Research by the University of Innsbruck has shown that the use of a hybrid fish barrier paired with a pulsed electric field can distinctly increase fish protection rates at hydropower plants
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Addressing the need for cost-effective fish protection measures at hydropower plants, the hydraulic engineering research group at the University of Innsbruck invented a fish protection system called the Flexible Fish Fence.
This system consists of horizontally arranged steel cables in a slight (< 45◦) but adjustable orientation angled towards a bypass system to improve the guiding effect. However, the Flexible Fish Fence could not sufficiently fulfil fish protection requirements and acted as a physical barrier as fish protection rates primarily depended on cable clearance.
Therefore, the Flexible Fish Fence was additionally equipped with a moderate, pulsing electric field using a fish guidance and deterrence system. The horizontally arranged steel cables, in addition to their function as a physical barrier, are used as electrodes creating an electric field in the water. The intensity of the graduated electric field increases to the electrodes. Hence, an anxiety zone is followed by an avoidance zone, and finally an escape zone.
The primary objective of the study led by the University of Innsbruck was to analyse the protection capability of the fish fence considering a different combination of parameters. Two hypotheses were tested: (1) Fish protection rates increase if physical barriers are combined with pulsating electric fields serving as a behavioural barrier and, (2) bypass acceptance is not negatively influenced by applied electric fields.
The behavioural experiments were performed with wild and tagged brown trout, rainbow trout, grayling and chub. Measured body lengths were between 100 and 285 mm.
Fish protection
The study results demonstrate that the use of a hybrid barrier with horizontally tensioned steel cables paired with a pulsed electric field, can distinctly increase fish protection rates at hydropower plants. The performed ethohydraulic experiments show that the combination of the two barrier types provide the most effective fish protection efficacy. All hybrid setups show mean fish protection rates higher than 97%. The small electric field (approximately 10cm) shows slightly better results regarding the fish protection rate than the large electric field (approximately 20cm). The probability of passing through the hybrid barrier is low for both investigated cable clearances (30mm vs. 60mm) and for all investigated fish species. Thus, the spacing between the steel cables can be enlarged while maintaining high fish protection rates. The influence of the approach flow angle (20◦ vs. 40◦) on fish protection is not significant for the hybrid barrier.
Concerning the bypass frequency, a denser physical barrier (cable spacing of 30mm) induces higher bypass frequencies for all setups. One reason for this could be that a denser physical barrier increases the guiding efficiency. The authors say that further research is necessary and could include an evaluation of video data. However, the electric field does not negatively affect the bypass frequency and a small electric field results in the highest bypass frequency. An approach flow angle of 40◦ compared to 20◦ shows slightly higher bypass frequencies for conducted ethohydraulic experiments.
Due to the restricted dimensions of the 3m wide test flume, with physical barrier lengths of 3.9m and 7.3m and a water depth of 0.5m, the authors warn that the direct transfer of the results to real hydropower plants should be treated with caution. Nevertheless, they believe that based on “these promising results”, pilot projects should be carried out. These projects should include further experiments at real sites to observe the efficacy considering flow velocities, various fish species, and deeper water.
The authors conclude that although future research is necessary, “the findings of this study clearly demonstrate that the physical barrier (tensioned steel cables) combined with the behavioural barrier (pulsed electric field) is a promising facility to guarantee adequate fish protection at hydropower plants”.
This article first appeared in International Water Power magazine.