Fish guidance structures with wide bar spacing

This technology has been developed in the FIThydro project! See Innovative technologies from FIThydro for a complete list.

Note: This article will be finished with the submission of deliverable 3.3 and 3.4 in October, 2020

Introduction

The revised Swiss Waters Protection Act (WPA) of 2011 demands the restoration of water bodies and the elimination of negative impacts of hydropower plants (HPPs) regarding fish migration until 2030. Similar demands are stated by the European Water Framework Directive (WFD) of 2000.

To minimize fish injuries or mortality during turbine passage at Run-of-River (RoR) HPPs and thus to fulfill the demands of the WPA and WFD, mechanical behavioural Fish Guidance Structures (FGS) with wide bar spacing and vertical bars have been developed for using at RoR HPPs and water intakes with high design discharges. These are Louvers, Angled Bar Racks (ABR), Modified angled Bar Racks (MBR) and Curved-Bar Racks (CBR) (Figure 1, Bates and Vinsonhaler, 1957; Raynal et al., 2013a; Albayrak et al., 2018 & 2020; Beck et al., 2020a &b). The CBRs have been developed in FIThydro (Figure 1d, e and 2).

All four FGSs feature wide clear bar spacing ≥ 25 mm and are classified as mechanical behavioural fish protection barriers (Fig. 1). Depending on fish size and bar spacing, they may partly function as physical barriers, i.e. preventing fish with minimum body dimensions greater than the clear bar spacing from passage. Louvers are made of vertical straight bars placed at an angle β = 90° to the flow direction mounted in a rack (Fig. 1a). The rack is placed across an intake canal at an angle to the flow direction of typically α= 15° to 30°. Classical ABRs function similar to louvers but their bars are placed at 90° to the rack axis, so that β varies with the main angle α, i.e. β = 90°− α (Fig. 1b), whereas MBRs have an independent variation of α and β with β ≠ 90°−α (Fig. 1c; Raynal et al., 2014; Albayrak et al., 2018 & 2020,). The CBRs consist of a series of vertical curved-bars instead of straight bars. The plan view angle between the upstream bar tip and the flow direction ranges from β = 45° to 90°, while the angle at the downstream end of the bar is optimally δ = 0°, i.e. parallel to the flow direction in the power canal (Fig. 1d, e and 2). All these four FGS types with clear bar spacings of s ≥ 25 mm guide fish to a bypass with hydrodynamic cues created by the bars instead of physically blocking fish from a water intake. When approaching the structure, fish should be able to perceive the elevated pressure and velocity gradients around and between the bars, resulting in avoidance behaviours. The velocity component parallel to the rack Vp, guides the fish towards the bypass. Effective guidance of such FGSs depends also on maintaining the ratio between Vp and rack normal velocity Vn above 1, i.e. Vp / Vn > 1 upstream of the bypass (Courret & Larinier, 2008). Furthermore, to ensure that fish can swim actively along the FGS 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., 2013b) as a first proxy. In general, the value of Vsustained = 0.50 m/s is recommended for the design of a FGS, if the fish fauna is not specified; else, Vsustained should be target fish specific. USBR (2006) recommends the ratio of mean bypass velocity Uby,in to the mean approach flow velocity Uo, between1.1 and 1.5 for louvers.

Detailed information and case study performance evaluation of louver systems are presented in USBR (2006). In addition, Albayrak et al. (2018, 2020) investigated the hydraulics and fish protection and guidance efficiencies of Louvers and MBRs in the laboratory. Albayrak et al. (2018) developed a headloss prediction equation for Louver, ABR and MBR. Furthermore, Albayrak et al. (2020) reported the flow fields and fish guidance efficiencies of a Louver with α = 15° and s = 50 mm and MBR configurations with α = 15° and 30°, s = 50 mm and with and without bottom overlays tested with barbel, spirlin, European grayling, European eel and brown trout. The results show that MBRs with α = 15° with and without overlay successfully guided 90% and 80% of the tested fish species, respectively. Furthermore, MBRs with α = 30° with an overlay guided 95% of the fish. Compared to Louvers and ABRs, MBRs reduce the head losses by ~5 times and ~2, respectively (see Table 1). Despite low head losses for MBRs, losses are still relatively high compared to conventional trash racks and the downstream flow field is still asymmetrical, which may negatively affect turbine efficiency.

Due to the flow straightening effect, the new CBR results in ~20 and ~4 folds lower head losses compared to the same Louver and MBR configurations (Table 1) and in quasi-symmetrical downstream flow (Beck, 2020 & Beck et al., 2020b), improving the rack downstream flow field and possibly HPP turbine efficiency. Furthermore, systematic ethohydraulic tests for a hydraulically optimized CBR configuration with β = 45° and s = 50 mm show fish protection and guidance efficiencies above 75% for spirlin, barbel, nase and salmon parr . The efficiency of the laboratory tests were below 75% for brown trout and eel (Beck, 2019 and 2020). Given the significantly reduced head losses and high fish guidance and protection efficiencies, CBRs developed in FIThydro present a high potential over Louvers, ABRs and MBRs with straight bars for a safe downstream fish migration at hydropower plants at minimum negative economic impacts (for more on CBRs, see Beck, 2020 and FIThydro deliverable 3.4).

Table 1: Comparison of head losses of FGSs with wide bar spacing for the rack configuration of α = 30°, s = 50 mm, d =100 mm, t = 10 mm (Fig.1).

FGS type	Louver	ABR	MBR	CBR
Bar angle, β	90°	60°	45°	45°
Head loss coefficient, ξ	13.7	5.0	2.8	0. 7

Methods, tools, and devices

During planning

To design a FGS with wide bar spacing and its corresponding bypass system (BS) at a given HPP, detailed site-specific information is needed. The information can be obtained from construction plans and measurements on site. It is recommended to (I) identify and utilize fish migration corridors using radio or acoustic telemetry technique (FIThydro Deliverable 2.2); (II) consider behaviour and biomechanical properties of target fish species; and (III) match the hydraulic conditions of a FGS-BS to (I) and (II). In order to assess the hydraulics of a FGS-BS, velocity and bathymetry measurements using e.g. an Acoustic Doppler Current Profiler (ADCP, see FITHydro Deliverable 2.2) should be conducted (exemplary velocity data from the test case HPP Bannwil, Figure 3). Based on such data, a physical or numerical model of the HPP (Feigenwinter et al., 2019) can be constructed. With either model, positioning and geometric optimization of FGS-BS can be done (numerical model results for HPP Bannwil, Figure 4, see FIThydro deliverable 2.2). Finally, it is recommended to integrate the HPP’s operating conditions and the hydrological boundary conditions of the studied site.

The construction of a FGS-BS at an existing HPP will in most cases lead to a temporary interruption of the HPP operation and thus to production losses. The construction of the rack itself is comparable to the construction of a conventional HPP trash rack. An additional bridge carrying the rack cleaning machine, which in most cases is analogue to conventional machines used at classical intake trashracks (Beck, 2020), should be installed above the FGS-BS.

During operation

Similar to the planning phase, after the construction of a FGS-BS at a HPP site, velocity measurements - using e.g. an ADCP - and fish monitoring using radio/acoustic telemetry or PIT-tagging are recommended to evaluate the effect of the FGS-BS on the flow field and its fish protection and guidance efficiencies. Based on the monitoring results, further optimization of the FGS-BS should be made, if needed.

Relevant MTDs and test cases

Relevant MTDs
N/A
Relevant test cases	Applied in test case?
Bannwil test case	-
Schiffmühle test case	-

Classification table

Classification	Selection
Fish species for the measure	All
Does the measure require loss of power production	Operational (requires flow release outside turbine)
	-
	-
Recurrence of maintenance	Yearly
Which life-stage of fish is measure aimed at	-
	-
	-
	Movements of migration of fish
Which physical parameter is addressed	N/A
	-
	-
	-
	-
	-
	-
	-
Hydropower type the measure is suitable for	Plant in dam
	Plant with bypass section
Dam height (m) the measure is suitable for	Up to 20
Section in the regulated system measure is designed for	In dam/power plant
	-
	-
	-
River type implemented	Steep gradient (up to 0.4 %)
	Fairly steep with rocks, boulders (from 0.4 to 0.05 %)
	Slow flowing, lowland, sandy (less than 0.05 %)
Level of certainty in effect	Very uncertain
Technology readiness level	TRL 9: actual system proven in operational environment
Cost of solution	See cost table