Assessment of fish pass design parameters to meet required hydraulic criteria

The removal of migration barriers on streams is crucial for conserving aquatic ecosystems, in line with the European Union's habitat protection goals. This study assesses the design and performance of a fish pass on the Myjava River, focusing on its compliance with project specifications and regulatory standards. Using HEC-RAS 2D software, three simulations were created based on the project’s original geometry, modelled for different flow rates: Q270, Q180, and Q90. These rates correspond to peak migration periods for various species, particularly from April to May. The simulation results were evaluated against project targets, legislative standards, and ecological assessments. The study highlighted the longitudinal slope of the fish channel, which was four times steeper than recommended by the valid Methodology, reducing the channel length to 19.4 meters instead of the recommended 81.0 meters. Post-construction evaluations and this study showed that the construction's roughness levels were not appropriate to meet the required velocity criteria, indicating that additional roughing elements were needed to achieve safe fish migration in terms of depths and velocities. While adding isolated boulders may help reduce velocity and increase depth, a milder slope would be preferable, necessitating significant reconstruction and is economically demanding.