Experimental study on the flow characteristics of unstructured block

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Experimental study on the flow characteristics of unstructured block ramps a

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Simona Tamagni , Volker Weitbrecht & Robert M. Boes a

Research Assistant, Laboratory of Hydraulics, Hydrology and Glaciology (VAW), Swiss Federal Institute of Technology (ETH) Zurich, CH-8093 Zürich, Switzerland Email: b

(IAHR Member), Head of the River Engineering Division, Laboratory of Hydraulics, Hydrology and Glaciology (VAW), Swiss Federal Institute of Technology (ETH) Zurich, CH-8093 Zürich, Switzerland c

(IAHR Member) Professor, Director of VAW, Laboratory of Hydraulics, Hydrology and Glaciology (VAW), Swiss Federal Institute of Technology (ETH) Zurich, CH-8093 Zürich, Switzerland Email: Published online: 03 Oct 2014.

To cite this article: Simona Tamagni, Volker Weitbrecht & Robert M. Boes (2014) Experimental study on the flow characteristics of unstructured block ramps, Journal of Hydraulic Research, 52:5, 600-613, DOI: 10.1080/00221686.2014.950610 To link to this article: http://dx.doi.org/10.1080/00221686.2014.950610

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Journal of Hydraulic Research Vol. 52, No. 5 (2014), pp. 600–613 http://dx.doi.org/10.1080/00221686.2014.950610 c 2014 International Association for Hydro-Environment Engineering and Research 

Research paper

Experimental study on the flow characteristics of unstructured block ramps SIMONA TAMAGNI, Research Assistant, Laboratory of Hydraulics, Hydrology and Glaciology (VAW), Swiss Federal Institute of Technology (ETH) Zurich, CH-8093 Zürich, Switzerland Email: [email protected]

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VOLKER WEITBRECHT (IAHR Member), Head of the River Engineering Division, Laboratory of Hydraulics, Hydrology and Glaciology (VAW), Swiss Federal Institute of Technology (ETH) Zurich, CH-8093 Zürich, Switzerland Email: [email protected] (author for correspondence) ROBERT M. BOES (IAHR Member), Professor, Director of VAW, Laboratory of Hydraulics, Hydrology and Glaciology (VAW), Swiss Federal Institute of Technology (ETH) Zurich, CH-8093 Zürich, Switzerland Email: [email protected] ABSTRACT Unstructured block ramps (UBR) are fish-friendly structures to stabilize a river bed. They are increasingly used to replace drops and represent large macro-roughness elements randomly placed on the river bed. Laser Doppler Anemometry measurements were conducted to determine the mean flow characteristics in a laboratory channel covered by block ramps. The resulting local time-averaged flow quantities allow for considerations of the flow field heterogeneity and for the identification of migration corridors for a certain target fish. Double-averaging (in time and in space) was applied to characterize the flow conditions taking into account the strongly heterogeneous bed of an unstructured block ramp. By applying this method, the characteristic vertical profiles describing the general flow conditions are presented. Based on geometrical and physical considerations, a suggestion is made to sub-divide the interfacial sublayer of macro-rough beds into upper and lower parts. The aim of this research is to present not only the flow field, but also to highlight the ecological value of UBR.

Keywords: Double-averaging; fish migration; flow resistance; LDA; macro-roughness elements; low relative submergence flows; unstructured block ramp the laboratory with the help of 2D Laser Doppler Anemometry (LDA) measurements. During the past decades various types of block ramps were studied. Most of these studies are published in German only (DWA, 2009; Hunziker, Zarn, & Partner, 2008), and within these publications only a few pages are dedicated to UBR. Comprehensive design guidelines for block ramps especially for UBR in English are limited. The two basic classes of block ramps are (Tamagni, Weitbrecht, & Boes, 2010): (A) block carpet type or riprap type, involving tightly packed blocks forming a block carpet covering the entire river width and (B) block cluster type (including UBR), characterized by different dispersed configurations of block clusters. Ramps of type (A) are subdivided into interlocked block ramps (riprap stairway) characterized by one layer of blocks vertically placed closely together, leading to

1 Introduction Unstructured Block Ramps (UBR) are river engineering structures to stabilize a river bed without interrupting fish migration in the upstream direction. In Switzerland about 100,000 drops or sills with an elevation difference larger than 0.50 m exist (Zeh Weissman, Könitzer, & Bertiller, 2009), leading to heavily fragmented habitat conditions. To improve the latter on a larger scale, drops and sills need to be replaced by ecologically effective measures assuring the streamwise river connectivity. Therefore, physical laboratory experiments were performed to analyse in a first step stability aspects (Tamagni, 2013) and in a second step flow characteristics as well as ecological criteria of UBR during flood conditions with the final goal to develop specific design criteria. This paper focuses on the detailed description of the flow characteristics determined in

Received 23 May 2013; accepted 28 July 2014/Open for discussion until 30 April 2015. ISSN 0022-1686 print/ISSN 1814-2079 online http://www.tandfonline.com 600

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Figure 1 UBR at Wyna River (Switzerland) of specific discharge q = 0.05 m2 s−1 and bed slope S = 2.7%

a compact, hydraulic solid but rigid construction; and dumped block ramps where the blocks are randomly dumped into two or more layers, leading to a heavier and more heterogeneous construction. Experiences with ramps of type (A) show that these are used up to a bed slope of S = 10% (Hunziker, Zarn & Partner, 2008), although this block ramp type was investigated up to slopes of S = 40% (Robinson, Rice, & Kadavy, 1998). Ramps of type (B) are subdivided into structured block ramps where the block clusters correspond to regular geometrical structures as rows or arches (Pagliara & Chiavaccini, 2007) generating a steppool system; and UBR where the blocks are isolated roughness elements randomly placed on the bed material (Fig. 1). According to LUBW (2006), the maximum slope for structured block ramps is S ∼ = 6%, whereas the collected Swiss experience indicates that the maximum slope for UBR is S ∼ = 3% (Hunziker, Zarn & Partner, 2008). Despite previous studies (Section 2.1), comprehensive design guidelines for UBR are still limited to the application range of the existing design rules (Pagliara & Chiavaccini, 2006; Whittaker, Hickman, & Croad, 1988), and research on UBR with natural blocks placed on a sediment mixture has not yet been performed. Additionally, their functionality in terms of fish migration is not yet proven (Section 2.3). The aim of this paper is to describe the mean flow and turbulence characteristics on UBR to serve as a basis for quantifying the ecological efficiency of UBR in terms of fish migration.

2 Background 2.1

Stability and flow resistance

Along with the development of the various types of block ramps, their flow resistance and stability have been studied. Whittaker & Jäggi (1986) investigated the stability of type (A) block ramps with different block diameters, bed materials of different characteristic grain sizes, bed slopes and ramp lengths. They defined three different failure mechanisms, corresponding to the stability criteria for this ramp type, namely: (1) destabilization of single blocks, (2) entrainment and washout of bed material from

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below the blocks and (3) scour and failure of ramp toe. The block ramps with dumped blocks were experimentally studied by Rice, Kadavy, & Robinson (1998), who derived a relationship for the flow resistance of this type of ramps, which is suggested by the German guidelines (DWA, 2009) for design. Aberle & Smart (2003) studied the behaviour of steep mountain rivers in the presence of large boulders forming step-pool systems similar to structured block ramps. The experiments were carried out for S = 2, 4, 8 and 9.8% and two different bed materials with maximum grain sizes of 32 and 64 mm. The standard deviation of the river bed-level changes, including boulders, was defined as the characteristic roughness parameter, to better parameterize the logarithmic law of resistance compared with approaches where roughness is parameterized with a characteristic grain size dc . The flow resistance of both structured and UBR was studied by Pagliara & Chiavaccini (2006), involving three widths 0.25, 0.35 and 0.50 m, 8% < S < 40%, and seven almost uniform bed materials fixed on the flume bed. The blocks were placed on the bed material and fixed either randomly or along rows, consisting of smooth or rough hemispheres of diameters 29 or 38 mm. The considered protrusion of the blocks into the water body was half the spheres diameter with a block placement density l < 30%; l is hereby defined as the ratio between the area covered by blocks and the entire ramp area. They observed that the flow resistance depends mainly on l, the block arrangement and the specific roughness of the block surface. The effects of Froude number and relative submergence, defined here as the ratio between water depth and median radius of the boulders, were found negligible in the tested data range. Note that due to the flat bed and the fixed blocks, natural bed morphology and the related flow heterogeneity were not investigated. UBR consist of bimodal sediment mixtures as the grain size distribution has a peak for the bed material with characteristic diameter d and one for the much larger blocks with the characteristic diameter D. In general, the most important parameters describing the flow conditions on UBR are the ratio between the equivalent spherical block diameter and the characteristic grain diameter of the bed material D/d and the block placement density l. D/d is decisive in terms of the corresponding failure mechanism. Two dominant processes occurring for bimodal bed material may be expressed as a function of D/d defined by Raudkivi & Ettema (1982): (1) dominant overpassing of the blocks for D/d < 6 and (2) dominant embedding of the blocks in the finer bed material for D/d > 17. In Alpine or other gravel bed conditions, where the river bed tends to armouring, the characteristic grain size regarding bed stability is d90 of the grain size distribution. In our following considerations, d stands for d90 . Applying this result on UBR to obtain a gradual failure mechanism, D/d90 should be set between 6 and 17, to avoid any dominant failure process. In general, the gradual failure of UBR during overload is an advantage compared with other ramp types and to drops and sills where the structure may collapse abruptly.

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The block placement density l is an important parameter for the design of UBR balancing ramp stability and ramp costs (higher l, higher costs) and strongly influences the flow conditions. Rouse (1965) suggested an optimum block placement density in terms of flow resistance of l = 0.26 using spheres. For lower l, the maximum relative roughness and the maximum form drag, respectively, are not achieved. For higher l, the roughness elements are too close to each other: the area affected by one single block is “disturbed” by the presence of the next block and the flow separation cannot develop completely, leading to reduced energy dissipation compared with lower l. For optimum l, each single block contributes with its maximum form drag to the flow resistance. It should be noted that maximum energy dissipation may not be adequate in terms of fish migration. Design rules for block ramps with respect to hydraulic stability and ecological criteria are summarized in the design manuals of Hunziker, Zarn & Partner (2008) and DWA (2009). For UBR, Hunziker, Zarn & Partner (2008) suggest the approach of Whittaker, Hickman, & Croad (1988) for block placement densities 1, −u w  decreases stronger in the present research, due to more heterogeneous flow field dominated by accelerating flows over and between the blocks and by regions of low turbulence and low velocities. Under such heterogeneous hydraulic conditions, the momentum exchange is higher than over a regularly rough bed as in the case of Ghisalberti & Nepf (2006). It should be noted that the experiments by Ghisalberti & Nepf (2006) are performed under low Froude number conditions (F ≈ 0.013 for the experiment R9), whereas in the present case, the flow at the block crest and above is close to critical and sometimes becomes locally supercritical. In the Ghisalberti & Nepf (2006) case, a mixing layer develops, where coherent flow structures can evolve and travel with the mean flow velocity. In the present study, the flow at and above the block crests has more a jet-like behaviour with changing water depth due to surface waves. The vertical profiles of the spatially-averaged RMS-values of both velocity components uRMS and wRMS , respectively,

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Figure 15 Vertical distributions of normalized spatially-averaged Reynolds shear stress −u w /u2∗,I from the present research for hm,3 /P = 1.5 and Ghisalberti & Nepf (2006). Notation as in Fig. 12

Figure 16 Vertical distribution of the standard deviation of the velocities uRMS and wRMS , respectively, normalized with the bulk velocity ub . Empty symbols: data from the present research; filled symbols: data from Ghisalberti & Nepf (2006). Notation as in Fig. 12

normalized with ub,3 for hm,3 /P = 1.5 have a double-curved shape (Fig. 16). The shape is similar for the u and w component with uRMS approximately 1.2–1.5 times larger than wRMS . In the macro-roughness sublayer, the RMS-values reach their maximum below the representative block crest height zc , at the point where the Reynolds stresses are maximum (Fig. 15). From this point upwards, uRMS and wRMS decrease until elevation Z = 1.46, and increase again towards the water surface, because of the surface waves. This increase towards the water surface should be interpreted with care. Only a few points could be measured near the water surface (see considerations about the statistical significance made in Section 4). The spatially-averaged RMS-values derived from the same data set of Ghisalberti & Nepf (2006) considered in Figs. 13 and 15 are also plotted in Fig. 16. Except for the sedimentary sublayer, the shape of both profiles is nearly triangular with the maximum approximately at the roughness crests. The velocity fluctuation uRMS of the Ghisalberti & Nepf (2006) data are approximately 1.5 times than wRMS . This difference is slightly higher than for the present data, where the irregular

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placed blocks strongly affect also w, leading to higher turbulence intensities compared with the more regular rough beds of Ghisalberti & Nepf (2006). Looking at the x–z-plane, the flow is more isotropic in the present study due to the stronger vertical momentum exchange.

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6 Conclusions Detailed velocity measurements with a 2D LDA system were conducted for three relative submergences over a fixed rough bed with large macro-roughness elements, randomly distributed on top of it, representing an unstructured block ramp. The data were analysed from a hydraulic point of view, and an ecological analysis in terms of fish migration was suggested. The analysis is based on time-averaged and spatially (double)-averaged velocities, using measured Reynolds stresses, form-induced stresses and RMS-values. A new subdivision of the interfacial sublayer was suggested based on geometrical and physical characteristics, to characterize the flow above a heterogeneous bed with distinct macro-roughness elements. The upper part is called macro-roughness sublayer and is dominated by a flow that interacts with the blocks, so that form drag plays an important role. The lower part is called sedimentary sublayer and is characterized by variations in the bed elevation and single gravel grains, where form drag and viscous drag are assumed to dominate the flow. The influence of viscous drag on the sedimentary sublayer could not be verified with the present measurements. Using representative slices within the measurement area, the local flow variability is presented with time-averaged velocities and turbulence intensities. These variables are decisive to quantify the ecological effectiveness in terms of fish migration, for which the local, rather than the spatially-averaged conditions are crucial. The measurements allow to quantify the flow heterogeneity and can finally be used to predict the ecological effectiveness in terms of fish migration. The double-averaged Navier–Stokes equations were applied by including the effect of the heterogeneous bed with the roughness geometry function. In this way, the heterogeneity due to the influence of both irregular bed and macro-roughness elements could be included. Spatially-averaged velocity, Reynolds stress and form-induced stress profiles were determined, discussed and compared with literature values. The present data set with a high spatial and temporal resolution can be used to validate numerical simulations of strong heterogeneous bed conditions and to predict the behaviour of different fish species in such an environment.

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Notation = characteristic grain sizes of bed material (e.g., dm , d16 , d84 , d90 ) (m) D = equivalent spherical block diameter (m) F = Froude number (–) g = gravity acceleration (m s−2 ) = mean flow depth (m) hm p = pressure (Nm−2 ) P = mean block protrusion (–) q = specific discharge (m2 s−1 ) R = Reynolds number (–) = roughness Reynolds number (–) R∗ S = ramp slope (–) = equilibrium ramp slope (–) Se = initial ramp slope (–) So TI = turbulence intensity (–) = bulk velocity measured with salt dilution ub method (m s−1 ) u, w = instantaneous longitudinal and vertical flow velocity components (m s−1 ) u¯ , w¯ = time-averaged longitudinal and vertical velocity components (m s−1 )   = velocity fluctuation of longitudinal and u,w vertical components (m s−1 ) uRMS , wRMS = RMS-value of the longitudinal and vertical velocity components (m s−1 ) = bulk shear velocity = (ghm S)0.5 (m s−1 ) u∗ = shear velocity derived from u w max = u∗,I (−u w max )0.5 (m s−1 ) ¯u, w ¯ = double-averaged longitudinal and vertical velocity components (m s−1 ) = spatially-averaged Reynolds stress −u w  2 −2 (m s ) u˜ , w ˜ = spatial variation of time-averaged longitudinal and vertical velocity components (m s−1 ) ˜uw ˜ = form-induced stress (m2 s−2 ) = representative block crest height (m) zc = mean bed level without accounting for blocks (m) zm zMR , zt = characteristic bed levels (m) = zero plane (m) zz Z = normalized height = z/zc (–) l = block placement density (–) ν = kinematic fluid viscosity (m2 s−1 ) ρ = fluid density (kg m−3 ) = standard deviation of bed material (–) σs = standard deviation of mean bed level zm (–) σb φ = roughness geometry function (–)

dc

Acknowledgements Prof. V. Nikora is gratefully acknowledged for his comments and suggestions, as well as the three reviewers and the Associate Editor. The authors thank the Swiss Federal Office for the Environment (FOEN) for the financial support.

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