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Racetrack flumes are established experimental settings in ecohydraulics and sediment studies. Their experimental results are often coupled with numerical simulations. The two-equation turbulence closures of the Reynolds-Averaged Navier-Stokes equations are applied widely in such engineering applications. They are preferred for their ease of use and low computational costs compared with more sophisticated turbulence models involving large-scale fluvial simulations. Here, three variants of two-equation models, i.e. the standard k-ε model and two variants of k-ω models, were applied to tackle mean flow and turbulence in a racetrack flume and a river reach. Regarding model performances, we found an overall reasonable agreement between simulated and measured mean velocity values. Nonetheless, the simulated turbulent kinetic energy exhibited discrepancies to the measured values. The computational costs of all investigated model variants are comparable. Therefore, there is no significant preference among models. The findings confirm the application of these model approaches for mean flow-related investigations (e.g. habitat modeling) but suggest exercising caution for applications sensitive to turbulent processes. ARTICLE HISTORY

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The Ziller River, a tributary to the Inn in Tyrol, Austria, is affected by hydropower generation. Macroinvertebrate habitat‐preferences were investigated in the field and corresponding habitat suitability curves were applied successfully to a novel integrative assessment method based on hydrodynamic modelling. Here, a reach of the Ziller River is simulated using a three‐dimensional unsteady hydrodynamic model. The three‐dimensional simulation results are applied to a predictive macroinvertebrate habitat model. Habitat suitably indices (HSI) are calculated based on temporal alterations of bed shear stress values. Model results are free from two‐dimensional flow‐averaging approximations and demonstrate the critical information lost in lower‐order models due to simplification procedures. Shear stress values, three‐dimensional simulated velocity components, and temporal alterations of flow are provided in the current study. Considering the simulation results, the implementation of the provided supplementary simulation details in macroinvertebrate studies and the integration into further habitat investigations is discussed.

Turbulence has a significant influence on fluvial processes. Especially in the sharp bends of geometrically complex natural streams, the interaction of turbulence and flow three-dimensionalities is considerable. Here, generation and evolution of streamwise vorticity in an open channel bend is assessed by means of term-by-term experimental investigations. As a result, corresponding mechanisms are shown and analysed. In addition, based on the homogeneity term of vorticity equation, the applicability of a widely used simplified equation of streamwise vorticity is discussed. The presented comparison reaffirms the inadequacy of the equation in order to estimate near bank vorticities.