Velocity Fields at Horizontal Bar Racks as Fish Guidance Structures

Julian Meister,Helge Fuchs,Robert M Boes,Ismail Albayrak,Claudia Beck

doi:10.3390/w12010280

Abstract

Horizontal bar racks used as fish protection measures at hydropower plants have rapidly gained importance in recent years. Despite the large number of installed racks in Europe, systematic investigations of the hydraulic losses and velocity fields were missing. To fill these research gaps, the hydraulic performance of horizontal bar racks was systematically investigated in a laboratory flume for a large number of rack parameters and different hydropower plant layouts. The results of the head loss assessment are published in a paper entitled Head Losses of Horizontal Bar Racks as Fish Guidance Structures, whereas the present paper focuses on the velocity fields. The measurements show that the bar shape, the horizontal approach flow angle, and the clear bar spacing have only a minor effect on the velocity fields. In contrast, bottom and top overlays might enhance the fish guidance efficiency for bottom and surface oriented fish, while the asymmetric downstream velocity field can reduce turbine efficiencies. The hydropower plant layout strongly affects the approach flow field to horizontal bar racks. For block-type hydropower plants, the installation of a dividing pier or partial opening of the spillways improves the flow field for better fish guidance.

Highlights

In the last 130 years, many hydropower plants (HPPs) were built, interrupting the ecological river continuum
This paper focuses on the velocity fields up- and downstream of Horizontal bar racks (HBRs), which are important for the guidance of fish, floating debris, and sediments, as well as the turbine admission flow, while the accompanying paper focuses on the head losses of HBRs [13]
The velocity fields at horizontal bar racks were presented for four different bar shapes, with approach flow angles α = 30◦, 45◦ and 90◦, clear bar spacings sb = 10, 20, and 30 mm, and various overlay configurations

Summary

Introduction

In the last 130 years, many hydropower plants (HPPs) were built, interrupting the ecological river continuum. The European Water Framework Directive and the revised Swiss Waters Protection Act demand a free fish migration continuum. Horizontal bar racks (HBRs) in combination with a bypass are an effective measure for protecting and guiding downstream migrating fish, thereby restoring the downstream migration continuum. Current design guidelines are based on practical experience at pilot HPPs. The clear bar spacing, sb , is chosen such that the HBR acts as a physical barrier to the target fish species and sizes, typically leading to sb = 10–30 mm [1]. The approach flow angle α and the rack length are selected to match two different design criteria, which are typically based on the flow velocity components normal (Vn ) and parallel (Vp ) to the rack: Turnpenny and O’Keeffe [4] suggest designing fish guidance structures (FGSs) such that Vn does not exceed the 90th percentile of the maximum sustained swimming speed, related to the smallest fish and the lowest water temperatures during the migration period. For salmonids with a total length of TL > 10 cm, Turnpenny and O’Keeffe [4]

Methods

Results

Discussion

Conclusion