Nature-like fish passes are commonly designed as a preferred way to overcome barriers in rivers. However, meeting the recommended hydraulic parameters for these passes can be challenging. As a result, boulders or sills are often incorporated to supplement their structure. From a hydraulic standpoint, the crucial parameters under investigation are the depths and velocities (or the corresponding velocity field). In this study, a comprehensive analysis was conducted on a full-width rock-ramp fish pass constructed in a river, specifically targeting the barbel zone. The achieved parameters were assessed through direct field measurements, complemented by mathematical modeling using 1D and 2D HEC-RAS models (version 6.3.1). For the assessment of model accuracy, the error indices root mean square error (RMSE) and mean absolute error (MAE) were used. Based on their evaluation, the 1D model provides more precise results in the assessed profiles (the RMSE for depths (m) was 0.0663 (for velocities (m∙s⁻¹) 0.293) compared to the 2D model, where the RMSE for depths (m) was 0.070 (for velocities (m∙s⁻¹) 0.398), the MAE for the 1D model for depths (m) was 0.0350 (for velocities (m∙s⁻¹) 0.185) compared to the 2D model, where the MAE for depths (m) was 0.0375 (for velocities (m∙s⁻¹) 0.274)). The findings highlight the differences in the problem-solving approaches between the models as well as the potential for simplifications in practical applications.

The accurate estimation of Manning’s roughness coefficient (n) is critical for hydraulic modeling in open channels. In fish passes designed as close-to-nature structures, this coefficient has a strong influence on the overall design and operation. This study evaluates n for the Veľké Kozmálovce fish pass using high-resolution drone imagery and image analysis techniques to determine riverbed surface characteristics and extract a grain size distribution curve. Various empirical equations based on Strickler’s formula were applied to specific grain sizes, yielding average n values of 0.036 and 0.037. Cowan’s method, which considers surface material, irregularities, vegetation, obstructions, and meandering, provided an upper-bound estimate of 0.040. However, this method is known to overestimate roughness in some cases. The Step-by-Step method, applied with hydraulic field measurements, resulted in a narrower range of n from 0.027 to 0.037. Overall, estimated values across all methods ranged between 0.023 and 0.040, reflecting the structural complexity of the fish pass, which includes boulders embedded in concrete and coarse gravel infill. These findings highlight the limitations of using generalized tabulated values for artificial channels and demonstrate that drone-based photogrammetry combined with empirical and analytical approaches can effectively capture spatial variability in hydraulic roughness.

Passing barriers on streams is nowadays an important activity for the protection and restoration of water ecosystems, and one of the goals of the European Union in the field of water habitat protection, that means barriers on waterways should be made suitable for the migration of aquatic fauna. The design and hydraulic parameters of these structures are fundamental in their design process, determining the effectiveness of fish passes in facilitating safe and efficient fish migration. These parameters should consider the specific needs and behaviours of the target fish species. It should provide suitable water depths, velocities, and facilitate fish movement upstream or downstream. The choice between technical fish passes, such as slot pass, and nature-like ones, such as rock ramps, is crucial. The selection of the right type also depends on various factors including the topography, hydrology and the species of fish being targeted. However, it is easier to achieve the parameters by technical fish pass. The proposed fish pass and its parameters were simulated by 1D mathematical model HEC-RAS. It was very difficult to design and fulfil all parameters according to the legislation. For more accurate results is recommended use of 2D or 3D models, or physical models.

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.

This research focuses on the verification of a nature-like fish pass in the Myjava River to facilitate fish migration by imitating natural environments. Utilizing the HEC-RAS 2D model, three proposed design alternatives were simulated to evaluate and improve water velocities, depths, and flooded width that were evaluated as insufficient during measurements provided by the Water Research Institute in Bratislava, Slovakia during fish pass monitoring. The study found that while boulders effectively influenced minimum water depths, they can also affect water velocities. A milder slope combined with perturbation boulders is an effective solution. The research recommended mathematical modeling tools to assess design alternatives before implementation, minimizing the economic costs associated with trial-and-error methods.