Multi slot versus single slot pool-type fishways: A modelling approach to compare hydrodynamics

Ecological Engineering 122 (2018) 197–206

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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng

Multi slot versus single slot pool-type fishways: A modelling approach to compare hydrodynamics

T

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Ana L. Quaresmaa, , Filipe Romãoa, Paulo Brancob, Maria Teresa Ferreirab, António N. Pinheiroa a b

CERIS – Civil Engineering for Research and Innovation for Sustainability, Instituto Superior Técnico (IST), Universidade de Lisboa, Lisboa, Portugal CEF – Forest Research Centre, Instituto Superior de Agronomia (ISA), Universidade de Lisboa, Lisboa, Portugal

A R T I C LE I N FO

A B S T R A C T

Keywords: Fish passage Vertical slot fishway Multi slot fishway Holistic fishway CFD Ecohydraulics Discharge efficiency

Adequately designed fishways mitigate the negative effects of barriers in rivers, restoring longitudinal connectivity. To do so, they must present suitable hydraulic conditions, with velocity and turbulence fields adequate for multiple fish species. In the present study, numerical modelling was used to compare the hydrodynamics of two widely used vertical slot fishways (VSF) configurations and three multi slot fishway (MSF) configurations. The MSF configuration needs a lower discharge to operate than the VSF, while keeping similar flow depths, which is vital in regions where water availability is limited, such as most Mediterranean regions. By reducing the discharge, the velocity, turbulent kinetic energy, and Reynolds shear stress values in the studied MSF configurations are much lower than the values for VSF configurations. Thus, besides requiring smaller discharges than similar VSF geometries, the MSF additionally could be less selective for fish species, particularly smaller-sized individuals, and individuals with weaker swimming capacities, and a better option for a multiple-species fishway.

1. Introduction

salmonids in overcoming the barriers. One possible explanation for this inefficiency is that the design of fishways has often targeted larger sized high priority and commercially important species, such as salmonids (Katopodis and Williams, 2012; Roscoe and Hinch, 2010), which have strong swimming capacities (Webb, 1975) contrarily to smaller sized and non-salmonids, which have weaker swimming and jumping capacities and may respond to hydraulic conditions differently from salmonid species (Branco et al., 2013; Santos et al., 2012; Silva et al., 2012; Williams et al., 2012). Although, potamodromous species have less migratory requirements than diadromous species, there is still a need for a large part of the population to move along the river, therefore the hydraulic conditions in fishways must be equally carefully designed (Porcher and Travade, 2002). In fact, the transfer of salmonid fishway designs to non-salmonid rivers has resulted in poor success (Branco et al., 2013; Katopodis and Williams, 2012; Mallen-Cooper and Brand, 2007). Non-salmonid species play a substantial role on fish assemblages, and free instream movement is crucial for their survival (Lucas et al., 2000). It is increasingly important to consider the swimming capacities and behaviour of the whole fish community as the goal of conservation activities shifts from a maximum-sustainable-yield to a biodiversity-protection approach (Noonan et al., 2012; Oldani et al., 2007). To achieve the goal of good ecological status, the European

Fishways are hydraulic structures that mitigate the negative effects of anthropogenic barriers in rivers by allowing fish to move upstream and downstream, thus restoring longitudinal connectivity. Pool-type fishways are one of the oldest, most common and widespread type of fishways (Clay, 1995; Larinier, 2002a; Santos et al., 2012). Larinier (2008) refers to the alternate deep notch and submerged orifice design as the most commonly used at small-scale hydropower plants in France. In Portugal the alternate surface notch and submerged orifice design is the most common pool-type fishway (Santos et al., 2012). These configurations need relatively low discharge to operate. In Mediterranean regions and other similar river systems that have extended low flow periods (Gasith and Resh, 1999) this may be a significant advantage. However, according to FAO/DWK (2002), vertical slot fishways (VSF) are the best type of technical fishway when multiple species are present and should be given preference over other technical fish passes. VSF allow fish to negotiate the fishway at their desired depth, and are less prone to clogging. On the other hand, VSF usually requires more water to operate than surface notch and submerged orifice design. In a review of fish passage efficiency in existing fishways, Noonan et al. (2012) found that non-salmonid species were less successful than

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Corresponding author. E-mail addresses: [email protected] (A.L. Quaresma), fi[email protected] (F. Romão), [email protected] (P. Branco), [email protected] (M.T. Ferreira), [email protected] (A.N. Pinheiro). https://doi.org/10.1016/j.ecoleng.2018.08.006 Received 5 March 2018; Received in revised form 2 August 2018; Accepted 9 August 2018 0925-8574/ © 2018 Elsevier B.V. All rights reserved.

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discharge, maximum flow velocities, and mean turbulent kinetic energy by up to 44%, 27%, and 19%, respectively, when compared to a standard VSF. Therefore, its configuration looks promising for use in regions with water scarcity problems by reducing the fishway discharge while maintaining its potential effectiveness, particularly to species with lower swimming abilities. There are currently, and since 2011, more than a dozen fishways of this type in operation, mainly in Austria (“Maba Fishpass – Our projects”, 2018). The main goal of this study is to compare the hydrodynamics and assess the hydraulic suitability for multiple fish species of two commonly used VSF configurations, and of the MSF configuration. For this purpose, a computational fluid dynamics (CFD) model was used to examine the flow field characteristics and turbulence of the two VSF and of three MSF (three variants of the Maba) configurations, with the same slope, water depth, pool dimensions and slot size.

Water Framework Directive (WFD, European commission, 2000) requires migration possibility for fish populations, even the ones with lower swimming abilities. To meet this current goal, multiple species individual features of fish behaviour, ecology and fishway hydrodynamics must be accounted for to improve the design of fishways (Pavlov et al., 2008; Williams et al., 2012). Developing multi-species fishways with dimensions and hydraulic characteristics adequate for native freshwater species with different morphological and ecological traits is thus vital. Designing fishways that offer suitable hydraulic conditions with velocity and turbulence fields adequate for multiple fish species is rather challenging. Numerous studies emphasize the role of hydraulic variables, such as velocity distribution, turbulent kinetic energy, shear stress, vorticity, and turbulence intensity in the efficiency of fishways (e.g. Enders et al., 2003; Liao et al., 2003; Katopodis, 2005; Katopodis and Williams, 2012; Silva et al., 2011, 2012; Wang et al., 2010). Additionally, developing effective low discharge fishways is of utmost importance in Mediterranean type river systems where water may be scarce to fulfil the needs. Modelling and testing new fishway designs are essential to reduce the operational costs without compromising its effectiveness for the target species (Bombač et al., 2017; Branco et al., 2013). Furthermore, to help designers in achieving maximum passage scenarios, research on flow fields which match fish stimuli required for fish to approach, enter and ascend a structure is needed (Silva et al., 2017). VSF is considered one of the best type of technical fishways for potamodromous and diadromous species (Clay, 1995; FAO/DVWK, 2002). However there is still room for improvement, to accommodate a wider range of species (Romão et al., 2017; Wang et al., 2010). Tauber and Mader (2010) and Mader and Tauber (2011) developed a multi slot fishway (MSF), designated by Maba, that reduces the discharge needed to operate. The Maba consists of alternating multi slot structures constituted by two slots separated by a local widening extending vertically over the full height of the baffles (Mader and Tauber; 2011), with the overall head drop between the pools divided into two when comparing with a standard VSF. According to Tauber and Mader (2010), while still maintaining the water depth in the pools, the MSF reduces the

2. Materials and methods 2.1. Geometry In this study, five fishway configurations were simulated and compared: two widely used VSFs, and three MSF. Fig. 1 shows the pool geometries of the five fishway configurations. The VSF1 configuration includes a central and a lateral baffle (Fig. 1a), whereas VSF2 has only a lateral baffle (Fig. 1b). These configurations are based, respectively, on design 1 and 11 included in Rajaratnam et al. (1992) study. The MSFs configurations geometries differ in the inner wall of the pool entrance chamber (Fig. 1): MSF1 inner wall has a small baffle to deflect the flow (Fig. 1c); MSF2 inner wall has no baffle (Fig. 1d); and MSF3 has a shorter inner wall and no baffle (Fig. 1e). 2.2. Experimental procedure The VSFs and MSF1 configurations (Fig. 1) were tested in a full-scale facility located at the National Laboratory for Civil Engineering (LNEC). The steel frame flume, with glass sidewalls, is 10 m long, 1 m wide, and

Fig. 1. Vertical slot fishways (VSF) and Multi slot fishway (MSF) configurations pool detail: (a) VSF1; (b) VSF2; (c) MSF1; (d) MSF2; (e) MSF3; (dimensions in meters). 198

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free surface dynamics, the volume of fluid (VOF) method is applied (Hirt and Nichols, 1981). Flow Science (2016) presents additional details regarding the theoretical and numerical fundamentals of FLOW3D®, which has been used in recent years in fishway research (e.g. Duguay and Lacey, 2016; Feurich et al., 2012; Kim et al., 2012; Kolden et al., 2016). The five fishways geometries (Fig. 1) were generated using AutoCAD and were imported into the code as stereolithography (STL) files. The computational domain was discretized using multi-block grids to optimize the mesh according to the simulated geometry. The grid blocks were created with uniform cells size. For VSF1 and VSF2 the mesh was constituted by eight grid blocks: one, which contained the entire fishway geometry, with a uniform mesh size of 0.04 m, and the other seven nested blocks, two with half the mesh size of the overall block and five with a quarter of the mesh size of the overall block. One of these blocks contained pools 2 to 4, while another one contained the upstream cross-wall, and the other five blocks contained the slots area (Fig. 2). For MSF1, MSF2, and MSF3 the mesh included three grid blocks: one, which contained the entire fishway geometry, with a uniform mesh size of 0.04 m, and two nested blocks with half the mesh size of the overall block. One of these blocks contained pools 2 to 4, and the other one contained the upstream pool cross-wall and pool entrance chamber (Fig. 2). The mesh sizes were of similar size or smaller than those adopted in previous studies (e.g. Fuentes-Pérez et al., 2018; Marriner et al., 2014; Quaranta et al., 2017) For VSF1, VSF2, and MSF1 one coarser mesh (m1) and a refined one (m2), halving the size of the mesh cells (doubling the number of cells in each direction), were tested to check the influence of the grid resolution on the results. Since for MSF2 and MSF3 only some minor alterations were introduced in the geometry (Fig. 1) and the mesh was the same of the MSF1 model, only the coarser mesh was used. Table S1 (in Supplementary material) shows the characteristics of the numerical meshes used in this study. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.ecoleng.2018.08.006. The acceleration of gravity was applied in the negative z-direction and in the positive x-direction so that the x-axis was parallel to the fishway bottom. The upstream and downstream boundaries were specified as pressure boundary conditions, based on the water depths observed experimentally (for the MSF fishways that were not tested experimentally – MSF2 and MSF3 – these boundary conditions were chosen to be the same as MSF1). The side walls and the bottom were modelled as no-slip boundaries, and, at the top, a symmetry boundary condition (zero value for normal velocity, zero gradients for the other quantities) was applied. The initial pressure was set to the atmospheric pressure.

1.2 m high, has adjustable slope and is equipped with a recirculation circuit. The tested fishway (a full-scale model) was composed of six pools, four of which complete, with 1.85 m long and 1.0 m wide, divided by five wood cross-walls (22 mm thick). The fishway slope was set at 8.5%, which is within the common range of slopes for this type of fishway (FAO/DVWK, 2002; Larinier, 2008). The flow discharge was controlled by a pump frequency converter, and measured by an electromagnetic flow meter installed in the recirculation pipe. The downstream flow depth was controlled by a slot gate. The three configurations maintained the same slot width, pool mean water depth (hm) and head drop between pools (Δh), but different flow discharges (Q). For all the configurations, the mean water depth in the pools and the head drop between pools were 0.8 m and 0.16 m, respectively. For MSF1, this head drop is divided into two, becoming approximately 0.08 m per slot since this configuration presents a twofold number of slots compared with the VSFs configurations (Fig. 1). The tested discharges were 110, 81, and 56 Ls-1, respectively for VSF1, VSF2, and MSF1. A Vectrino Acoustic Doppler Velocimeter (ADV) (Nortek AS) was used to measure the three-dimensional instantaneous water velocity in the VSFs configurations, since this instrument is capable of accurately estimating mean flow field, velocities, and turbulence in pool-type fishway flows (Quaresma et al., 2017). These measurements were performed for both VSF configurations in the second pool (in the upstream direction) in two planes parallel to the fishway bottom at flow depths 0.50 m, and 0.625 m, corresponding, to 62.5 and 78% of hm, respectively. These measurements are presented in more detail in Romão et al. (2017). ADV data was post processed for despiking and noise reduction. Spikes were removed using phase-space threshold despiking method (Goring and Nikora, 2002) and replaced by linear interpolation. Doppler noise reduction was then applied through the method of Hurther and Lemmin (2001). The coordinate system defined for this study has its longitudinal x-axis parallel to the bed and the flume axis, the y-axis in the transversal direction, and the z-axis perpendicular to the bed. The instantaneous velocity components u, v, w, correspond to the x-, y- and z-axis, respectively. 2.3. Numerical modelling The flow field numerical modelling was performed with FLOW-3D®, a CFD commercial software that solves the governing equations of fluid motion in a Cartesian staggered grid using finite volumes method. One of the major features of FLOW-3D® is the Fractional Area/Volume Obstacle Representation (Hirt and Sicilian, 1985), named FAVORTM method. This method is used to represent obstacles through fractional areas and volumes in a fixed orthogonal grid. To accurately capture the

Fig. 2. Vertical slot fishways (VSF) and Multi slot fishway (MSF) configurations plan view– mesh blocks used: (a) VSF1; (b) VSF2; (c) MSF1; (d) MSF2; (e) MSF3 (the green points are the ones where LES Index of Resolution Quality (LES IQ) was computed). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 199

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The generalized minimum residual (GMRES) implicit solver, which is the default solver of FLOW-3D®, was used (Yao, 2004). GMRES is a non-stationary iterative method, having iteration-dependent coefficients (Barrett et al., 1994), highly accurate and efficient for a wide range of problems (Flow Science, 2016). The roughness effect was investigated by testing a range of Manning surface roughness coefficients, n, that cover the experimental facility materials possible values. FLOW3D® sets the wall roughness using the roughness height, which was computed from the Strickler formula considering n = 0.01 m−1/3 s−1 (Souders and Hirt, 2002). The selected value led to the best agreement between simulated and measured discharges and flow depths. The second order monotonicity preserving method and the large-eddy simulation (LES) model were used. The momentum advection method and turbulence model choices were made following the results obtained in a previous study, where several model parameters were evaluated, and a quantitative and qualitative comparison of experimental and numerical model results were performed for a pool-type fishway with bottom orifices flow (Quaresma and Pinheiro, 2014, 2015). The turbulence model chosen was the large-eddy simulation (LES) model and simulations were performed for a number of time steps enough to achieve a statistically stationary solution, and obtain converged timeaveraged values. All the numerical results refer to Pool 3, which is used as a reference for a typical pool. Volume averaged and maximum values of mean velocity magnitude (U¯ ), turbulent kinetic energy (k), and parallel to the bottom component of the Reynolds shear stress (τuv) were evaluated for this pool. Considering mean velocity magnitude the voN − 1 − lume averaged values were computed as Uaverage = V ∑1 UdV i i , where total Vtotal is the total volume of water in pool 3, N is the total number of mesh − cells of pool 3, Ui is the mean velocity magnitude in each mesh cell of pool 3, and dVi is the volume of each mesh cell of pool 3. The maximum − values were computed by determining the maximum value of Ui. This procedure was similarly applied to k and τuv. The pool mean water depths were obtained by averaging the flow depths of all the 3rd pool area cells. The relative differences were computed by dividing the difference between each configuration and VSF1 values by the latter ones. VSF1 was used as a reference, since this is the most used and widespread configuration. Based on the critical swimming speed of different fish species (Alexandre et al., 2016; Almeida et al., 2007; Mateus et al., 2008; Quintella et al., 2010; Romão et al., 2012) and the thresholds values found by Silva et al. (2011) for k and τuv, the percentages of pool 3 volume with values lower than those thresholds were determined.

Table 1 Vertical slot fishways (VSF) and Multi slot fishway (MSF) configurations comparison between measured and FLOW-3D® discharges and pool mean water depths. Fishway configuration

Source

Discharge (L s−1)

Relative difference (%)

Pool mean water depth (m)

Relative difference (%)

VSF1

Experimental Numerical model Experimental Numerical model Experimental Numerical model

110 112 81 80 56 58

2.3

0.80 0.80 0.80 0.81 0.80 0.83

0.1

VSF2 MSF1

−1.3 3.3

1.8 4.2

numerical model accurately reproduced the discharges and pool mean water depths verified in the laboratory experiments, as shown in Table 1. − When comparing U , k and τuv obtained with the ADV measurements and with the numerical model, for VSF1 and VSF2 configurations, mean absolute differences (MAD) of 0.1 m s−1, 0.03 m2 s−2 and 7 Pa, respectively, were observed for both configurations and meshes. These differences are within the same order of magnitude of differences observed in previous studies (Fuentes-Pérez et al., 2018). Moreover, for both VSF configurations the maximum and average mean velocity magnitudes differ by a maximum of 5% from the experimental values. Fig. 3 shows a comparison of the mean velocity magnitudes and streamlines measured with ADV and obtained with FLOW-3D® for the VSF configurations for the coarser mesh (m1). The numerical results agree with the experimental data for both VSF configurations and the circulation pattern and the location of the recirculation zones are well predicted by the numerical model. Since both meshes showed sufficient grid resolution and similar agreement with experimental data, in this paper only the results obtained with the coarse mesh size (m1) are presented. Given these verification and validation results and the fact that this model had previously been thoroughly verified and validated for a pooltype fishway with bottom orifices flow (Quaresma and Pinheiro, 2014, 2015), which presents a more complex flow field, this numerical model was considered adequate for the objectives of this study. 3. Results

2.3.1. Numerical model verification and validation To verify the numerical model quality, the LES Index of Resolution Quality (LES IQ) proposed by Celik et al. (2005) was computed in 90 points (shown in Fig. 2) for VSF1, VSF2 and MSF1 configurations. The coarser mesh (m1) has an average LES IQ of 0.81 for all configurations and the finer mesh (m2) has an average LES IQ of 0.91, 0.92 and 0.92 for VSF1, VSF2 and MSF1 configurations, respectively. According to Pope (2000), a good LES should have a LES IQ higher than 0.8, which means that 80% of the TKE was resolved. Celik et al. (2005) consider that a LES IQ of 0.75 to 0.85 may already be considered adequate for most engineering applications that typically occur at high Reynolds numbers. Thus, the values obtained for both meshes (m1 and m2) indicate that sufficient grid resolution was used. It should, however, be emphasized that this index is a verification index which only assesses mesh resolution quality. To assess model accuracy, a comparison with experimental data is still necessary. Hence, to assess the ability of the numerical model to reproduce the hydrodynamics of VSF and MSF flow and validate model accuracy, the numerical results were compared with the measurements obtained from the laboratory experiments. Table 1 presents VSF1, VSF2 and MSF1 measured and predicted discharges and pool mean water depths and also the relative differences, computed by dividing the difference between numerical and experimental values by the latter ones. The

Table 2 shows the discharges, the pool mean water depths, the volume averaged and the maximum mean velocity magnitudes, turbulent kinetic energies and τuv component of Reynolds shear stress obtained with the numerical model for the studied fishway configurations. The MSF configuration operates with a smaller discharge (almost 50% less when compared to VSF1). The MSF1 and MSF2 operate with similar discharges, while MSF3 needs a larger value. On the other hand, the pool mean water depths are similar for all the studied configurations with small differences among them, thus reproducing well what was fixed in the performed experimental tests. MSF configurations also present smaller values of velocity, turbulent kinetic energy and Reynolds shear stress magnitudes. VSF1 configuration presents the highest volume averaged mean velocity magnitude, turbulent kinetic energy and Reynolds shear stress magnitudes of the studied configurations. VSF2 volume averaged mean velocity magnitude, turbulent kinetic energy and Reynolds shear stress are 29%, 23%, and 20% lower, respectively. For all the analysed volume averaged hydraulic parameters MSF configurations present values more than 50% lower, with the exception of MSF3 volume averaged turbulent kinetic energy that is 44% lower than VSF1 value. Regarding the maximum values, the differences are not so large for the velocity magnitude, with VSF2 showing a value 16% lower than 200

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Fig. 3. Vertical slot fishway configurations (VSF) mean velocity magnitude and streamlines (left: Acoustic Doppler Velocimeter (ADV), right: FLOW-3D mesh m1): (a) and (b) VSF1 at z = 0.50 m; (c) and (d) VSF2 at z = 0.50 m.

configurations present lower velocities than the VSFs ones with the highest velocities occurring along the slots, the pool entrance chambers walls and the right wall of the pool. The two VSF configurations have similar flow patterns, with two large recirculation regions located on either side of the jet. In both configurations, the jet is directed to the sidewall and part of the flow is directed to the next slot, whereas the remaining flow creates two recirculation regions, a smaller counter clockwise recirculation zone in the upstream part of the pool and a larger clockwise recirculation zone limited by the convex portion of the jet. Regarding the MSF configurations, a recirculation zone appears at the pool entrance chambers: a clockwise swirl is formed in the right-hand side pool entrance chambers, whereas a counter clockwise swirl is formed in the left-hand side pool entrance chambers. The flow passing through the slot is partly directed towards the opposite pool entrance chamber, where it flows to

VSF1, and the MSF configurations values being around 37–45% lower. As to the maximum turbulent kinetic energy, VSF2 shows the maximum value with a higher value (4%) than VSF1, MSF1 presents the lowest maximum value. Considering the maximum Reynolds shear stress, VSF2 has the highest value, although the difference to VSF1 is small (1%). MSF1 and MSF2 present values more than 60% lower than VSF1, whereas MSF3 has a maximum value 35% lower. Fig. 4 shows the mean velocity magnitude and the 2-D time-averaged streamline patterns of the studied fishway configurations at z = 0.4 m (50% hm). VSF1 presents the highest velocities near the slot and central baffle with the velocity decreasing as the jet propagates in the pool, with the lower velocities occurring in the recirculation zones. VSF2 has slightly lower mean velocity magnitudes than VSF1 with the higher velocities occurring between the slot and the lateral baffle. The MSF

Table 2 Vertical slot fishways (VSF) and Multi slot fishway (MSF) configurations Discharges (Q), pool mean water depths (hm), volume averaged and maximum mean velocity − magnitudes (U ), turbulent kinetic energy (k), and parallel to the bottom component of the Reynolds shear stress (τuv). Fishway configuration Q hm ± SD

− Uaverage − Umax

k average

kmax |τuv |average

|τuv |max

Magnitude RD (%) Magnitude RD (%) Magnitude RD (%) Magnitude RD (%) Magnitude RD (%) Magnitude RD (%) Magnitude RD (%) Magnitude RD (%)

(L s−1) (m) (m s−1) (m s−1) (m2 s−2) (m2 s−2) (Pa) (Pa)

VSF1

VSF2

MSF1

MSF2

MSF3

112.5 – 0.80 ± 0.07 – 0.58 – 2.2 – 0.054 – 0.34 – 10.3 – 145 –

80.0 −28.9 0.81 ± 0.06 1.2 0.41 −29.3 1.9 −16.4 0.042 –23.0 0.35 3.8 8.2 −20.3 147 1.3

57.9 −48.5 0.84 ± 0.08 5.0 0.26 −55.4 1.4 −37.1 0.026 −52.9 0.12 −65.8 5.1 −50.3 52 −64.3

57.5 −48.9 0.84 ± 0.08 5.2 0.26 −56.0 1.3 −41.0 0.025 −54.5 0.18 −48.3 4.6 −55.5 45 −68.8

63.0 −44.0 0.84 ± 0.08 4.9 0.24 −59.5 1.2 −45.3 0.031 −43.6 0.20 −42.2 5.1 −50.2 94 −35.0

− − RD (relative difference to VSF1); Q (discharge); hm (pool mean water depth); SD (standard deviation); Uaverage (volume averaged mean velocity magnitude); Umax (maximum velocity magnitude); kaverage (volume averaged turbulent kinetic energy); kmax (maximum turbulent kinetic energy); |τuv |average (volume averaged Reynolds shear stress parallel to the bottom component); |τuv|max (maximum Reynolds shear stress parallel to the bottom component). 201

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Fig. 4. Vertical slot fishway (VSF) and Multi slot fishway (MSF) configurations mean velocity magnitude and 2-D time-averaged streamline patterns at z = 0.40 m (50%hm): (a) VSF1; (b) VSF2; (c) MSF1; (d) MSF2; (e) MSF3.

Fig. 5. Vertical slot fishways and Multi slot fishway configurations turbulent kinetic energy at z = 0.40 m (50%hm): (a) VSF1; (b) VSF2; (c) MSF1; (d) MSF2; (e) MSF3. 202

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passage time through the fishway might increase as this may also be determined by the number of pools to be negotiated. As observed by White et al. (2011), for a given water level difference across a fishway, a higher number of slots and pools, slows the ascent rates of larger sizeclasses fish. As shown in this study, and also by Romão et al. (2017) different slot configurations may, for the same slopes, water depth, pool dimensions and slot size lower the discharge in almost 30%. Bombač et al. (2017) showed that optimizing the slot layout by changing, for example, the angle of deflection between the small and large baffles in a VSF could lead to a reduction of up to 42% in the fishway discharge. Even so, with the studied MSF configurations a reduction of more than 48% in the fishway discharge can be achieved, while keeping the same slope, water depth, pool dimensions and slot size. Additionally, the studied MSF configurations lowered the maximum velocities, turbulent kinetic energy, and parallel to the bottom Reynolds shear stresses in up to 45, 65 and 68%, respectively. According to Tudorache et al. (2008), the critical swimming speed (Ucrit) data can define acceptable velocity limits for the target species in fishways, although it cannot be used to estimate maximum allowable velocities. Owing to increasing river degradation, Iberian endemic freshwater fish are among the most threatened species in the world (Santos et al., 2017). Considering the mean critical swimming speed of four Iberian native cyprinid species, the Iberian barbel (Luciobarbus bocagei, Steindachner, 1864) – 0.81 m s−1 (Mateus et al., 2008), the Northern Iberian chub (Squalius carolitertii, Doadrio, 1988) – 0.54 m s−1 (Romão et al., 2012), the Tagus nase (Pseudochondrostoma polylepis, Steindachner, 1864) – 0.78 m s−1 (Romão et al., 2012), and the Southern straight-mouth nase (Pseudochondrostoma willkommii, Steindachner, 1866) – 0.54 m s−1 (Alexandre et al., 2016) – the tested MSF configurations seem more suitable for those cyprinid species comparatively to the VSF configurations, especially to VSF1. Since velocities in the pools should be lower than the critical swimming speed of the target species (Romão et al., 2012), when considering the weak swimmers (the Northern Iberian chub and the Southern straight-mouse nase, which is listed as Vulnerable in the IUCN Red List of Threatened Species), more than 90% of the MSFs pool volume (Table 3) is favourable for these species swimming capacities while only 60 and 79% of VSF1 and VSF2 pool volume, respectively is favourable for these species. When considering the stronger swimmers (the Iberian barbel and the Tagus nase) the percentage of the pool volume that presents velocities lower than the critical swimming speeds of these species is not so different, when comparing the MSFs (more than 96%) to VSF2 (more than 90%). On the other hand, VSF1 presents a significantly smaller value (75%). Fish passage criteria for anguilliform species, considered weak swimmers (Almeida et al., 2007; Porcher, 2002; Quintella et al., 2009, 2010), like the European eel (Anguilla anguilla, Linnaeus, 1758), and lamprey species, like the river lamprey (Lampetra fluviatilis, Linnaeus, 1758), and the sea lamprey (Petromyzon marinus, Linnaeus, 1758) are still poorly developed (Kemp et al., 2011). All these species are of conservation concern, with the European eel (Anguilla anguilla, Linnaeus, 1758) being classified as Critically Endangered by the IUCN Red List of Threatened Species. Although special technical fishway facilities were developed for eels, in dams and weirs (FAO/DWK, 2002), in places where these facilities are absent, the pool-type fishways should be suitable for these species, given the importance of longitudinal connectivity to these diadromous species, with obstacles to migration being considered as one of the main causes for the populations decline (Mateus et al., 2012). The consideration of these species, of conservation importance, on the development of appropriate fishways should therefore be considered a priority. Considering the critical swimming speed of two distinct life stages of European eels, the yellow-phase eels – 0.43 m s−1 (Quintella et al., 2010), and the silver-phase eels – 0.66 m s−1 (Quintella et al., 2010) – the tested MSF configurations seem more suitable for this species life stages, having velocities lower than the critical swimming speed in more than 85 and 93% of the pool

Table 3 Vertical slot fishways and Multi slot fishway configurations % of pool volume − where mean velocity magnitude (U ), turbulent kinetic energy (k), and parallel to the bottom component of the Reynolds shear stress (τuv) are lower than a given threshold value. Fishway configuration

VSF1

VSF2

MSF1

MSF2

MSF3

VU− (%)

U ⩽ 0.43 m s−1

48

65

85

85

87

U ⩽ 0.54 m s−1

60

79

90

90

91

U ⩽ 0.66 m s−1

70

87

93

94

94

U ⩽ 0.78 m s−1

75

90

96

96

96

U ⩽ 0.81 m s−1

76

91

96

96

97

U ⩽ 1.00 m s−1

82

94

99

99

99

k ⩽ 0.05 m2 s - 2 |τuv | ⩽ 10 Pa

59

73

94

94

85

69

77

83

85

85

Vk (%) V|τuv | (%)

VU− (% of pool volume where mean velocity magnitude is lower than a given threshold value); Vk (% of pool volume where turbulent kinetic energy is lower than a given value); V|τuv | (% of pool where Reynolds shear stress parallel to the bottom component ( τuv ) is lower than a given value).

the next pool. Another part is directed to the left sidewall, creating a recirculation region which occupies most of the pool, and the rest is directed to the opposite cross wall, where it creates a swirl with reverse rotation direction. This swirl is smaller, although it occupies almost all the width of the pool between the sidewall and the pool entrance chamber inner wall. Fig. 5 shows the turbulent kinetic energy of the studied fishway configurations at z = 0.40 m (50% hm). VSFs present the highest turbulent kinetic energy values along the jet boundary, which rapidly dissipates as the jet penetrates into the pool. The MSF configurations present lower turbulent kinetic energy than the VSF ones, with MSF3 presenting the highest values. Table 3 shows the percentages of pool volume, where an analysed − hydraulic parameter (mean velocity magnitude (U ), turbulent kinetic energy (k), and parallel to the bottom component of the Reynolds shear stress (τuv)) is lower than a given threshold value. In the MSFs more than 85% of the pool volume has mean velocity magnitudes lower than 0.43 m s−1 whereas this percentage drops to 48 and 65% in VSF1 and VSF2, respectively. VSF1 has 18% of the pool volume with mean velocity magnitudes higher than 1.0 m s−1 and in VSF2 this value drops to 6%. In MSFs 1% of the pool presents velocities higher than 1.0 m s−1. Regarding turbulent kinetic energy, 94% of MSF1 and MSF2 pool volume presents values lower than 0.05 m2 s−2. This value decreases to 85% in MSF3 and to 59 and 73% in VSF1 and VSF2, respectively. More than 83% of the pool volumes of the MSF configurations present |τuv| magnitudes lower than 10°Pa, this percentage drops to 69 and 77% in VSF1 and VSF2, respectively. 4. Discussion The VSF was originally developed for salmonid passage. However, this design is also adequate for other species, including cyprinids (Clay, 1995; Romão et al., 2017; White et al., 2011). Nevertheless, there is still room to improve these devices, as well as, reduce their operational costs, by reducing the discharge, thus developing more cost-effective fishways (Romão et al., 2017, 2018; Wang et al., 2010; White et al., 2011). Within the more widespread VSF designs, it is possible to achieve discharge and velocity reductions, for example by means of lower slopes (Barton et al., 2009), lower length to width (L/B) ratios (Bermúdez et al., 2010; Puertas et al., 2012), slot size and layout modifications (Bombač et al., 2017), and slot configuration (Romão et al., 2017). However, if lower slopes, or lower L/B ratios are adopted although reduced discharge, velocity and turbulence, is achieved, more space and/or pools (with increased numbers of pools for a given height, or a costly increase in pool volume) are needed for the construction of the fishway which is more expensive and not always possible. Also 203

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with bottom orifices, Iberian barbel used low turbulent kinetic energy locations (< 0.05 m2 s−2) as resting areas before subsequent efforts to negotiate areas of higher velocity and turbulence. Thus, a large portion of the pool should have turbulent kinetic energy values lower than 0.05 m2 s−2. Concurrently, Smith et al. (2005) also showed that juvenile rainbow trout (Oncorhynchus mykiss) chooses locations with lower turbulence levels. Additionally, Reynolds shear stress has a strong impact on fish swimming performance and stability (Odeh et al., 2002; Silva et al. 2011) and, at extremely high levels, it may cause severe injury or mortality (Cada et al., 1999; Odeh et al., 2002). Silva et al. (2011) also found that the τuv component of Reynolds shear stress was the hydraulic parameter that most strongly affected fish movements, particularly smaller-size individuals, and fish used mainly areas with low |τuv | (< °10°Pa). Turbulent environments are also likely to be particularly challenging for anguilliform species such as eels and lampreys (Liao, 2007; Kemp et al., 2011). Considering these previous studies with fishes, the MSF configurations seem more suitable than the VSF configurations for anguilliforms, smaller sized individuals and species with weaker swimming capacities given the lower values of the turbulent parameters (Table 2). Also, when considering the values observed by Silva et al. (2011), the MSF configurations present, comparatively to VSF configurations, a much larger pool volume with the turbulent kinetic energy values lower than 0.05 m2s-2 and |τuv | values lower than 10 Pa, thus, being more suitable for cyprinid species, especially for smaller sized and individuals with weaker swimming capacities. Comparatively to the VSF configurations, MSF allows fish to negotiate the fishway with lower energy expenditure, given the lower maximum and average values of velocities and turbulent parameters. Thus MSF configurations seem less selective being more suitable for multiple fish species, especially for smaller sized and individuals with weaker swimming capacities. Furthermore, the modelled MSF configurations, for the same slope, water depth, pool dimensions and slot size need a much smaller discharge to operate than the VSF configurations which is of utmost importance in Mediterranean type river systems where water may be scarce to fulfil the needs. Further studies, using numerical modelling, should be performed considering higher slopes for the MSF configuration, since, by increasing the slope of MSFs smaller fishways could be attained with maximum values of velocities and turbulence similar to the VSF.

volume (Table 3), respectively. The VSF1 configuration has less than half of the pool volume (48%) with velocities lower than the critical swimming speed of the yellow-phase eels. According to Quintella et al. (2009), pool-type fishways can maintain a high rate of sea lamprey (Petromyzon marinus, Linnaeus, 1758) successful passage with an average velocity lower than 1.0 m s−1, the critical swimming speed of this specie (Almeida et al., 2007) and with the maximum velocity of 3.9 m s−1. All configurations have maximum velocities lower than this limit, with the MSF configurations having almost all the pool volume (> 99%) with characteristics favourable for a successful sea lamprey passage. According to Larinier (2002b), for most species, a velocity of the order of 1 m s−1 should be the minimum at the entrance to the fishway. Almeida et al. (2007) when evaluating sea lamprey (Petromyzon marinus, Linnaeus, 1758) swimming performance observed that this specie showed the strongest reotaxic response for velocities between 0.4 m s−1 and 0.8 m s−1. Given these values, all configurations maximum velocities seem to be suitable for fish to sustain their positive rheotaxis behaviour and promote their migratory success. In this study the comparison between configurations was made for the uniform flow, the ideal condition for a fishway to operate. However, when designing a fishway one must also check its operation under non-uniform conditions caused by hydrological variability, inadequate execution, or even operational reasons (Fuentes-Pérez et al., 2018; Marriner et al., 2014). As shown by Rajaratnam et al. (1986), if the tailwater level increases above the uniform flow level there will be a decrease in the maximum velocity at the downstream slots, thus potentially decreasing the attractivity of the fishway. Given the maximum velocities of the studied configurations, the MSFs would be more prone to the loss of attraction than the VSFs. Still, since the MSFs operate with relatively low discharge, an additional attraction flow could be provided at the entrance of the fishway, especially during high river discharge seasons, thus overcoming this issue and reducing the influence of competing flows. On the other hand, the division into two of the overall head drop between pools of the MSFs can dampen the natural seasonal changes in the river boundary conditions and contribute to the reduction in amplitude of the non-uniform water level profiles in the fishway, thus benefiting fishway negotiation. It should be referred that VSF1, VSF2 and MSF1 have maximum velocities in the slot higher than the maximum theoretical velocity − (Umaxtheo = 2g Δh ) . When performing field measurements of flow in a VSF, Bombač et al. (2015) also found maximum velocities 50% higher than the maximum theoretical value, pointing out that the theoretical value is an approximation, only valid if the velocity component along the slot axis in the upstream pool is neglected, as also corroborated by Bermúdez et al. (2010). Bombač et al. (2017) also showed that the angle of deflection between baffles and the slot size affect the dissipation of kinetic energy of the flow (due to the different contraction of the jet) leading to maximum velocities significantly larger than the theoretical value (up to 62% larger, in the configurations studied by these authors). The VSFs present higher turbulent kinetic energy in the jet boundaries (Fig. 5) with its rapid decay caused by the entrainment of the recirculating flow on both sides of the jet. The MSFs have lower turbulent kinetic energy values than VSF, with MSF3 presenting the highest values (Fig. 5). These occur mainly in the pool entrance chambers, where, for MSF3, the jet is not so well aligned with the inner wall and the recirculation zone is smaller and not completely developed as in the other two MSF configurations. This might explain the higher values, since the entrainment of the recirculating flow and jet mixing is not so well achieved as in the other MSF configurations. Researchers have found that high levels of turbulence may prevent displacement of resident fish (Lupandin, 2005; Montgomery et al., 1997; Odeh et al., 2002) and may even disrupt their upstream movements and hence define the upstream boundaries of fish population (Haro et al., 2004). Silva et al. (2011) found that when negotiating a pool-type fishway

5. Conclusions The present study compared the hydrodynamics of two of the most widely used VSF and three MSF configurations. Additionally, this study highlighted the value of CFD models, and the utility of software like FLOW-3D®, as a tool for fishway design, as new configurations or different solutions for retrofitting existing non-operational fishways can be modelled and optimized, in shorter periods and in a less expensive way prior to their testing or construction. Variants of a specific configuration, resulting from slight geometry changes (e.g. fishway slope and width, pool length and depth or slots width) or from different hydraulic conditions (e.g. non-uniform regimes), can be swiftly analysed. The results showed that the MSF configurations require much lower discharges to operate than the VSFs, for similar slope, pool and slot sizes, while keeping similar flow depths. The differences are more significant relatively to the VSF1 configuration. This may represent a huge advantage in regions where water availability is limited, such as some Mediterranean regions. MSF configurations provide the opportunity for fish to negotiate the fishway at their preferred depth, with the additional advantage of still remaining operational with discharges significantly lower than the design discharge. Comparatively to MSF configurations, maximum and average values of velocities and turbulent parameters are higher in VSF configurations, which means, that fish can experience more fatigue while negotiating a VSF. Thus MSF configurations seem to be less selective and more suitable for multiple fish species, especially for smaller sized and 204

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individuals with weaker swimming capacities. Nevertheless, further research is needed and fish species should be tested in laboratory to assess fish performance under these multi slot configurations in comparison with the standard VSF configurations.

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