A new fish passage has been recently opened at the Poutu weir in Finland, allowing the migratory fish to travel upstream without obstacles. The technically challenging project was successfully implemented thanks to close collaboration among the South Ostrobothnia Centre for Economic Development, Transport and the Environment, Kone-Kovera Oy, and Pipelife Finland. The installed culvert has restored the migration routes of salmon, trout, grayling and other aquatic species in the Lapua river.
The Poutu weir is located about 3 kilometers downstream from the center of Lapua town. Built in 1991, the weir protects the surrounding area from flooding and slows down shore erosion in the Lapua river.
"The weir helps control the water levels, prevents landslides and allows using the nearby area for recreation," explains Pekka Hyytiäinen, water management specialist at the South Ostrobothnia Centre for Economic Development, Transport and the Environment.
However, the weir also forms an obstacle to migrating fish and other aquatic life in the river. Particularly affected species are salmon, trout and grayling which have to swim upstream to reach suitable habitats for spawning.
In order to improve the ecological health of the river, it was decided to construct a fish passage, allowing the aquatic life to migrate easily past the weir.
To successfully implement the project, the South Ostrobothnia Centre for Economic Development, Transport and the Environment worked together with the contractor Kone-Kovera Oy who was responsible for earthworks and Pipelife Finland who supplied tailor-made corrugated pipes for building the fishway.
"Pipelife was chosen as the supplier for this project because they had the design expertise and knowledge of the best materials and installation methods for culvert fishways. Together, we were able to find all the necessary solutions," says Jarkko Yli-Kojola from Kone-Kovera Oy.
The construction of the fishway began in November 2021. For the project, Pipelife Finland manufactured and supplied corrugated PE pipes with a diameter of 1.6 meters.
"The fishway was designed and constructed in a way to mimic the natural river conditions as closely as possible," says Joni Vihanta, product manager at Pipelife Finland. "The Lapua is a river with thousands of cubic meters of water passing per minute. Therefore, the 40-meter-long culvert has cross-walls every 5 meters that help hold the pipe in place and ensure optimal hydraulic conditions for fish. The bottom of the pipe is covered with stones and gravel, imitating a riverbed, and there are three lightwells, letting in the sunlight."
The installation of the fishway posed additional challenges as excavations had to be carried out on a riverbank with soft soil. Furthermore, the works continued throughout the flooding season, and the changing water levels had to be taken into account when constructing the passage.
The pipe was installed and secured at an average depth of 4-5 meters. The works were completed by July 2022 to ensure the fishway would be open well before the salmon migration in autumn.
To the delight of the project team, the first fish were observed at the end of the new passage already shortly after its completion.
Yli-Kojola recalls the opening of the fishway: "We saw both bream and perch at the end of the fish pass. The water was flowing well, and according to experts from fisheries, everything was in excellent order."
According to recent estimations, there are currently more than 1 million barriers on European rivers, heavily affecting the local freshwater ecosystems. Finland is one of the European countries that have adopted a national program supporting the removal of river barriers and restoring migratory fish routes.
While obsolete dams can be fully removed, other solutions are needed for river barriers that are crucial for purposes like hydropower, navigation, flood protection or water supply. The construction of fish migratory aids allows improving river ecosystems where complete barrier removal is not feasible, and the new fishway at the Poutu weir serves as a successful case.
"After the first flooding season, we can now assess the project results," says Hyytiäinen. "The fishway remains in great condition, showing no signs of damage and looking even better than we expected. We hope that such fish passages will be replicated and used in other locations, too."
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