Hydrobiologia (2008) 609:189–196 DOI 10.1007/s10750-008-9413-1
EIFAC 2006: DAMS, WEIRS AND FISH
Numerical flow simulation of pool-type fishways: new ways with well-known tools Stephan Heimerl Æ Margit Hagmeyer Æ Christof Echteler
Ó Springer Science+Business Media B.V. and FAO 2008
Abstract Fish passage structures are built to restore the connectivity of rivers and allow the migration of aquatic fauna. In order to assess the functioning of a pool-type fishway, it is necessary, inter alia, to possess detailed knowledge of its flow structure, since observations of fishways, and in particular of the visible water surface, can only provide a rough idea of the actual conditions inside the pools. Numerical simulation has been used for many years to support engineering sciences. Especially, the modeling of flow processes in hydraulic machines can, on the one hand, help avoid major problems during the design stage of fish passage structures and, on the other, improve the structure’s hydraulic
Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries S. Heimerl (&) Fichtner GmbH & Co. KG, Sarweystr. 3, 70191 Stuttgart, Germany e-mail:
[email protected] M. Hagmeyer VOITH SIEMENS Hydro Power Generation, Alexanderstr. 11, 89522 Heidenheim, Germany e-mail:
[email protected] C. Echteler Ed. Zueblin AG, Albstadtweg 3, 70567 Stuttgart, Germany e-mail:
[email protected]
performance. To this end, two diploma theses within the framework of a research project of the local energy supplier Energie Baden-Wu¨rttemberg AG (EnBW) employed modeling tools for 3D flow simulation, primarily for pool-slot fishways (PSF), and for traditional vertical slot fishways (VSF). Keywords Fishway Fish passage structure Numerical flow simulation Pool-type fishway Vertical slot fishway Velocity distribution
Introduction The visualization of numerically simulated flow in pool-type fish passages can mean significant simplification for cooperation with the different specialists involved in planning a fishway, who are often not familiar with the theory. These simulations help make the often non-trivial flow patterns visible for everyone. Hydraulics in fish passage structures have been investigated for more than 20 years. The main focus of researchers dealing with pool-type fishways has been on the vertical slot structures that are widespread in North America. Rajaratnam et al. (1986), Wu et al. (1999) and Puertas et al. (2004) conducted more or less extensive measurements of the velocity fields in single pools of fishways in separate studies. The disadvantage here is that the data cannot be collected at all points simultaneously because of the
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excessive cost; therefore, it is not known how the different turbulent velocities relate to each other. Over the past few years, numerical models have been set up occasionally to depict flow structures. Most of these published investigations (Fujihara et al., 2002; Barton & Keller, 2003; Tarrade et al., 2005) have only been visualized through their velocity factors in conventional laboratory studies or only set up in two dimensions.
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The results obtained with CFX show higher velocities but do not exceed the theoretical values, compared to the results obtained using FENFLOSS. Primarily, different main types of fishways have been simulated to visualize the flow structures (Hagmeyer, 2005). In a further step, the detailed influence of the position and geometry of the guide elements was investigated (Echteler, 2006). Fishway design variants
Materials and methods The first numerical steps were made using FENFLOSS (Hagmeyer, 2005; Heimerl et al., 2005), a Reynolds averaged Navier-Stokes solver that has been in use for more than 10 years at the Institute for Fluid Mechanics and Hydraulic Machinery (IHS) at the University of Stuttgart. To adapt a numerical model to reality correctly is not easy. The main problem is the fixed grid of finite elements, and therefore, the overfall geometry as well as the number of pools and the discharge. Hydraulics in open channels are often characterized by a free surface in the form of a layer between air and water at which special boundary conditions are valid. FENFLOSS has been set up for fixed grids and is nearly exclusively applied for modelling of closed and totally water-filled systems like water turbines or draft tubes. This means that the geometry of the fish passage structure is more or less a pipe as the free surface has to be simulated as a fixed boundary wall. A cascade of several pools was investigated to define which pools are touched by inlet and outlet influences. This indicates the minimum number of pools necessary for adequate modelling of both poolslot and vertical-slot fishways. This was fixed at four entire pools. These and additional results were calculated with CFX (Echteler, 2006; Heimerl et al., 2006) to check the results from FENFLOSS and the limits of the calculation methods, and to apply CFX-specific methods to special hydraulic questions such as a free surface. The decision to use CFX software was justified by the implemented multi-phase model being able to simulate a boundary layer between two fluids. The surface structure calculated with this method is very close to reality and the time consuming set up of the overfall geometry is no longer necessary.
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An entire fishway was simulated to compare the advantages and disadvantages of different types of constructions. This covered the fishway from water inlet to outlet and included a number of baffles and typical problem zones in the form of bends. In addition to investigating the flow pattern, the focus here was on modelling an entire fish passage structure and on the effects of different types of construction. The curved inlet section is succeeded by five pools, with a 30° bend after pool 3. The pools have a length of 2.90 m and a width of 1.20 m with a mean water depth of 0.90 m. As a pool-slot fishway (PSF), the facility requires 300 l s-1 and as a vertical slot fishway, it requires 350 l s-1. Vertical slot fishway The hydraulics of a fish passage structure are defined not only by the flow through the entire channel geometry but also by details of individual features. The geometry of the vertical slot studied (Fig. 1) was taken from earlier investigations by Larinier
Fig. 1 Vertical slot (Echteler, 2006)
fishway—whole
simulated
cascade
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et al. (2002). At a slot width of 0.17 m, the pools have a length of 1.38 m and a width of 1.13 m. With a discharge of 350 l s-1 and an average water depth of 0.9 m, the fall between two pools is 0.2 m. Global influence of guide elements Guide elements are often installed, particularly in vertical slot fishways, to provide a positive influence on the flow in the individual pools and especially on the flow through the slot. Investigations were carried out to clarify the influence of guide elements using a straight channel geometry with or without such elements. Positioning of guide elements In general, guide elements are installed in pool-type fishways to lead smooth hydraulic flow into the next slot. The energy dissipation causes a decrease of the maximum velocity in the slot, thus easing the upstream migration for fish. The positioning of the guide elements in a vertical slot fishway was carried out at three different locations. The guide element is moved by an identical distance to and away from the slot (Fig. 2) in accordance with the procedure described by Larinier et al. (2002). Length of guide elements Variation of the length of the guide elements is based on the original geometry, which may be shortened or lengthened by half the original dimension (Fig. 3).
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Influence of bottom substrate In German-speaking countries, bottom substrate is normally installed in virtually all types of fishways to allow stream benthos and fish that are not strong swimmers to ascend upstream (Heimerl et al., 2001; Heimerl and Ittel, 2002). Fish passage structures were modelled with a bottom substrate to show the corresponding changes in the velocity distribution.
Results Fishway design variants Since the primary aim was to study PSFs, two design variants were investigated (Fig. 4). The variants differ with respect to the arrangement of the first slot, which occupies a position either along the inner or the outer wall. Subsequently, two vertical slot fishway (VSF) designs were investigated with slots close to the inner or the outer bend (Fig. 5). The corresponding figures always show a view of the flow along the surface and a streamline image below. The flow structure in vertical slot passes remains mainly 2D (e.g. Puertas et al., 2004), in contrast to the 3D flows in PSFs, which was investigated as well by Hagmeyer (2005) and Echteler (2006). Closer examination of the velocities reveals some interesting points. Most publications do not define the position of maximum velocity, supposing it to be at the centre of the slot, and no previous papers could be
Fig. 2 Flow pattern at different guide element positions (Echteler, 2006)
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Fig. 3 Flow pattern at different guide element lengths (Echteler, 2006)
Fig. 4 Pool-slot fishway (PSF) with first overfall on the outer side (left) versus PSF with first overfall on the inner side (right) (Hagmeyer, 2005)
Fig. 5 Visualization of computation results for a vertical slot fishway (VSF) with slots on the outer side (left) versus VSF with slots on the inner side (right) (Hagmeyer, 2005)
found that specifically address this question. Velocity distribution around the slot can be studied in detail using the 3D simulations and the associated possibility of vizualizing different layers. Figure 6 shows that the highest velocities are not reached in but directly behind the slot. Furthermore, the velocity is not uniformly distributed over the slot height but decreases in the direction of the water surface as
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indicated by the results of 2D studies (e.g. Fujihara et al., 2002). The highest velocities can be found close to the basin floor and the lower half of the pool. At the water surface (Fig. 6), the maximum velocity is reached approximately 25 cm behind the slot exit in the direction of the pool centre. This means that the highest surface velocities occur when the overfall flow plunges into the surface.
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Fig. 6 Velocity distribution in the slot: in direction of the main flow (left); at the slot exit (middle); at the water surface (right) (Echteler, 2006)
Fig. 7 Streamlines at the slot: without guide element (left), with guide element (right) (Hagmeyer, 2005)
Slot flow structure Comparing the different slot flows, without a guide element (Fig. 7, left), the flow lines describe a narrow curve that is almost a kink. With a guide element (Fig. 7, right), in contrast, the water moves along a well-established ‘‘soft’’ bend and flows evenly into and through the slot. Shifting the guide element towards the slot results in a more uniform velocity field with lower peak
values. The proximity to the slot means that the flow velocity in the slot is maximal at its centre, thus positively supporting upstream migration. The velocity vectors (Fig. 8) show that the position of the guide element influences in which angle the main flow reaches the slot. The main flow is directed more or less into the centre of the pool with the result that it influences the whole pool volume. The influence of the length of the guide element is similar to that described for variations in position.
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Fig. 8 Flow pattern with velocity vectors at different guide element positions (Echteler, 2006)
Fig. 9 Flow pattern with velocity vectors at different guide element lengths (Echteler, 2006)
The modifications mainly affect the angle in which the main flow reaches the next slot. The dimensions and positions of the original geometry as shown by the images in the middle of Figs. 2, 3, 8 and 9 create a well-formed flow similar to a bow. Narrow bends are avoided with this geometry. In the case of vertical slot fishways without a bottom substrate, the highest velocities occur, as expected, in the middle of the slot close to the bottom (Fig. 10, left). Figure 10, left and right, shows that the velocities in the zone close to the bottom are reduced by the simplified bottom substrate model and that a different velocity profile is formed as a result.
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Discussion The investigation of several variants using the same channel geometry allows the engineer to identify hydraulic problems and choose an appropriate type of construction. Numerical flow simulation has proved to be a useful technique that can clearly show which version is to be preferred. In the literature, the maximum velocity of fish passage structures is restricted to figures calculated for the channel bottom at the centre of the slot, e.g. Larinier et al. (2002). This calculation is based on the head difference between two pools, whereas in reality, this is not produced at one particular point but across the entire length of the overfall. The
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Fig. 10 Velocity distribution in the slot: without bottom substrate (left), with bottom substrate (right) (on different representation scales) (Hagmeyer, 2005)
consideration of the dimensions of the two occurring velocity maxima is crucial for the design of pool-type fishways, as it affects the number of pools and consequently, the construction costs. The results demonstrate that the extreme values only occur locally and that the fish encounter a differentiated flow pattern. For this reason, the objective to build a well-functioning migration facility will not be fully attained if the computed maximum velocity is the only factor considered, as it is frequently the case. Furthermore, a computed maximum velocity may not be applicable to all types of construction. The difference between baffles with or without guide elements has also been verified in situ on a constructed fish pass. The field test confirmed the results of the numerical investigations. The deflection of the water into the slot is less abrupt and the overfall is calmed and structured due to the guide element, which also has a positive effect on the reduced velocity area for the ascending fish. Therefore, the installation of guide elements is recommended. The different guide element positions show that a compromise must be found when planning a fishway between the lowest possible velocity and the adaptation to the main flow structure.
Conclusion The successful completion and functioning of the fishway is the final goal of every hydraulic engineer responsible for the design and construction of such a scheme. However, problems cannot always be avoided with an interdisciplinary project of this kind, particularly during the design phase. Numerical simulation of different fishway types and features can be extremely helpful in this context and provide a detailed, easily understandable and close-to-reality picture of the flow through the pool.
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196 Water Engineering: Proceedings 13th IAHR-APD Congress, Singapore. World Scientific, Vol. II: 1019–1024. Hagmeyer, M., 2005. Hydraulische Aspekte beckenartiger Fischaufstiegsanlagen. Universita¨t Stuttgart, Institut fu¨r Wasserbau, Diploma Thesis (unpublished). Heimerl, S., M. Hagmeyer & B. Kohler, 2005. En route with water particles in fishways—how to explain flow structure in a pool-type fishway? In Hydro 2005—Policy into Practice. Conference Proceedings, Villach. Heimerl, S., M. Hagmeyer & C. Echteler, 2006: Numerische Stro¨mungssimulation von beckenartigen Fischaufstiegsanlagen—Werkzeug bei der Beurteilung der Durchga¨ngigkeit. In Schriftenreihe zur Wasserwirtschaft. Technische Universita¨t Graz. Wasserbausymposium Graz 2006. Band 46. Heimerl, S. & G. Ittel, 2002. Becken-Schlitz-Pa¨sse als zukunftstra¨chtige Bauweise fu¨r technische Verbindungsgewa¨sser. In Wasserwirtschaft 92, Heft 4/5: 54–55. Heimerl, S., G. Ittel & G. Urban, 2001. First operational experiences with one of the largest fish passage structures
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