Full scale performance evaluation of a partial-depth floating fish guidance bar rack

Migratory fish are commonly hampered or harmed by Hydropower plants (HPP). To mitigate these impacts, protective and guidance structures have been developed. Modified Bar Rack (MBR) systems, tested under laboratorial conditions, have emerged as promising structures to guide migratory fish past HPP intakes. We evaluated the downstream passage and guiding efficiency of a partial depth floating MBR (89 m long x 1,5 m deep, 50 mm bar spacing) at the River Mandal, Norway. Passage efficiency and behavioural responses were examined by tracking 118 Atlantic salmon smolts with 2D and 3D telemetry. Flow dynamics and behavioural observations were combined to relate hydrodynamic conditions to guidance efficiency.

The installation of the MBR system, combined with a flow reduction at the HPP (to 50 % of the river flow), resulted in a downstream passage efficiency of 72.5 %, representing a 22 percentage point increase compared to predictions with flow reduction alone. Success was limited by the partial-depth design, as some smolts passed beneath the rack. Of the 56 tracked smolts that interacted with the MBR (some descended and passed under), 67 % were guided. Based on smolt depth distribution and behavioural responses, guiding efficiency could be approximated at 76 %, almost matching laboratory results. The relatively high guidance was likely due to the sweeping velocity created by the design of the MBR, that also prevented rack clogging.

This study provides support for the concept of guiding fish into a migration corridor using an MBR system and presents the first full-scale evaluation of a MBR.