8.1 Introduction

Horizontally or vertically inclined FGS with narrow bar spacing of sb = 10–30 mm described in Chap. 7 are not recommended for medium- to large-scale HPPs with a design discharge Qd > 100 m3/s because of their velocity limitations to avoid fish impingement (Ebel 2016) and relatively high clogging risk by floating debris and hence operational problems. For these HPPs, mechanical behavioural FGS with wide bar spacing of sb = 25–100 mm present a promising alternative (Albayrak et al. 2018, 2020). They guide fish to a bypass with hydrodynamic cues created by the vertical bars instead of physically blocking fish from entering the water intake. When approaching the FGS, fish should perceive high turbulence zones and spatial velocity and pressure gradients around and between the bars and avoid passing the FGS. The velocity component parallel to the FGS guides fish towards the bypass located at the downstream end of the FGS. Louvers belong to this type of FGS with straight vertical bars placed normal to the approach flow, i.e., with a bar angle of β = 90°, and a rack angle to the approach flow of α = 10–45° (Amaral 2003, Bates and Vinsonhaler 1957, EPRI and DML 2001, Fig. 8.1a). They are widely used to bypass anadromous fish around HPPs and water intakes in the northeast USA and Canada. Furthermore, classical angled bar racks are also used for fish guidance similar to louvers, but their bars are placed at 90° to the rack axis, so that β varies with the rack angle α, i.e., β = 90° − α (Fig. 8.1b). Upon the design of louvers, Albayrak et al. (2018, 2020) developed a Modified angled Bar Rack (MBR) with β independent of α, preferably β = 45° instead of 90°. Such a reduction of the bar angle reduces the head loss and improves the rack downstream flow field (Fig. 8.1c). Albayrak et al. (2020) reported the flow fields and fish guidance efficiencies (FGEs) of a louver with α = 15° and sb = 50 mm and MBR configurations with α = 15° and 30°, sb = 50 mm and with and without bottom overlays for barbel (Barbus barbus), spirlin (Alburnoides bipunctatus), European grayling (Thymallus thymallus), European eel (Anguilla anguilla) and brown trout (Salmo trutta). The results show that MBR with α = 15° with and without overlay successfully guided 90% and 80% of the tested fish species, respectively. Furthermore, MBR with α = 30° with an overlay guided 95% of the tested fish. Such high FGEs and improved flow field of MBR led to the development of an innovative Curved-Bar Rack-Bypass System (CBR-BS) for a safe downstream fish passage at small- to large-scale HPPs and water intakes (Beck 2020; Beck et al. 2020a, b, c, Figs. 8.1 and 8.2).

Fig. 8.1
figure 1

Different fish guidance structure layouts with wide bar spacing a louver, b angled bar rack, c modified angled bar rack (MBR) and d curved-bar rack (CBR)

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Fig. 8.2
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Illustration (a) and detailed geometry (b) of Curved-Bar Rack-Bypass System and curved bar cross-section (c)

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8.2 Curved-Bar Rack-Bypass System

A CBR consists of vertical curved bars instead of straight bars used in louvers and MBR. They are arranged with equidistant spacing along the rack axis and mounted in a rack frame. The rack is placed across an intake canal at a rack angle typically α = 15–30° (Fig. 8.2a, b). A curved-bar is designed to have a bar angle to the flow direction ranging from β = 45–90° at the upstream bar tip and an outflow angle of δ = 0°, i.e. parallel to the flow direction in the power canal at the downstream end of the bar (Fig. 8.2d). The clear spacing between the bars is sb ≥ 25 mm, the bar thickness t = 10 mm, and the bar depth d = 100 mm. The upstream and downstream bar tips are typically rounded to avoid fish injuries.

A CBR creates hydrodynamic cues of turbulence, high velocity and pressure gradients by its bars similar to the working principle of louvers and MBR (Albayrak et al. 2020; Beck et al. 2020c). Such flow structures in front and between the bars are perceived and avoided by fish approaching the rack. Thanks to the angled rack arrangement, the velocity component parallel to the rack, Vp, guides the fish towards the bypass system (BS) without causing a shock from a major physical contact at the rack. A CBR acts as behavioural barrier for smaller fish while it functions as a physical barrier for fish whose width is larger than the bar spacing (Fig. 8.2b). For an effective guidance of the CBR, the ratio between Vp and the rack normal velocity Vn should be above 1 along the rack, i.e. Vn < Vp (Courret and Larinier 2008). Furthermore, to ensure that fish can swim actively along the CBR without exhaustion, the rack normal velocity should be smaller than the sustained swimming speed of fish, i.e. Vn < Vsustained. A general value of Vsustained = 0.50 m/s is recommended for smolts and silver eels (Raynal et al. 2013) as a first proxy.

Laboratory tests by Beck et al. (2020c) confirm the behavioural guiding effect of the CBR for several fish species except the European eel. They reported that above 75% of spirlin, barbel, nase (Chondrostoma nasus) and Atlantic salmon parr (Salmo salar) and below 75% of brown trout and eel were efficiently guided by a hydraulically optimized CBR configuration with sb = 50 mm, α = 30° to a full depth BS in the laboratory tests. The use of bottom and top overlays may improve the FGE of the CBR-BS for bottom and surface-oriented fish species, respectively. The effectiveness of such overlays was demonstrated and recommended by EPRI and DML (2001) and Amaral (2003) for louvers and by Albayrak et al. (2020) for MBR. Furthermore, overlays can mitigate operational problems of driftwood, organic fine material and sediment by guiding them to the bypass (Beck 2020).

The curved-bars of a CBR cause a flow straightening effect, which results in ~20 and ~5 folds lower head losses compared to the same Louver and MBR configurations and in quasi-symmetrical downstream flow (Beck et al. 2020b), improving the rack downstream flow field and possibly HPP turbine efficiency. A head loss prediction equation for louvers, MBR and CBR is presented in Beck et al. (2020a).

Successful CBR design requires a good bypass system design, which should attract, safely collect and transport the fish and return them unharmed to the river downstream of a HPP. Full depth, surface, bottom and both surface and bottom bypasses are the main types and should be selected based on the biomechanical requirements of the target fish species and HPP layout. The ratio of the bypass entrance flow velocity to the approach flow velocity VR = Uby/Uo and a gradual velocity increase along the rack to the bypass are crucial parameters for fish guidance and bypass acceptance (e.g. Simmons 2000; Albayrak et al. 2020; Beck 2020; Beck et al. 2020c). To this end, USBR (2006) recommends 1.1 ≤ VR ≤ 1.5 for louver-BS, Ebel (2016) recommends 1.0 ≤ VR ≤ 2.0 for horizontal bar rack-BS, while Beck et al. (2020c) recommend VR = 1.1 ≤ VR ≤ 1.2 for CBR-BS or other FGS to protect and guide fish of all species, life stages and sizes.

8.3 Conclusions and Outlook

Given the significantly reduced head losses and high fish guidance and protection efficiencies, CBR-BS presents a high potential over Louvers and MBRs for a safe downstream fish movement at HPPs at minimum negative economic impacts. Cost-effective engineering design recommendations for CBR-BS are given in-detail by Beck (2020). The first CBR-BS variant is currently installed, and its effectiveness will be assessed at the pilot HPP of Herrentöbeli located on River Thur in Switzerland. More projects at HPPs of different sizes and layouts are needed to evaluate the CBR-BS effectiveness under various flow conditions and for different fish species and to further improve its design.