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Introduction

The hydropower plant (HPP) is on the river Aare and located in Bannwil, some 46 km downstream of Lake Biel. There are two upstream HPPs on the Aare river below Lake Biel, HPP Brügg at the lake outflow and HPP Flumenthal about 12 km from Bannwil.

The river Aare is a 291 km long tributary of the Rhine and the longest river within Switzerland. The Aare River passes through three major lakes: Lake of Brienz, Lake of Thun and Lake of Biel.

In total there are 10 hydropower plants between Lake of Biel and the junction with the Rhine river.The other nine HPPs as well as two nuclear power plants are located downstream of the Bannwil HPP.

The altitude of the lowest and highest points of this river reach are ca. 312 m a.s.l. and 429 m a.s.l., respectively. The average altitude of the whole catchment amounts to 1060 m a.s.l. and the whole catchment area is 17687 km2, of which 1.4 % are covered by glaciers. On the river Aare, the mean monthly discharge increases from February to June and then decreases from July to October. The annual discharge in 2015 was 260 m3/s.

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Bannwil hydropower plant

The run-of-river hydropower plant is located in Bannwil, downstream of Lake Biel and the HPP Brügg and Flumenthal. There are 9 more power plants between HPP Brügg and the junction with Rhine river as well as two nuclear power plants with water abstraction for cooling.

The weir is located on the right side of the river, while the Powerhouse is located on the left side. The reservoir is about 7 km long. With thre bulb turbines, the HPP has an installed capacity of 28.5 MW and an average annaul production of 150 GWh. The gross head amounts to 5.5 - 8.5 m depending on the up- and downstream water level with a designed discharge of 450 m3/s. The annual Production is about 150 GWh.

Characteristics
CountrySwitzerland
RiverAare
OperatorBKW Energie AG
Capacity28.5 MW
Head5.5 - 8.5 m
Inter-annual discharge260 m³/s
3 Bulb turbines

The Operator: BKW
The HPP Bannwil is operated by the BKW group. The Group plans, builds and operates infrastructure to produce and supply energy to businesses, households and the public sector, and offers digital business models for renewable energies. Read more

Pressures on the water body's ecosystem
The river Aare is located in the Rhine river catchment, which was historically one of the most important Atlantic salmon (Salmo Salar) rivers in Europe. The upstream migration of Salmons in the Rhine catchment became almost impossible after hydropower plant constructions. This includes the river Aare. All of the occurring fish species present in the Aare river (total of 44 fish species) face potentially high mortality during downstream migration or difficulties during upstream migration.

Furthermore, the river Aare is highly influenced by hydropower and considered as a heavily modified water body. Moreover, there are three nuclear power plants on the Aare river, two of them reintroducing the used cooling water, which induces an increase of the river water temperature. The river has a moderate ecological potential. Measures for sediment control, fish migration, flow changes, habitat in-channel and morphology off-channel have been implemented in the water body.

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Test Case topics

Fish population
There are a total of 44 fish species in the river Aare, some of which are: Eel (Anguilla anguilla), brown trout (Salmo trutta), chub (Squalius cephalus), grayling (Tymallus Thymallus), spirlin (Alburnus alburnus), common barbel (Barbus Barbus). Salmon is expected in the next 10 - 20 years.

Upstream migration
The current fish pass is pool design with bottom and top openings. The entrance is located at the left side of river shortly downstream of the powerhouse. The bottom slope of the technical fish pass is on average 6 %. Most of the head difference is accomplished in the first half of the fishway. It consists of chambers with a spiral fishway. The upper part of the fishway consists of near horizontal canal that lead to the exit roughly 100 m upstream of the HPP. The mean discharge in the fishway amounts to 350 l/s.

The fish pass needs to be restructured to accommodate larger fish in the near future. Current plans include replacing the lower part with a vertical fish pass design and the upper part with a nature like open channel.

Fish migration
44 known species
eel, brown trout, chub grayling, spirlin, common barbel
New vertical slot pass

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Research objectives and tasks

At Bannwil, downstream fish migration measures are investigated by means of field and numerical studies. The current situation and the efficiency of spill flow or water release as an operational measure at the HPP Bannwil will be investigated through field monitoring and 3-D numerical modelling in the area near the powerhouse and weir. Applying this model to different structural and/or operational scenarios will allow to come up with solutions to improve fish migration at reduced energy losses. Dynamic pressure fluctuations experienced by fish during the turbine and spillway passages will be studied at HPP Bannwil using a Barotrauma Detection System (BDS) developed at TUT. Based on the data a CFD-model will be developed, which can then be used to evaluate the fish passage in order to judge the possibility of adapting the hydropower operation for certain time periods

Research tasks
The research tasks and field studies conducted at Bannwil are:

  • Velocity & hydraulic measurements
  • 3D Turbine & HPP models with Biological Performance Assessment and Barotrauma Detection System
  • Fish monitoring; Survey of fish movement using ARIS Sonar and radio telemetry techniques
  • Variant Study
Research objectives
Downstream fish migration
Operational measures
Turbine passage of fish

Main FIThydro partners involved in the research at Schiffmühle

Research topicsFIThydro partners
Dynamic pressure fluctuations experienced by fish during the turbine and spillway passages using a Barotrauma Detection SystemTUT, Voith
Flow during turbine passage with CFD modelVoith
Different operating modes for safe fish passageTUT, Voith
Fish turbine mortality assessmentTUM

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pictures: © BKW Energie AG