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 × 8 cm, 10 × 10 cm and 12 × 12 cm) in a model scaled 1:3 to the prototype. Since Reynold’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. Reynold’s shear stress data were analyzed according to the injury (>50 N/m2) and disorientation (>30 N/m2) 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.
Highlights
Physical alterations of rivers, e.g., river training works, water regime alterations or water abstractions, are the main source of hydromorphological pressures in rivers [1], among which large European rivers fall into those that are strongly affected [2]
The aim of this paper is to investigate the hydraulic environment of the three-dimensional flow and turbulence 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 biocriteria boundaries defined in the literature
The influence of turbulence in the fishway is analyzed through 3D scatterplots of mutual dependence between calculated values of Reynold’s shear stress (RSS) for all three planes, streamwise (τ uw ), horizontal (τ uv ), and vertical (τ vw )
Summary
E.g., river training works, water regime alterations or water abstractions, are the main source of hydromorphological pressures in rivers [1], among which large European rivers fall into those that are strongly affected [2]. Hydromorphological pressures result in the fragmentation of rivers, i.e., reducing sediment transport and disabling migratory species to travel along the river course, which in turn has a negative impact on the river’s ecological status [3], as sometimes the needs of fish populations cannot be met within a single habitat [4]. 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. 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]. 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])
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