New fish pass for Flumenthal hydropower plant, Switzerland

3 March 2023

Alpiq has announced it is investing 10.5 million Swiss francs in a completely new fish pass at the Flumenthal hydropower plant, to fulfil the current standards for fish migration in the River Aare. 

The original fish pass at Flumenthal was built in 1970. In the original design, fish were able to overcome the difference in elevation between the headwater and tailwater of approximately 8m, but this pass no longer meets modern requirements.

Alpiq will now build a 480m bypass watercourse which will ensure 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.

From the end of 2023, fish and other aquatic life will be able to overcome the weir at the Flumenthal 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 5m3 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.

In addition to aquatic passability, the watercourse will provide 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.

While Alpiq Hydro Aare AG will be building the fish passage, the federal government is set to reimburse the costs from the grid surcharge fund.

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