New fish pass at the Flumenthal hydropower plant

Following the construction of the Flumenthal run-of-over power plant near Riedholz in the canton of Solothurn, a fish pass was built in 1970. The fish were able to overcome the difference in elevation between the headwater and tailwater of approximately 8 metres using what is known as a basin pass. This fish pass does no longer meet modern standards for fish migration. Alpiq Hydro Aare AG owns and operates the run-of-river power plant and is replacing the fish ladder with a 480-metre bypass watercourse. This watercourse ensures the upward migration of fish according to state-of-the-art science and technology. Construction work began in autumn 2022 and is scheduled for completion in late 2023.

Near-natural watercourse

From the end of 2023, fish and other aquatic life will be able to overcome the weir at the Flumenthal power plant in a largely near-natural watercourse. The watercourse consists of three different stretches: a natural watercourse with rapids, a longer ramp with boulders, horizontal slabs and individual basins in the form of a riffle channel, and a basin-like concrete vertical slot pass at the mouth. Depending on the actual water level in the Aare, an average of 3 to 5 cubic metres of water flow through the fish pass every second. The intake structure regulates the water flow depending on the level of the Aare in the headwater.

The near-natural watercourse provides in addition to aquatic passability a diverse habitat for aquatic fauna. To ensure that the fish pass functions at varying water levels in the Aare, the lowest stretch by the mouth is designed as a vertical slot pass. In order to enhance the leading flow, an additional outflow from the headwater will be led via a pipeline to a weir turbine and discharged into the mouth basin.

Upgrading of the fish pass

In total, Alpiq Hydro Aare AG is investing approximately 10.5 million Swiss francs in the construction of the fish pass. However, the federal government reimburses the costs from the grid surcharge fund, which means that the upgrading of the fish pass at the Flumenthal power plant will be funded by all electricity consumers via the electricity price. 

The Flumenthal hydropower plant with its three horizontal bulb turbines was commissioned in 1970, and at full capacity, it processes roughly 386 cubic metres of water per second. The three machines have an output of 25 MW, generating on average some 146 million kWh of electricity per year. 

Further information: Construction site webcam