Introduction

The hydropower plant (HPP) is on river Aare and located in Bannwil, some 46 km downstream of Lake of 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 totally within Switzerland. There are in total 10 hydropower plants between Lake Biel and the junction with the Rhine river.

The altitude of the lowest and highest points of this river reach are ca. 312 m asl and 429 m asl, respectively. The average altitude of the whole catchment amounts to 1060 m asl and the whole catchment area is 17687 km2, of which 1.4 % are covered by glaciers. The Aare River passes through three major lakes: Lake of Brienz, Lake of Thun and Lake of Biel.

On 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.

Bannwil hydropower plant

The power plant is located in Bannwil, downstream Lake of Biel and the HPP Brügg and Flumenthal.

There are 9 more power plants between HPP Brügg and the junction with Rhine river. The weir is located on the right side of the river, while the Powerhouse is located on the left side. The HPP has an installed capacity of 450 m3/s (3x 150 m3/s Kaplan tuubines ) with a head of 5.5 – 8.5 m. The annual Production is about 150 GWh. The reservoir is about 7 km long.

Layout

Topview of HPP Bannwil

Overview of the HPP Bannwil

Characteristics
Country Switzerland
River Aare
Operator BKW Energie AG
Capacity 28.5 MW
Head 5.5 - 8.5 m
Inter-annual discharge 260 m³/s
3 Bulb turbines

The Operator: Onxy / BKW
Onyx is a Power Company founded in 1895. BKW Energie AG is the main Stakeholder of the company. The company owns HPP Bannwil and the downstream-located HPP Wynau Schwarzhäusern.

Challenges at the Test Case

Pressures on the water body's ecosystem
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 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 putting back 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.

Fish population
There’s 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 technical fish pass is on average 6 %. Most of the head difference is accomplished in the first half of the fish way. It consists of chamber with a spiral fish way. The upper part of the fish way consists of near horizontal canal that lead to the exit roughly 100 m upstream of the HPP. The mean discharge in the fish way 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

Research objectives and tasks

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

  • 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

Main FIThydro partners involved in the research at the Bannwil HPP

Research topics FIThydro partners
Dynamic pressure fluctuations experienced by fish during the turbine and spillway passages using a Barotrauma Detection System TUT, Voith
Flow during turbine passage with CFD model Voith
Different operating modes for safe fish passage TUT, Voith
Fish turbine mortality assessment TUM

Research objectives
Downstream fish migration
Operational measures
Turbine passage of fish

Update and results
ADCP measurements at HPP Bannwil

ADCP measurements at HPP Bannwil

Results of the numerical simulation

Results of the numerical simulation

The results of the studies on downstream fish migration at HPP Bannwil indicate that no fish injury is expected during fish spillway passage since fish terminal velocity is higher than the weir velocity. However, due to back-roll in the stilling basin of spillway, fish might be exposed to increased predation or strike to baffle blocks. Numerical model results also indicate that hydraulic conditions at potential positions of a fish guidance structure in front of the turbines are unfavorable.

Analysis of the Biological Performance Assessment (BioPA) based on the CFD simulation of the turbines indicate that the average fish friendliness scores of the HPP Bannwil are 9.1 and 8.5 out of 10 at an acclimation depth of 5 m and 10 m, respectively. The former i.e. 5 m is more realistic for the HPP regarding the depth of the river. For an absolute fish survival rating, this factor plays an important role.

picture rights: Areal view of HPP © ETHZ; Pictures of the HPP © BKW Energie AG, Philipp Zinniker; trash rack and fish pass © Jeffrey Tuhtan; Updates and results © ETHZ