bank. The design includes two fish locks and a fish ladder nearly 700-m in length.

Guilaume Morier-Genoud, project engineer of Poyry’s Hydraulic Department explained what will be done to attract fish to the upstream fish-migration facilities.

“Migrating fish swim instinctively against the current,” he said. “We need to provide attraction currents at the entrances for the fish to detect and guide them through the collecting galleries, the locks and eventually into the reservoir.”

Fish migrating upstream will enter the fish-collecting gallery from the spillway or through numerous entrances at the powerhouse. The gallery will lead them to the fish ramps that are U-shaped, creating a low stop between two large pools. Water will pass through at a low velocity made variable with slots. The ramp will lead fish 5-m and higher. At the top, a fish crowder will push them into the fish lock, which will lift them 30-m higher to the upper level. When they arrive, a gate will open and they will swim past a monitoring station and back into the river.

A water pumping system had to be added so the fish passage facilities would not take water from the hydropower plant. Tailwater from below the barrage will be pumped back up for the fish passage. To compensate for natural variations in the level of the Mekong, water also must be brought to cover all of the fish entrances year around.

If the upstream-migration fish ramp does not attract enough fish, the design allows for a fish lift that would carry fish the entire 35-m from the bottom to the top. The navigation lock for ships, on the right side of the project, will also act as a fish lift.

In the downstream fish migration system, there will be a bypass that will lead surface fish and bigger fish, even the biggest of fish like the Mekong giant catfish, across the powerhouse to avoid the intake. The spillway will also serve to let surface-level fish through.

Trash rack bars spaced horizontally 120 mm apart were designed to keep sticks and garbage out of the intake system. These bars will also prevent larger fish from entering.

“Most of the bottom fish smaller than 120 mm and less than 1-metre long will be able to pass the trash rack, and pass through the turbine safely,” he said.

The seven 175 MW turbines of 8.6-m diameter will be installed on a vertical axis. Similar to fish-friendly turbines on the Columbia River in the United States, the turbines at Xayaburi will be equipped with a special vane design and other fish-injury prevention mechanisms.

Downstream, migrating fish on the surface will arrive at a rest area with a flap gate that will open periodically to release them. Construction of the surface-level bypass exit chute is nearing completion.