Methods to safeguard downstream migration of salmonids past hydropower structures
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Sustainable development of hydropower production must include measures to reduce the negative consequences of river fragmentations. Migratory Atlantic salmon (Salmo salar, L) is one of the species that can be severely affected since it is dependent of up- and downstream migration over hydropower structures between ocean and freshwater habitats. While general migration of Atlantic salmon past hydropower intakes is a well-studied topic, the integrated research on juveniles combining hydraulics and behavioural monitoring is not common in literature. The SafePASS project, funded by the Research Council of Norway, has the purpose of developing solutions to enhance upstream and downstream migration of fish in regulated rivers. This thesis is a part of that project that aims at investigating both general and sitespecific solutions to mitigating issues related to downstream migrating Atlantic salmon smolts at hydropower intakes in order to avoid their failed passage. Smolts that encounter intake channels to hydropower plants face multiple challenges, which are likely to affect their migration. In order to guide them towards the ocean, alternative trashrack designs could offer a solution at low operational cost. Three different configurations of angled fish-friendly trash-racks were tested with experimental hydraulics to assess their operational performance and further relate the characterized flow environment to ecological factors. The configurations differed in bar positions (vertical-angled, vertical-streamwise or horizontally oriented) and each of them were examined with two bar shapes under steady flow conditions in a flume experiment. The racks were positioned 30° to the wall towards the outlet of the flume following findings from literature regarding higher guidance efficiency. Results from published studies were adapted to design the bypass entrance, which was placed at the downstream end of the racks. Operational challenges were addressed like flow resistance, blockage ratio, diverted discharge to the bypass and maintenance complexity in order to assess the performance of the three configurations from a hydropower production point of view. In addition, the flow field were characterized using volumetric particle image velocimetry at the upstream side of the trash-racks and acoustic Doppler velocimeter at the bypass entrance to evaluate the hydraulic conditions from the perspective of the target fish species, Atlantic salmon smolts and European eel. The measured hydrodynamic parameters were linked with known responses of fish to flow parameters reported in literature in order to evaluate flow effects on fish responses. According to the results, the study promotes the horizontal bar configuration as the best candidate, following by the vertical-streamwise bar type, which are more likely to provide a suitable environment for the downstream migrating fish at an acceptable operational cost. The question of maintenance complexity has been considered but would need to be addressed in further studies for a well-functioning system. The third rack configuration provided 3-8 times larger flow resistance in addition to the questionable flow environment for target species at both investigated locations. At some river sites fish-friendly design of intake structures is not a feasible option due to reasons related to retrofitting new racks and building bypass solutions. Therefore, preventing smolts to enter into the intake channel may be the best practical solution at such cases. At the intake to the Svorkmo power plant in the River Orkla a combination of methods were used to identify and evaluate feasible mitigation measures in order to increase the rate of successful passage of smolts past the hydropower intake. In a positional telemetry study, nearly hundred smolts were tagged and tracked at the intake site firstly to explore their passage rate towards to the hydropower intake and secondly to determine the key factors of failed passages. The the telemetry and the operational and hydrological data. Meanwhile, the flow environment was characterized by using computational fluid dynamics. The combination of the findings from the route model and the hydraulic model formed the basis of the selection a number of mitigation measures that were further investigated by numerical simulations. The main aim of the study was not to perform detailed design of a solution for Orkla, but to illustrate the value of interdisciplinary methods to develop and test mitigation measures in prior to field implementation. One key to successfully mitigate the anthropogenic impact on smolt migration is to follow the “Think like a fish” concept and enhance our knowledge on fish behaviour in relation to its local environment. At the same river site on Orkla, the acquired telemetry data for each smolts were further linked with the extensive hydrodynamic parameters along their travel paths derived from hydraulic modelling. The numerical simulations were used to characterize the conditions observed during the smolt run. The individual fish tracks linked with the experienced flow variables were used to evaluate model performance of two smolt behavioural models, which were recently developed at River Mandalselva as part of the SafePASS project. The two models consider the swimming speed and the swim orientation of the smolts as a function of several hydraulic and fish related parameters. Further attempts were made to improve the performance of the two models through adjustment of velocity vectors to represent local conditions aligned with the traveling path of the smolts. Afterwards the improved models were cross-checked with data collected from River Mandalselva to evaluate their transferability across river systems. According to the findings, the swim orientation model performed significantly better at tests on both study sites, while the swimming speed model require calibration at the study site where it is intended to be used. The work in this thesis shows how a combination of hydraulics and biology can be used to solve problems that may not be solved by one discipline alone.