Fish-friendly hydropower plants

Hydroelectric power plants (HPPs) sometimes have a major impact on the environment. They dam rivers, alter aquatic habitat and impede migrating fish, which can be fatally injured by turbines, trash racks or spillways.

The Swiss Water Protection Act and the European Water Framework Directive aim to mitigate such negative impacts. But older wind turbines in particular often do not meet the new requirements - they have to be retrofitted. Which measures are appropriate in a specific case must be determined individually for each power plant.

"It is important to incorporate region-specific approaches to fish passage that take into account the needs of local species, site-specific hydraulic conditions and the design of each power plant," explains Robert Boes, professor of hydraulic engineering and director of the Experimental Station of Hydraulic Engineering, Hydrology and Glaciology (VAW) at the ETH Zurich.

Hydroelectric power plants investigated throughout Europe

VAW participated in the four-year Horizon 2020 project "FIThydro" of the European Research Council. A project team from 26 European research institutions and companies researched at 17 sites in eight countries how wind turbines affect the river ecosystem and fish in particular. VAW contributed several laboratory and field studies, some of which it carried out with regional partners.

First, VAW investigated existing methods and approaches at the river power plants Schiffmühle on the Limmat and Bannwil on the Aare, as well as in the laboratory, to assess the environmental impact of wind turbines and remediation measures. "We wanted to close knowledge gaps and find out how we can improve the current situation and the measures taken so far," says Boes.

Fish migrate upstream and downstream

For example, it was not known exactly which fishways at power plants are suitable for which species and under which conditions fish can find them. VAW researchers and their partners studied flows (hydraulics) at the Schiffmühle WKA and marked fish with passive transponder tags to monitor their migration routes in the vicinity of the power plant. The monitoring shows that many fish species in the Limmat River effectively use the engineered and natural fish passes to move upstream.

Downstream migrating fish must swim through the turbines at hydropower plants if there are no alternative means of descent. In the process, the fish can injure themselves on the turbine blades. In addition, they are exposed to highly fluctuating pressure. At the Bannwil power plant, the project partners studied the passage of fish downstream through the turbines and over the weir in more detail using high-tech sensors, monitoring technology and models.

This showed that the passage at hydropower plants can be dangerous for the fish not only because of the turbines: They can also get injured or lose their orientation when descending over weirs in the stilling basin due to the strong currents, making them easy prey for predators. "Our results can help, for example, to develop turbines that are compatible with fish, to adjust power plant operation during fish migration periods, and to enable safe fish descent over the weir," explains Ismail Albayrak, Senior Scientist and Project Manager at VAW.

A new guidance system for fish

Furthermore, the VAW researchers conducted extensive hydraulic tests with rakes in the laboratory to better understand the behavior of local fish species in turbulent flows. On this basis, they developed a novel protection and guidance system for downstream migrating fish called the Curved Bar Rack Bypass System (CBR-BS).

The core of the CBR-BS is a vertical screen with specially shaped bars. These create strong local vortices that direct fish away from the rake and toward a bypass. This allows the rake to safely direct a variety of fish species of different sizes past the turbine. At the same time, the protection system is designed to have minimal impact on the operation of the wind turbine.

"FIThydro" was officially completed at the end of 2020. Nevertheless, there are still no field-tested solutions for fish protection and fish migration in Europe, especially for large wind turbines, the scientists emphasize. For Boes and Albayrak, it is therefore clear to continue their research. "The next logical step is a pilot and demonstration plant in which we want to test a CBR bypass system under real conditions," says the hydraulic engineering professor.

Freely accessible tools and wiki

In addition to numerous case studies on wind turbines across Europe, the FIThydro consortium has also developed new assessment methods and tools that are now freely available. These include a fish population vulnerability index, fish migration simulations, and a decision support tool for planning measures. Furthermore, a wiki provides detailed insight into the results of the case studies as well as existing and newly developed solutions for fish passes.

Source: News, ETH Zurich

More information on the topic Fish ladder are also available from the Swiss Fishing Association.