7.1 Introduction

Fish migration in regulated rivers is often hampered by Hydropower plants (HPPs), dams, weirs and spillways. The main risks associated with the presence of such structures include: blocking or delaying of up- and downstream fish migrations, and damage or mortality of fish when passing turbines, weirs or spillways. For an efficient restoration of water bodies, the European Water Framework Directive was enacted in 2000 and the revised Swiss Waters Protection Act (WPA) and Waters Protection Ordinance were introduced in 2011. For HPPs, fish passage facilities and connections to adjoining water bodies must be upgraded or newly erected.

Downstream fish passage still poses challenges to scientists, engineers, authorities and HPP operators due to the lack of design standards and related basic information on behaviour of various fish species. To this end, FIThydro has improved and developed downstream fish passage technologies for a range of fish species and size of HPPs based on laboratory and field investigations. These are classified into two groups, namely fish guidance structures with narrow bar spacing (i) and wide bar spacing (ii). The former group includes vertically inclined bar rack-bypass system and horizontal bar rack-bypass system, which work as a physical barrier and potentially applied for small to medium size HPPs with a design discharge <120 m3/s and this chapter deals with the hydraulics, fish guidance efficiency and design recommendations of this type of fish guidance structures. The latter group includes an innovative curved-bar rack-bypass system, which functions as a combination of mechanical behavioural barrier for small-to-large HPPs as addressed in Chap. 8.

7.2 Physical Barriers

Different solutions have been widely studied in multiple designs (inclined, angled, vertical or horizontal bars, different bar shapes and bar spacings) during the FIThydro project. They proved their efficiency in avoiding fish passage through and impingement risks at the rack and additionally guide fishes towards a downstream bypass. However, narrow bar spacing racks cause problems of head losses and clogging by floating debris such as leaves and wood, requiring efficient cleaning systems and generating additional maintenance costs compared to classical intake trash racks. In this sub chapter, different solutions with narrow bar spacing are explained and summarized.

The first type of such racks is the Vertically Inclined Bar Racks (VIBR). They consist of plane screens composed of elongated flat bars positioned in vertical planes aligned with the flow (Fig. 7.1). The plane screen is inclined with an angle β with respect to the river bed to guide fish towards one or several surface bypass inlets located at the top of the rack (Raynal et al. 2013a). Another configuration consists of a perforated plate instead of bars, which is called Vertically Inclined Perforated Plate (VIPP). Detailed information on the design and efficiency of both VIBR and VIPP is given in FIThydro Deliverable 3.4 (Albayrak et al. 2020) and Lemkecher (2020).

Fig. 7.1
figure 1

Longitudinal profile of a vertically inclined bar rack (from Courret and Larinier 2008)

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The second type of solutions are angled bar racks. Angled bar racks are installed at an angle α to the approach flow direction in plan view to guide fish towards a bypass located at the downstream end of the rack. Three types of angled racks with narrow bar spacing, sb ≤30 mm, can be distinguished (Fig. 7.2):

Fig. 7.2
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Type of fish guidance structures with narrow bar spacing: angled bar rack with vertical bars (a), vertical streamwise bars (b) and horizontal bars (c) (adapted from Lemkecher et al. 2022)

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  • “Classical” angled bar rack, with vertical bars angled (Raynal et al. 2013b)

  • Angled bar rack with vertical bars oriented in streamwise direction (Raynal et al. 2014)

  • Horizontal Bar Rack (HBR)’ (Albayrak et al. 2019, 2020, Meister 2020; Meister et al. 2020a, b; Lemkecher et al. 2022)

Figure 7.3 shows the horizontal bar rack—bypass system (HBR-BS) of the FIThydro case study residual HPP Schiffmühle on the Limmat River, Switzerland, during revision work in 2018. The design discharge of the HPP is Qd = 14 m3/s and the HBR was built in 2013 with foil-shaped bars, a clear bar spacing of sb = 20 mm, and a pipe bypass.

Fig. 7.3
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(Source Julian Meister, VAW) and b principle sketch of an HBR-BS (Source VAW, adapted from Ebel 2016)

a Horizontal bar rack—bypass system at the residual flow HPP Schiffmühle, Switzerland, during revision work in July 2018.

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Inclined bar racks (VIBRs) and horizontal bar racks (HBRs) are characterized by narrow bar spacing typically ranging between sb = 10 and 30 mm, such that they are physically not passable for a large share of the fish population. VIBRs and HBRs are thus designed as physical fish exclusion and guidance barriers to prevent fish from entering water intakes or the turbines at run-of-river HPPs. As a rule of thumb, the rack constitutes a physical barrier when the bar spacing is lower than 1/10 of the total length for most species including salmonids, but except for eels, which require bar spacing lower than 3% of their length (Ebel 2016). For fish smaller than the threshold size, VIBRs and HBRs act as behavioural barriers. The lower the bar spacing, the higher the fish will be reluctant to go through the rack.

Bottom and top overlays can be used to enhance the guidance efficiency of sediments, floating debris, and bottom and surface oriented fish, respectively Fig. 7.3. An automated rack cleaning machine is needed to prevent the rack from clogging. In case of HBRs, Fig. 7.3 illustrates that the bypass discharge is usually controlled with a restrictor and/or a ramp. In case of VIBRs, Fig. 7.4 illustrates that one or several bypass entrances, depending on the intake width, are collected in transversal galleries with growing hydraulic sections in the downstream direction.

Fig. 7.4
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Views of VIBR installed at Las Rives HPP, France: inclined rack and bypass entrances viewed from upstream (a), (water intake out of water) and gallery collecting the 3 surface bypass entrances (b)

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The bars of VIBRs and HBRs can be built with different bar shapes, such as rectangular, rectangular with a circular tip, rectangular with an ellipsoidal tip & tail, and foil-shaped (Fig. 7.5). Most modern VIBRs and HBRs are equipped with foil-shaped bars or rectangular bars with an ellipsoidal tip & tail because of the reduced head losses (Lemkecher et al. 2020; Meister et al. 2020a). Additionally, these bars can be cleaned more easily than rectangular bars due to the thickness reduction from tip to tail (Meister 2020). Figure 7.5 shows the different rack parameters of an HBR, including the clear bar spacing sb, the bar thickness tb, and the bar depth db (see Albayrak et al. 2020 for more information on HBR-BS).

Fig. 7.5
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(Source VAW, adapted from Meister et al. (2020a))

Cross-section of an HBR illustrating different rack parameters; ho: approach flow depth, hds: downstream flow depth, Uo: mean approach flow velocity, Uds: mean downstream flow velocity, hBo: bottom overlay height, hTo: top overlay height, sb: clear bar spacing, tb: bar thickness at thickest point, db: bar depth.

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To prevent fish from passing through the Flow Guidance Structure (FGS) with narrow bar spacing, there are three design criteria: (i) the bar spacing, (ii) the normal velocity (Vn; velocity component normal to the rack axis), which is directly linked to the rack surface, and (iii) the ratio of the rack parallel velocity (Vp) to the rack normal velocity, which should be higher than 1 or even 2, i.e. Vp/Vn > 1 (or 2). The maximum values of the first two parameters depend on the species taken into account.

The recommended bar spacing and normal velocity (Vn) are the same for inclined racks (VIBR), angled racks with horizontal bars (HBR) and with vertical streamwise bars, as the behavioural “louver effect” is not considered strong enough in such configuration.

For salmonid smolts, the bar spacing (for inclined and angled bar racks) has to be smaller than 10–15 mm to constitute a physical barrier (based on the rule 1/10 of body width), but a strong behavioral repulsion can be obtained with bar spacing up to 25 mm (Courret and Larinier 2008). As eels do not show strong behavioural repulsion and are therefore likely to pass through trash racks, it appeared necessary to implement physical barriers. In France, the recommended bar spacing (for inclined and angled bar racks) is generally 20 mm to stop female eels longer than 50–60 cm. The bar spacing can be reduced to 15 mm in case of a significant presence of males upstream of the HPP (Courret and Larinier 2008). In Germany, the authorities in some regions even go below these values, with prescribed thresholds down to 10–12 mm.

For HBRs, the horizontal approach flow angle α, is selected such that the velocity component normal to the rack Vn does not exceed the sustained swimming speed of the target fish species. Approach flow velocities, typically varying between Uo = 0.40 and 0.80 m/s, lead to α = 20 ÷ 40°. The rack angle is therefore a compromise between limiting Vn on the one hand and the rack length on the other hand. For Vertically Inclined Bar Racks (VIBRs), approach velocities have to respect the same criteria as the HBRs regarding Vn and rack inclinations of the order of 25° are necessary to guide fish towards surface bypass entrances—thus confirming existing recommendations (Vp/Vn > 2)—and helping to limit head losses (Courret and Larinier 2008; Courret et al. 2015).

The head losses induced by HBRs can be predicted with the equations published in Meister et al. (2020a) and Lemkecher et al. (2022). These equations do not only take rack parameters, as defined in Fig. 7.5, into account, but also different approach flow configurations as determined by the HPP layout such as diversion HPP or block-type HPP. If an HBR is installed in a straight headrace channel of a diversion HPP, the velocities are typically nearly homogeneously distributed, which means that the criterion of Vp/Vn > 1 is fulfilled for HBRs with α < 45° (Meister et al. 2020b). If an HBR is installed at a block-type HPP, the streamline pattern is usually complex and Vp/Vn along the rack decreases towards the downstream rack end (Meister et al. 2020b). Likewise, Vn will be underestimated at the downstream rack end if the velocity components are calculated from continuity, which could lead to fish impingements or passages through the rack. It is therefore recommended to determine the optimal HBR position with numerical simulations such as described in Feigenwinter et al. (2019).

The head losses of VIBRs and VIPP can be predicted using the equations developed by Lemkecher (2020).

In addition to the design of a FGS with narrow bar spacing, the bypass design is important to safely collect and transport the fish and to return them unharmed to the river downstream of an HPP. Different bypass designs are described in literature such as the full depth open channel bypass, a bypass with a vertical axis gate consisting of bottom and top openings, and a pipe bypass (Beck 2020; Meister 2020). The latter is not recommended because it can clog easily and fish avoid large velocity gradients at the inlet of the pipe bypass (Dewitte and David 2019).

The height and the width of the turbine intake influence the choice of the solution (inclined or angled). In addition, the possible location of the bypasses could modify the final solution. To reduce head losses, a particular attention has to be paid on the bar shape, the spacers and the support structures of the bar rack. For more details, please see the FIThydro Deliverables 2.2 (Dewitte and David 2019) and 3.4 (Albayrak et al. 2020); and the FIThydro Wiki on FGSs with narrow bar openings.