Chapter Title

The development and evaluation of downstream bypasses for juvenile salmonids at small hydroelectric plants in France

Publication Date



bypass, downstream bypass, hydroelectric, hydroelectric plants, juvenile, salmonids, fish passage, salmon, Salmon River, efficiency, hydraulic conditions, trout, smolt, intake, canal, surface bypass, radio telemetry, telemetry, entrance, turbulence, video, bypass systems, flow patterns, mitigation

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Book Title

Innovations in Fish Passage Technology


Odeh M;

Publication Place

Bethesda, MD


American Fisheries Society


Experiments were conducted from 1992 to 1996 at four small-scale hydro-electricplants on salmon rivers in the southwest of France to relate downstream bypass efficiency tohydraulic conditions and to the behaviour of salmon (Salmo salar) and seat trout (S. trutta) smoltsin the intake canal.The maximum turbine discharge varied from 20 to 85 m3/s and the width of intakes varied from 11to 30 m depending on the plant. The surface bypasses were located laterally along the intake atone end of the trashrack. The mean bypass discharges varied from 0.4 to 4 m3/s, or an averageof 2 to 8% of the turbine discharge. The efficiency of the devices was evaluated by the markrecapturetechnique. Radio telemetry was used to monitor movement patterns of salmon andseat trout smolts in front of the intake and near bypass entrances. Depending on the site, themean bypass efficiency was found to be between 17 and 80%. Behaviour of fish in the vicinity ofthe trashrack and the bypass seemed to be largely influenced by the flow pattern. Poor hydraulicconditions (turbulence, strong acceleration, upwellings) and insufficient discharge were identifiedthrough direct and video observations as being responsible for many aborted passages at thebypass entrances.The results suggest that siting of surface bypass systems must take into account flow patterns inboth the trashrack area and intake canal. It is suggested that surface bypasses associated withexisting trashracks may be an acceptable mitigation technology at small-scale hydroelectricprojects where it is not necessary to guarantee a highly efficient downstream passage protection.

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