The combination of hydrology, ecology and engineering has resulted in the smart design of a fish guidance system that changes the water flow and directs salmon past power plants.
The Mandal River was once a river teeming with salmon, but acid rain wiped out the population in the first half of the 20th century. As a result, fish were not a major concern when power plants were built here in the 1970s and 80s. However, after the water quality was significantly improved by adding lime to the river, the salmon have returned – but the fish face a new challenge: The Laudal power plant.
– Even though the power plant owner, Å Energi, has done a lot to improve conditions for the fish, many salmon still end up in the turbines, where we know up to 70% of them die, says Halvor Kjærås.
Measures such as increased water flow and the removal of weirs make it easier for salmon to pass the power plant on their way upstream to spawn. However, young salmon heading out to sea still risk their lives on their way past the turbines.
Kjærås did his PhD work at the research centre HydroCen where he focused on how to safely guide fish past such obstacles.
Floating fish fences that function!
Power plant owners have tried various barriers to keep fish away from turbines. Today’s recommended solution is to build fish racks with 15 mm spacing between the bars, which guide the fish toward a safe bypass nearby. But for large facilities like the Laudal power plant, such constructions are difficult to implement.
– Where it isn’t practical or economically feasible to build these racks, our experiments show that floating guidance racks can be one of several good alternatives, says Kjærås.
In the Mandal River, they tested such a system on a full scale.
The floating guide racks like a 90-meter-long barrier with slanted bars extending 1.5 meters down into the water, spaced 5 cm apart. The openings are large enough for small fish to pass through, but the smart design ensures that salmon are guided past:
–The bars are angled in a way that slows down the water. Fish react to changes in water speed and turbulence and are steered by the current along the rack toward a safe passage at the end, Kjærås explains.
He mapped the water flow around the rack and observed fish behavior as part of his doctoral research at NTNU.
The results from the Mandal River are now published:
72.5% of the salmon were guided safely past the turbines!
Without the rack, calculations show that only about half of the salmon would have made it through safely.
Stavene i gjerdet er vinklet på en måte som gjør at de bremser opp vannet. Foto: Juliet Landrø / NINA.
Challenge a common myth about where salmon swim
Kjærås and his colleagues from NINA also noticed that some fish crossed under the fence.
Previously, it has been assumed that salmon mainly swim near the surface to take advantage of faster currents. For that reason, the rack was not designed to extend more than 1.5 meters below the surface.
–We saw that most of the fish that crossed the rack did so by swimming underneath it—not through it, says Kjærås.
When they studied fish behavior elsewhere in the river, they were surprised to find that nearly half of the young salmon (smolt) swam deeper than 1.5 meters.
– This shows that the myth that salmon only swim near the surface isn’t true. That’s particularly interesting for me as a researcher and consultant, says Kjærås.
He adds that earlier in his career, he helped debunk the opposite myth - about eels. People used to believe they only swam along the bottom, but researchers found that eels also swim closer to the surface when the conditions are right.
–Such knowledge is crucial when designing fish passages, says Kjærås, who now works as a consultant at Sweco, combining hydraulics and biology to develop fish-friendly hydropower solutions.
Due to these findings, Å Energi has built up the riverbed to make the water shallower and begun installing deeper racks.
– We haven’t run new tests since then, but we have good reason to believe this will make the rack even more effective, Kjærås says.
Testing a guidance fence in the Orkla River
In September this year, a similar fish guidance system was installed at the Bjørset Dam in the river Orkla, where salmon can also risk entering a power plant on their migration from the river to the sea.
– This rack is based on the same principles as the one we used in the Mandal river but has a slightly different, patented design, says Torbjørn Forseth, project manager at NINA.
The design was developed and patented by researchers at the Swiss university ETH Zurich.
– We’ve been granted permission to use the solution because we’ll be the first to document how well it actually works at a full size power plant intake, says Forseth.
The project is run by TrønderEnergi, which operates the power plant and financed the construction. The rack was built and installed by Steis Mekaniske verksted.
–We’re optimistic about this solution as well. Our partners in Switzerland have run several flume experiments showing that fish can be guided across the current. In 2027, we plan to evaluate the system by tracking salmon that pass the dam, says Forseth.
Contact: Halvor Kjærås
Read the journal publication: Full scale performance evaluation of a partial-depth floating fish guidance bar rack av Halvor Kjærås m.fl.