Flumenthal – Alpiq Hydro Aare AG has replaced the fish ladder of the Flumenthal power plant in the canton of Solothurn with a near-natural bypass watercourse measuring 480 metres long. The modern facility, which has now been officially opened, allows fish in the Aare to safely bypass the power plant and creates new biotopes and spawning habitats.
Fish ladder in Flumenthal
At the Flumenthal power plant, fish and other aquatic organisms can now swim upstream the Aare from the tailwater to the supply canal in a near-natural fish pass. The fish pass meets the latest standards and is one of the largest newly created artificial waters in the canton of Solothurn. It is located on the left bank of the Aare in the direction of flow at the level of the power plants in the commune of Riedholz (SO) and covers a difference in altitude of around eight metres. After around two years of work, the new fish ladder will replace the old facility. This dates from 1970 and no longer met the requirements for fish migration. A total of around CHF 20 million was invested in the construction of this plant via the national fund “Netzzuschlagsfonds” as part of the ecological refurbishment of hydropower.
Alpiq Hydro Aare worked in collaboration with the federal government, the canton of Solothurn and environmental protection organisations to develop a suitable solution for the Flumenthal hydropower plant that is tailored to the species of fish that live in the Aare, such as barb, perch and chub. In addition to the aquatic continuity, the fish ladder offers a diverse habitat for the aquatic fauna.
It consists of three parts: a supply canal rock ramp with rapids and gravel banks, a longer ramp with 2–3 blocks each, various transverse bars and around 37 other basins in the form of a tailwater rock ramp, and the basin-like, concrete vertical slot pass at the mouth. A fish counting chamber means the effectiveness of the new fish ladder can be checked regularly. Depending on the run-off of the Aare, an average of 3 to 5 m³ of water flows through the fish ladder per second. The structure regulates the amount of water and adapts it to the level of the Aare in the supply canal.
Fish ladder in Flumenthal
The near-natural part of the fish ladder in the supply canal below the inlet structure is designed for larger discharge volumes than the subsequent rock ramp and provides a diverse habitat for aquatic fauna thanks to the tree stumps and other natural elements. The rock ramp basin ladder extends from the power plant access road to the tailwater. In this rock ramp, the fish cover most of the difference in altitude between the tailwater and the supply canal. To ensure that the fish ladder works at different water levels on the Aare, the lowest section at the mouth is designed as a vertical slot pass. The connection to the tailwater is made directly below the outlet structure parallel to the current. To this end, the bank wall below the mouth was removed and replaced by a new concrete wall. This set-back entrance contains a newly created riverbed connection that makes it possible for migratory fish moving close to the ground to find the entrance. An additional lock flow is also generated to make finding the entrance easier. To do this, part of the run-off from the near-natural rock ramp is led via a pipeline to a doping turbine and discharged at the entrance. Two additional entrances have also been created on both sides of the central pillar of the river weir in the tailwater. The fish from these two entrances are guided past the power plant via a collecting channel and then united with the other part of the fish ladder from the set-back entrance.
Thomas Fürst, managing director of Alpiq Hydro Aare AG, commented on the fish ladder project: “We are delighted to officially open this completely new and near-natural fish ladder. It will provide the Flumenthal power plant with a state-of-the-art facility that meets today’s fish migration requirements and promotes a diverse habitat for aquatic fauna.”
The construction of the new fish ladder at the Flumenthal power plant is the result of the revised Waters Protection Act of 2011, which calls for the restoration of free fish migration in several Swiss rivers. All power plants in Switzerland should meet the requirements of the new Waters Protection Act by 2030.
Alpiq Hydro Aare AG is a wholly owned subsidiary of Alpiq. It owns and operates the three hydropower plants Flumenthal, Ruppoldingen and Gösgen at the Aare River in the canton of Solothurn. The Flumenthal hydropower plant was commissioned in 1970 with three horizontal bulb turbines and processes around 386 m³ of water per second at full capacity. The performance of the three machines amounts to 25 MW and the average annual production is around 146 million kWh of electricity, which corresponds to the annual electricity consumption of 32,450 households. BKW owns 37.9% of the Flumenthal power plant.
