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The equations for double-mean, form-induced and spatially averaged turbulent energy budgets are employed to analyse data from direct numerical simulations of turbulent open-channel flows over trans...

There is uncertainty on the ecological effects of tidal stream turbines. Concerns include animal collision with turbine blades, disruption of migratory and foraging behaviour, attraction of animals to prey aggregating around turbines, or conversely displacement of animals from preferred habitat. This study used concurrent ecological and physical measurements to show the predictability of fish school characteristics (presence, school area and height above seabed) in a high energy tidal site across spring/neap, ebb/flood and daily cycles, and how this changed around a turbine structure. The rate of schools and school area per hour increased by 1.74 and 1.75 times respectively around a turbine structure compared to observations under similar conditions without a turbine structure. The largest schools occurred at peak flow speeds and the vertical distribution of schools over the diel cycle was altered around the turbine structure. While predictable attraction or aggregation of prey may increase prey availability and predator foraging efficiency, attraction of predators has the potential to increase animal collision risk. Predictable changes from the installation of turbine structures can be used to estimate cumulative effects on predators at a population level. This study can guide a strategic approach to the monitoring and management of turbines and arrays.

In the dynamic environments targeted for marine renewable energy extraction, such as tidal channels, the natural distribution of fish and behavioural impacts of marine renewable energy installations (MREIs) are poorly understood. This study builds on recent methodological developments to reveal the behaviour of fish schools using data collected by a seabed-mounted echosounder deployed in extreme tidal flows and in the wake of a MREI (composed of the foundation of a full-scale tidal stream energy turbine). The results show a significant change (p

Stereoscopic particle image velocimetry measurements of open-channel flows over streamwise-orientated triangular-shaped ridges were used to explore interactions between ridge-induced secondary currents (SCs) and turbulence. Terms in the double-averaged (in space and in time) momentum and energy conservation equations were analysed for a range of ridge spacings ( near the elevations of SC cell centres. Partitioning the total kinetic energy into double-mean (DMKE), dispersive (DKE) and turbulent (SATKE) components and considering the balance equation for each component indicated that at the elevation of SC cell centres the production rate of SATKE via exchange with DMKE was comparable in magnitude to the production rate via exchange with DKE (due to SCs). For all ridge spacings, SATKE was reduced compared to a no-ridge benchmark case due to suppression of very-large-scale turbulent motions by the SCs. Finally, it is demonstrated that energy is supplied to SCs by turbulence.

Streamwise ridges in open-channel flows induce secondary currents which alter the flow structure relative to the scenario with no ridges on the bed. We investigate the effects of ridge spacing, cross-section, surface roughness, and relative submergence on the hydraulic resistance in open-channel flows. Our results show that at small ridge spacing, the hydraulic resistance is reduced by up to 32% relative to a prediction based on the divided channel method. As the ridge spacing increased, the hydraulic resistance is amplified by up to 35% before reducing again and matching that of a case with no ridges. For comparable ridge spacings and flow depths, the hydraulic resistance over square ridges was less than that over triangular ridges. Increasing the ridge surface roughness increased the hydraulic resistance relative to the smooth ridge scenarios. Interestingly, the streamwise ridges were observed to suppress the water surface fluctuations relative to a case with no ridges.

International audience; Free-surface shallow mixing layers behind a splitter plate were studied in a flume of rectangular cross-section consisting of a bed and sidewalls made of glass. The focus is on the effects on the emergence of horizontal Kelvin-Helmholtz-type coherent structures (KHCSs) of vertical flow confinement (i.e., flow depth) and dimensionless velocity shear λ = (Ux2 − Ux1)/(Ux2+Ux1), where Ux1 and Ux2 are two flow velocities characterizing the low-speed stream and high-speed stream, respectively. Two flow depths were tested, with the λ-value at the flume entrance varying between 0.1 to 0.8, with a threshold value λt = 0.3 needed for the emergence of KHCSs in compound open-channel flows being inside this range. This paper reports new data that provide a better insight in the interplay between the constraining effect of flow depth, damping effect of three-dimensional bed-induced turbulence, and triggering effect of dimensionless shear on the formation of KHCSs.

International audience; The structure of free-surface flows in a straight compound channel was investigated in a laboratory flume, consisting of a central smooth-bed main channel (MC) and two adjacent rough-surface floodplains (FPs). The experiments covered both uniform and non-uniform flow conditions, with the latter generated by imposing an imbalance in the discharge distribution between MC and FPs at the flume entrance. The non-uniform cases involved transverse currents directed from MC to FPs and vice-versa. The focus of the study was on assessing the effects of transverse currents on: (i) transverse shear layer and horizontal Kelvin-Helmholtz type coherent structures (KHCSs) forming at the interfaces between MC and FPs; (ii) helical secondary currents (SCs) developing across the channel due to topography-induced flow heterogeneity; and (iii) turbulent large-and very-large-scale motions (VLSMs). Transverse currents can entirely displace the shear layer over FP or in MC, but they do not alter the KHCSs to the same degree, resulting in a mismatch between shear layer extent and KHCSs length-scales. KHCSs emerge once dimensionless velocity shear exceeds a critical value above which KHCSs length-scales increase with the shear. Three well-established SC cells, which are induced by turbulence anisotropy, are observed in uniform flow and non-uniform flow with transverse currents towards FP. They are replaced by a single cell in the presence of a transverse mean flow towards MC. The spectral signatures of VLSMs are visible at the upstream section of the flume but they quickly disappear along the flow being suppressed by simultaneous development of KHCSs and SCs.

Stereoscopic particle image velocimetry (PIV) configured in two orthogonal planes was utilised to capture the flow structure at the instant of entrainment of spherical bed particles in open-channel flow. Experiments were conducted with lightweight target particles amongst a bed of coplanar fixed spheres with diameters of 16 mm. The protrusions of the target particles were set to give an average entrainment rate of . These protrusions were established from extensive initial experiments which utilised an automated mechanism to place spheres on the bed of the flume and record the time elapsed until they were entrained by the flow. The results showed that at lower flow depth to particle diameter ratios, bed particles are more stable and require larger protrusions to entrain at the same rate as at a larger depth. This effect is consistent with observations of reduced velocity variance and reduced drag force variance for lower flow submergences. The PIV measurements indicated that particle entrainment is associated with very large-scale motions which extend up to 50 flow depths in the streamwise direction. Contributions of smaller scale velocity and pressure spatial fluctuations are suppressed by a spatial averaging effect related to the particle size, and a temporal averaging effect related to the time taken to fully entrain a particle from its resting pocket. These observations are relevant to sediment transport modelling. However, further data are required to clarify the role of particle lift forces and particle shape in the entrainment process.

It is widely acknowledged that streamwise ridges on the bed of open-channel flows generate secondary currents (SCs). A recent discovery of meandering long streamwise counter-rotating vortices in open-channel flows, known as very-large-scale motions (VLSMs), raises a question regarding the interrelations between VLSMs and SCs in flows over ridge-covered fully rough beds. To address it, we conducted long-duration experiments using stereoscopic particle image velocimetry, covering a range of ridge spacings (, two SC cells are formed between neighbouring ridges and VLSMs are entirely suppressed, suggesting that ridge-induced SCs prevent the formation of VLSMs by absorbing their energy or overpowering their formation. At the same time, velocity auto- and cross-spectra reveal a new feature that can be explained by low-amplitude meandering of the alternating low- and high-momentum flow regions associated with instantaneous manifestations of SCs. Two-point velocity correlations and smooth velocity field reconstructions using proper orthogonal decomposition further support the validity of this effect. Its origin is probably due to the instability related to the presence of inflection points in the spanwise distribution of the streamwise velocity within the SC cells. These results have implications for bed friction in open channels, where the friction factor may increase if depth-scale SCs are present, or decrease under conditions of sub-depth-scale SCs and suppressed VLSMs.

This Technical Note reports on new experiments focused on hydraulic resistance in open-channel flows over beds covered with streamwise ridges. Extensive bulk friction factor measurements, c...

Aquatic macrophytes are a key component of river systems around the world. Surveys of macrophyte surface cover, cross-sectional blockage and plant/patch sizes provide data for river manager...

Flow–vegetation interactions is an interdisciplinary research area with applications in the management of coastal waters, lakes, and watercourses. Due to an emerging interest in the cultivation of seaweeds, this study seeks to develop a sound understanding of the physical interactions between flow and seaweeds. This is achieved via experiments in a laboratory flume using plastic-made models of blades of the seaweed species Saccharina latissima. In the experiments, strain gages, a digital camera, and acoustic Doppler velocimeters were used for measuring drag forces, blade movements (reconfiguration), and flow velocities. The study involved experiments with single blades and with pairs of tandem blades at different spacing between the blades. The revealed mechanisms controlling the dynamics of seaweed blade models varied depending on the ratio of blade length to eddy length scale. The drag coefficient of seaweed blade models appeared to be dependent on the Reynolds number, the Cauchy number, and the ratio of blade length to integral turbulence length scale. Turbulence had a primary role in controlling blade model dynamics and its drag coefficient. Seaweed blade models affected the flow in their wakes by increasing the turbulence intensity and reducing the mean longitudinal velocity. These effects on the flow are the reason for which, in a pair of tandem blades, the drag force experienced by the downstream blade is lower than that experienced by the upstream blade.

Field measurements of the flow in the benthic boundary layer (BBL) of a tidal channel are presented which compare data collected in the wake of a marine renewable energy installation (MREI) with control data representative of the natural conditions. The results show significant flow modification in the wake of the MREI including a reduction in mean velocity, enhanced turbulence, and the breakdown of the natural structure and dynamics of the BBL. This study provides new information relevant to the environmental impact assessment of MREIs and to the design and consenting of marine renewable energy projects.

Interactions between water flow and aquatic vegetation strongly depend on morphological and biomechanical characteristics of vegetation. Although any physical or numerical model that aims to replicate flow-vegetation interactions requires these characteristics, information on morphology and mechanics of vegetation living in coastal waters remains insufficient. The present study investigates the mechanical properties of blades of Saccharina latissima, a seaweed species spread along the shores of the UK and North East Atlantic. More than 50 seaweed samples with lengths spanning from 150 mm to 650 mm were collected from Loch Fyne (Scotland) and tested. Seaweed blades had a natural ‘stretched droplet’ shape with bullations in the central fascia and ruffled edges in the area close to the stipe. Their morphological features showed high variability for samples longer than 400 mm. The blades were almost neutrally buoyant, their material was found to be very flexible and ductile, being stiffer in longer blades. The laboratory tests showed that estimates of tensile Young's modulus appeared to be similar to bending Young's modulus suggesting a reasonable degree of isotropy in studied seaweed tissues.

Long-duration particle image velocimetry measurements in rough-bed open-channel flows (OCFs) reveal that the pre-multiplied spectra of the streamwise velocity have a bimodal distribution due to the presence of large- and very-large-scale motions (LSMs and VLSMs, respectively). The existence of VLSMs in boundary layers, pipes and closed channels has been acknowledged for some time, but strong supporting evidence for their presence in OCF has been lacking. The data reported in this paper fill this gap. Length scales of the LSMs and VLSMs in OCF exhibit different scaling properties; whereas the streamwise length of the LSM scales with the flow depth, the VLSM streamwise length does not scale purely with flow depth and may additionally depend on other scales such as the channel width, roughness height or viscous length. The transverse extent of the LSMs was found to increase with increasing elevation, but the VLSM transverse scale is anchored around two flow depths. The origin and nature of LSMs and VLSMs are still to be resolved, but differences in their scaling suggest that VLSMs in rough-bed OCFs form independently rather than as a spatial alignment of LSMs.

This paper presents the application of the double-averaging methodology to actual data obtained for the mobile-bed conditions from direct numerical simulations using an immersed boundary method. The dimensions of the computational domain resemble those for open-channel flows with small relative submergence. The domain bottom is a plane covered with one layer of hexagonally packed, single-size spheres fixed to the bed. The fixed particles are covered by 2000 mobile particles of the same size that are free to move. Two simulation scenarios at distinctly different values of the Shields parameter are studied representing a fully mobilized and partially mobilized granular bed. The effects of the averaging time and the averaging domain size and shape on the double-averaged flow quantities are identified first. Then, the data analysis focuses on the detailed assessment of the key terms of the double-averaged momentum balance equation formulated for mobile-bed conditions. The paper demonstrates that the d...

Based on direct numerical simulations, the paper investigates momentum fluxes and hydrodynamic stresses within and above a mobile granular bed in a turbulent open-channel flow laden with monodisperse spherical particles. Two simulation scenarios are considered: one with partially mobilized particles (“heavy” particles) and another with all particles in motion (“light” particles). The momentum fluxes were computed as temporal and spatial averages following the double-averaging methodology for rough-bed flows. The analysis, hence, allows the DNS data to be convoluted in a rigorous way to provide detailed description and quantification of the physical mechanisms involved in momentum exchanges. In particular, it was found that heavy particles create streamwise bedforms causing heterogeneity in the spanwise direction. This yields significant form-induced momentum fluxes that cannot be neglected. For both scenarios, the mobile particles take up a substantial amount of the momentum supplied, which ultima...

Funded by University of Aberdeen Natural Environment Research Council (NERC) Department for Environment, Food, and Rural Affairs. Grant Number: NE/J004308/1 Marine Collaboration Research Forum

A theoretically based relationship for the Darcy–Weisbach friction factor $f$ for rough-bed open-channel flows is derived and discussed. The derivation procedure is based on the double averaging (in time and space) of the Navier–Stokes equation followed by repeated integration across the flow. The obtained relationship explicitly shows that the friction factor can be split into at least five additive components, due to: (i) viscous stress; (ii) turbulent stress; (iii) dispersive stress (which in turn can be subdivided into two parts, due to bed roughness and secondary currents); (iv) flow unsteadiness and non-uniformity; and (v) spatial heterogeneity of fluid stresses in a bed-parallel plane. These constitutive components account for the roughness geometry effect and highlight the significance of the turbulent and dispersive stresses in the near-bed region where their values are largest. To explore the potential of the proposed relationship, an extensive data set has been assembled by employing specially designed large-eddy simulations and laboratory experiments for a wide range of Reynolds numbers. Flows over self-affine rough boundaries, which are representative of natural and man-made surfaces, are considered. The data analysis focuses on the effects of roughness geometry (i.e. spectral slope in the bed elevation spectra), relative submergence of roughness elements and flow and roughness Reynolds numbers, all of which are found to be substantial. It is revealed that at sufficiently high Reynolds numbers the roughness-induced and secondary-currents-induced dispersive stresses may play significant roles in generating bed friction, complementing the dominant turbulent stress contribution.

Vegetation surrogates have been extensively used in laboratory experiments for studying flow–vegetation interactions. However, it remains unclear how accurately the surrogates replicate the prototy...

This paper reports the morphology of a natural patch of Ranunculus penicillatus and presents high-resolution measurements of flow velocities in its wake using a stereoscopic PIV field measurement system. The patch was 3.80 m long, 1.24 m wide and caused substantial changes to downstream mean velocities and turbulence. Vertical profiles of streamwise mean velocity were not logarithmic and flow was redirected under the positively buoyant canopy, enhancing vertical turbulent mixing in the wake and generating a large region where the velocity covariance u'w' was positive. Turbulent kinetic energy was enhanced downstream from the patch lateral shear layer, but not at the centre of the wake. Spectra downstream from the patch showed that turbulence was neither dominated by fine-scale nor large-scale structures, likely due to the low energy of the flow conditions and lack of a developed vortex street within the measurement domain. Sedimentation was observed at the upstream end of the patch, but not underneath the floating canopy. The methods and results of this work will be useful for planning other in situ studies. Also, the reported data on macrophyte geometry and biometrics will assist with the design of more realistic replicas for use in laboratory studies.

Physical interactions between seaweed blades of Saccharina latissima and unidirectional turbulent flow were examined in an open-channel flume, focussing on flow velocities, drag force acting on a blade, and blade reconfiguration. The data reveal that seaweed blades adjust to high-energy flow conditions relatively quickly, efficiently reducing flow-induced drag via compaction, a mechanism of blade reconfiguration. The drag coefficient of blades of S. latissima varied between 0.02 and 0.07 over a range of mean flow velocities from 0.1 to 0.55 m/s. Both flow action and blade biomechanical characteristics influenced the blade dynamics, with the flow role being predominant in highly energetic conditions. The interaction mechanisms and their strength were found to be scale-dependent, with the combined effect of reduced mean flow velocity and enhanced turbulence in blade wakes. The thickness of the diffusive boundary layer, an important factor in nutrient uptake from the surrounding water, was estimated to be in the range from 0.010 to 0.067 mm. Mechanisms of blade adjustment to the flow and scale-dependent dynamic interactions between blades and turbulent eddies have direct implications for seaweed growth, acclimation, and survival. The estimates of the drag coefficient and the thickness of the diffusive boundary layer will be useful for the development of bio-physical models, environmental assessments, and design of seaweed farms.

A novel technique has been developed to measure bed particle flight durations and amplitudes using the impact data from the records of instantaneous pressure acting on multiple bed particles. It has the potential to measure many more particle jumps compared to other methods, thus generating records long enough for well-converged statistics of the variables of interest. The proposed technique has been validated with independent measurements by a tracking method using a four camera particle image velocimetry system. The application of this technique in a laboratory study revealed that probability distributions and statistical moments of flight durations and heights are strongly dependent on flow submergence and particle density.

Seaweeds are primary producers that form a key component of coastal ecosystems. In estuaries and the intertidal zone they are frequently exposed to temporary hyposaline conditions. Previous research has shown that seaweed health status and photosynthetic activity are reduced due to hyposaline stress, but its effects on seaweed physical properties are unknown. This knowledge gap has important implications for the prediction of seaweed hydrodynamics and mechanical failure due to hydrodynamic stress, particularly in extreme conditions (e.g. storm events). In this study, we begin to address this knowledge gap by investigating the effects of hyposaline stress on the morphological and mechanical properties of the kelp Saccharina latissima (order Laminariales). We analysed the morphology of 23 seaweed blades and performed tension and bending tests on more than 90 samples prepared from them. The obtained data provide evidence that both morphology and mechanics of S. latissima are affected by hyposaline stress, i.e.: (i) blades bleach, develop blisters underneath the cortex, and change dimensions (increased volume and thickness, decreased width); and (ii) blade material becomes more flexible and more difficult to break (i.e. tougher). The results indicated a significant correlation between the time of exposure and changes in biomechanics of S. latissima with potential implications for seaweed hydrodynamics and survival strategies.

The scaling behaviour of landscape properties, including both morphological and landscape patchiness, is examined using monofractal and multifractal analysis. The study is confined to two neighbouring meso-scale catchments on the west coast of the South Island of New Zealand. The catchments offer a diverse but largely undisturbed landscape with population and development impacts being extremely low. Bulk landscape properties of the catchments (and their sub-basins) are examined and show that scaling of stream networks follow Hack’s empirical rule, with exponents ∼0.6. It is also found that the longitudinal and transverse scaling exponents of stream networks equate to νl ≈0.6 and νw≈ 0.4, indicative of self-affine scaling. Catchment shapes also show self-affine behaviour. Further, scaling of landscape patches show multifractal behaviour and the analysis of these variables yields the characteristic parabolic curves known as multifractal spectra. A novel analytical approach is adopted by using catchments as hydrological cells at various sizes, ranging from first to sixth order, as the unit of measure. This approach is presented as an alternative to the box-counting method as it may be much more representative of hydro-ecological processes at catchment scales. Multifractal spectra are generated for each landscape property and spectral parameters such as the range in α (Holder exponent) values and maximum dimension at α0, (also known as the capacity dimension Dcap), are obtained. Other fractal dimensions (information Dinf and correlation Dcor) are also calculated and compared. The dimensions are connected by the inequality Dcap≥Dinf≥Dcor. Such a relationship strongly suggests that the landscape patches are heterogeneous in nature and that their scaling behaviour can be described as multifractal. The quantitative parameters obtained from the spectra may provide the basis for improved parameterisation of ecological and hydrological models. Keywords: fractal, multifractal, scaling, landscape, patchiness

Long duration PIV measurements in rough-bed (glass beads) open-channel flow (OCF) reveal that the pre-multiplied spectra of the streamwise velocity has a bimodal distribution due to the presence of large and very large scale motions (LSMs and VLSMs, respectively). The existence of VLSMs in boundary layers, pipes and closed channels has been acknowledged for some time, but strong supporting evidence for their presence in OCF has been lacking. Length scales of the large and very large scale motions in OCF exhibit different scaling properties; whereas the streamwise length of the LSM scales with the flow depth, the VLSM streamwise length does not scale purely with flow depth and may additionally depend on other scales such as the channel width, roughness height, or viscous length. Supplementary data for flows over self-affine fractal rough beds support these findings and additionally indicate that the length of VLSMs may grow along the extensive distance from the channel entrance. The origin and nature of LSMs and VLSMs are still to be resolved, but differences in their scaling suggest that VLSMs in rough-bed open-channel flows form independently rather than as a spatial alignment of LSMs.

The motion of bedload particles is diffusive and occurs within at least three scale ranges: local, intermediate and global, each of which with a distinctly different diffusion regime. However, these regimes, extensions of the scale ranges and boundaries between them remain to be better defined and quantified. These issues are explored using a Lagrangian model of saltating grains over the uniform fixed bed. The model combines deterministic particle motion dynamics with stochastic characteristics such as probability distributions of step lengths and resting times. Specifically, it is proposed that a memoryless exponential distribution is an appropriate model for the distribution of rest periods while the probability that a particle stops after a current jump follows a binomial distribution, which is a distribution with lack of memory as well. These distributions are incorporated in the deterministic Lagrangian model of saltating grains and extensive numerical simulations are conducted for the identification of the diffusive behavior of particles at different time scales. Based on the simulations and physical considerations, the local, intermediate, and global scale ranges are quantified and the transitions from one range to another are studied for a spectrum of motion parameters. The obtained results demonstrate that two different time scales should be considered for parameterization of diffusive behavior within intermediate and global scale ranges and for defining the local–intermediate and intermediate–global boundaries. The simulations highlight the importance of the distributions of the step lengths and resting times for the identification of the boundaries (or transition intervals) between the scale ranges.

This is the first volume of a two-volume guide to designing, conducting and interpreting laboratory and field experiments in a broad range of topics associated with hydraulic engineering. Specific guidance is provided on methods and instruments currently used in experimental hydraulics, with emphasis on new and emerging measurement technologies and methods of analysis. Additionally, this book offers a concise outline of essential background theory, underscoring the intrinsic connection between theory and experiments. This book is much needed, as experimental hydraulicians have had to refer to guidance scattered in scientific papers or specialized monographs on essential aspects of laboratory and fieldwork practice. The book is the result of the first substantial effort in the community of hydraulic engineering to describe in one place all the components of experimental hydraulics. Included is the work of a team of more than 45 professional experimentalists, who explore innovative approaches to the vast array of experiments of differing complexity encountered by today’s hydraulic engineer, from laboratory to field, from simple but well-conceived to complex and well-instrumented. The style of this book is intentionally succinct, making frequent use of convenient summaries, tables and examples to present information. All researchers, practitioners, and students conducting or evaluating experiments in hydraulics will find this book useful.

Biomechanical properties and morphological characteristics of stems of eight species of submerged aquatic plants were studied to analyse (1) differences between river and lake specimens, (2) seasonal differences between winter/spring and summer/autumn specimens, and (3) change of biomechanical properties and morphological characteristics along the stems. The data show that river macrophytes display not only characteristic biomechanical traits and morphological characteristics specific to their hydraulic habitats, but also distinctive temporal changes due to seasonally varying water temperature, flow velocity, and growth phase. Furthermore, the data reveal differences between lake and river specimens that could be explained by wind exposure of the lake sampling sites and the species-specific flow requirements of the river macrophytes. Biomechanical properties and morphological characteristics varied along the stem with larger cross-sections and a higher resistance against tension and bending forces at the bottom compared to the top parts, being similar for both lake and river specimens. The acquired and analysed stem biomechanical and morphological data contribute to the plant biomechanics database to underpin a wide range of studies in aquatic ecology, river and wetland management.

The Author should be commended for an interesting paper on the concepts of self-similarity and scale invariance and their use in interpreting scale effects in Froude model-based studies. The Discus...

Vertical profile of longitudinal velocity in vegetated channels reflects complex mechanics of flow-vegetation interactions and determines the bulk flow velocity and flow rate. Most available models of velocity profiles in vegetated channels are based on a single physical concept that underpins theoretical considerations and data interpretation. However, measured velocity profiles suggest that the use of a single concept is not sufficient to cover all possible scenarios of flow-vegetation interactions. As a result, a number of models in which different concepts are applied to different flow regions have been recently developed. Within this framework, the overall velocity profile is represented with a set of linked segments. Although such segment-based models have improved velocity profile description, there is a need for more robust approaches and better analytical formulations. This paper proposes a new approach where a vertical velocity profile in vegetated channels is modelled as a linear superp...

A stereoscopic particle image velocimetry (PIV) system for use in shallow (${\sim }$0.5 m deep) rivers was developed and deployed in the Urie River, Scotland, to study the interactions between turbulent flow and a Ranunculus penicillatus plant patch in its native environment. Statistical moments of the velocity field were calculated utilizing a new method of reducing the contribution of measurement noise, based on the measurement redundancy inherent in the stereoscopic PIV method. Reynolds normal and shear stresses, their budget terms, and higher-order moments of the velocity probability distribution in the wake of the plant patch were found to be dominated by the presence of a free shear layer induced by the plant drag. Plant motion, estimated from the PIV images, was characterized by travelling waves that propagate along the plant with a velocity similar to the eddy convection velocity, suggesting a direct coupling between turbulence and the plant motion. The characteristic frequency of the plant velocity fluctuations (${\sim }$1 Hz) may suggest that the plant motion is dominated by large eddies with scale similar to the flow depth or plant length. Plant and fluid velocity fluctuations were, in contrast, found to be strongly correlated only over a narrow (${\sim }$30 mm) elevation range above the top of the plant, supporting a contribution of the shear layer turbulence to the plant motion. Many aspects of flow–aquatic plant interactions remain to be clarified, and the newly developed stereoscopic field PIV system should prove valuable in future studies.

Summary Proliferations of periphyton associated with the ecological degradation of streams have been linked to changes in flow regime, most frequently as a product of water abstraction and impoundment. We used descriptive and experimental studies to examine the relationship between water velocity and the distribution of two distinct, often high biomass, periphyton patch types and associated macroinvertebrate communities in the Waipara River, a meso-eutrophic stream in Canterbury, New Zealand. One patch type (‘Phormidium’) consisted mainly of two filamentous cyanobacterial taxa: a Phormidium/Lyngbya complex and Oscillatoria. The other type (‘filamentous greens’) consisted mainly of several filamentous chlorophyte taxa (Mougeotia sp.) along with various diatom epiphytes. Ash-free dry mass, chlorophyll a and particulate N and P concentrations were significantly greater in the Phormidium than in the filamentous green patches. There was a significant relationship between patch cover and water velocity, with filamentous greens negatively associated with velocity and dominant in areas with a velocity ∼0.40 m s−1. A 7-day experimental reduction in velocity in Phormidium-dominated habitats resulted in a significant reduction in% cover (84–21%) with a corresponding increase in filamentous greens (16–79%). There was a small but significant reduction in the % cover of filamentous greens (90–82%) with experimentally increased velocity, but no significant change in Phormidium cover (7–2%), perhaps due in part to the slow growth rate of taxa in this patch type. Characterisation of macroinvertebrate assemblages collected from Phormidium and filamentous green patches and the substratum immediately below showed patch-specific assemblages, although patch type was a less important determinant than vertical location, with 85% of the macroinvertebrates found in the substratum below algal mats. The differential responses of Phormidium and filamentous green patches to velocity are considered in the context of a subsidy-stress model that examines how trade-offs between flow-mediated biomass accrual and loss processes vary for different algal growth forms. Collectively, these results indicated that key ecological characteristics of rivers can be quite sensitive to spatial and temporal variations in hydraulic conditions that are much smaller than those typical of floods and droughts.

This article explores the interrelations between flow, fluctuating drag forces, and dynamic reconfiguration experienced by four freshwater plant species in response to turbulent open-channel flow. The study is based on experiments conducted in a laboratory flume using single specimens of Ranunculus penicillatus, Fontinalis antipyretica, Myriophyllum alterniflorum, and Callitriche stagnalis. A significant correlation between drag fluctuations, plant movements, and upstream turbulence was found. The data also revealed that the plants in general exhibited spatial flapping-like motion, with the propagation velocity of perturbations along the plant being approximately constant and comparable to the upstream flow velocity. The analysis of statistical moments, spectra, cross-correlations, and coherence functions provided strong support to the conclusion that the dynamic reconfiguration directly affects the drag force exerted on the plants, in addition to the effect of the upstream turbulence.

[1] This paper reports results from experiments with simultaneous measurement of areal bed-load sediment concentration and water velocity (horizontal and vertical components) made at 1.5 particle diameters above the mean bed. The same spatial and temporal scales were employed to measure the kinematic properties of flowing water and sediments. The obtained data, covering a range of low sediment discharges, were analyzed in terms of bulk statistics, autocorrelation and cross-correlation functions, and quadrant decomposition of fluctuations. At the lowest transport rates, the motion of sediment particles was found to be mostly associated with sweep events. However, with increasing flow rate and bed-load discharge, the process mechanics changed, and the transport of sediments was also found to be significantly correlated with ejections. The obtained results are integrated into a conceptual picture attempting to bridge previous depictions of the bed-load transport mechanics.