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Flumenthal hydropower plant, new watercourse for fish migration

The existing fish ladder at the Flumenthal power plant, which was built 50 years ago, does not meet the current standards for fish migration. Alpiq Hydro Aare AG is replacing the fish ladder with a 480-metre bypass watercourse. The construction work will be completed by the end of 2023. 

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. 
Following the construction of the power plant, the fish were able to overcome the difference in elevation between the headwater and tailwater using what is known as a basin pass. Since it no longer meets modern standards for fish migration, Alpiq Hydro Aare AG is implementing a completely new fish pass in the form of a bypass watercourse, scheduled for completion in late 2023. 


Who is Alpiq Hydro Aare AG?

Alpiq Hydro Aare AG is a wholly owned subsidiary of Alpiq. The company owns and operates the three run-of-river power plants Flumenthal, Ruppoldingen and Gösgen on the River Aare in the canton of Solothurn.

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The key elements of the bypass watercourse

a. Intake structure 

b. Near-natural watercourse

c. Riffle channel

d. Vertical slot pass and inlet structure with the weir turbine

e. Collecting channel

New watercourse improves fish migration

From the end of 2023, fish and other aquatic life will be able to overcome the approximately 8-metre difference in elevation between the headwater and tailwater via 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 is located in the municipality of Riedholz directly below the bridge on which the cantonal road passes over the Aare. The structure regulates the water flow depending on the level of the Aare in the headwater. 
The near-natural watercourse between the intake structure and the power plant is designed to accommodate higher flow rates than the subsequent riffle channel and, in addition to aquatic passability, it provides a diverse habitat for aquatic fauna. 
The riffle channel runs from the power plant access road to the tailwater.  In this channel, the fish overcome a large part of the difference in elevation between the headwater and tailwater. 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. The connection to the tailwater will be located directly below the turbine outlet, parallel to the flow. To achieve this, the embankment wall below the mouth will be removed (“riverbed connection”) and replaced by a new concrete wall. 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.
A collecting channel is connected directly to the power plant. This creates two additional entrances to the fish pass on both sides of the centre pier of the weir in the tailwater.