###### Abstract  
Fragmentation of rivers has a negative impact on river's ecological status which can be improved by the construction of fishways next to obstacles in rivers that prevent a free migration. Flow field characteristics are key factors in the design process of hydraulically efficient fishways--flow and turbulence patterns in a functional fishway allow fish to enter, progress through and exit with minimum time/energy expenditure. The aim of this paper is an experimental study of the flow field characteristics measured in the physical fishway model with the goal of providing information on the Reynold's shear stress distribution that would facilitate their design in accordance with the environmental requirements. The focus of the research was on the nominally hydraulically efficient con-figuration pool-type fishways--pool-orifice and vertical slot. Fishway geometry was varied for bottom slope (7.5%, 10% and 12.5%), pool length (45 cm, 60 cm and 90 cm) and orifice size (8 \(\times\) 8 cm, 10 \(\times\) 10 cm and 12 \(\times\) 12 cm) in a model scaled 1:3 to the prototype. Since [PERSON]'s shear stress has been identified as the main turbulent parameter affecting fish swimming performance and behavior, it is used as the basis for the analyses. The velocity data were collected with Vectrino ADV and processed in all three planes--streamwise, horizontal and vertical. [PERSON]'s shear stress data were analyzed according to the injury (>50 N/m\({}^{2}\)) and disorientation (>30 N/m\({}^{2}\)) biocriteria boundaries defined in the literature. The percentage of the flow field exceeding the boundaries were analyzed depending on the fishway geometry. The results obtained in this research suggest that the critical design parameter is the orifice size for the pool-orifice fishways and the pool length for the VS fishway. The Reynold's shear stress is generally the highest in the bottom layer for pool-orifice fishways and the surface layer for vertical slot fishways. ADV; turbulence; Reynold's shear stress; physical model; pool-type fishway; pool-orifice; vertical slot +
Footnote †: journal: Applied Sciences  
The river ecosystem status can be improved and in-stream connectivity partially restored by the construction of fishways next to obstacles in rivers that prevent a free migration [10]. A fishway is a low-head hydraulic structure that is hydraulically designed according to the needs of prevailing species in the ecosystem in such a way that flow conditions allow both upstream and downstream migration for a design range of flow conditions. The most distinctive classification of a fishway is technical and nature-like, with the former being an artificial channel with the most effective spatial footprint which allows hydraulic conditions for fish migration, and the latter simulating natural flow conditions utilizing a lower slope, a wider corridor and eco-friendly materials [11]. Common features of the technical fishway is a layout that consists of a series of pools formed by cross-walls on equidistant spacing positioned on a sloped bottom (Figure 1). This way the total difference between head- and tail-water level of the obstacle is reduced into a series of pools with smaller drops, providing favorable hydraulic conditions for fish migration [12]. Cross-walls used for dividing the pools can be designed with slots, weirs, orifices, notches, or a combination for connecting the successive pools in order to achieve the desired velocity, turbulence intensity, or energy dissipation. Technical fishways can be further classified into pool-type (pool-weir, pool-orifice, pool-notch, and vertical slot), Denil, and culvert fishways [13], with modifications for each of them being proposed to compensate for fish species that use a different mode of migration (e.g., [14; 15]). Flow field characteristics are key factors in the design process of hydraulically efficient fishways--flow and turbulence patterns in functional fishways allows fish to enter, progress through, and exit with minimum time/energy expenditure.