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1.
Detailed echo‐sounder and acoustic Doppler velocimeter measurements are used to assess the temporal and spatial structure of turbulent flow over a mobile dune in a wide, low‐gradient, alluvial reach of the Green River. Based on the geometric position of the sensor over the bedforms, measurements were taken in the wake, in transitional flow at the bedform crest, and in the internal boundary layer. Spatial distributions of Reynolds shear stress, turbulent kinetic energy, turbulence intensity, and correlation coefficient are qualitatively consistent with those over fixed, two‐dimensional bedforms in laboratory flows. Spectral and cospectral analysis demonstrates that energy levels in the lee of the crest (i.e. wake) are two to four times greater than over the crest itself, with minima over the stoss slope (within the developing internal boundary layer). The frequency structure in the wake is sharply defined with single, dominant peaks. Peak and total spectral and cross‐spectral energies vary over the bedform in a manner consistent with wave‐like perturbations that ‘break’ or ‘roll up’ into vortices that amalgamate, grow in size, and eventually diffuse as they are advected downstream. Fluid oscillations in the lee of the dune demonstrate Strouhal similarity between laboratory and field environments, and correspondence between the peak frequencies of these oscillations and the periodicity of surface boils was observed in the field. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

2.
Fluvial bedforms generate a turbulent wake that can impact suspended-sediment settling in the passing flow. This impact has implications for local suspended-sediment transport, bedform stability, and channel evolution; however, it is typically not well-considered in geomorphologic models. Our study uses a three-dimensional OpenFOAM hydrodynamic and particle-tracking model to investigate how turbulence generated from bedforms and the channel bed influences medium sand-sized particle settling, in terms of the distribution of suspended particles within the flow field and particle-settling velocities. The model resolved the effect of an engineered bedform, which altered the flow field in a manner similar to a natural dune. The modelling scenarios alternated bed morphology and the simulation of turbulence, using detached eddy simulation (DES), to differentiate the influence of bedform-generated turbulence relative to that of turbulence generated from the channel bed. The bedform generated a turbulent wake that was composed of eddies with significant anisotropic properties. The eddies and, to a lesser degree, turbulence arising from velocity shear at the bed substantially reduced settling velocities relative to the settling velocities predicted in the absence of turbulence. The eddies tended to advect sediment particles in their primary direction, diffuse particles throughout the flow column, and reduced settling likely due to production of a positively skewed vertical-velocity fluctuation distribution. Study results suggest that the bedform wake has a significant impact on particle-settling behaviour (up to a 50% reduction in settling velocity) at a scale capable of modulating local suspended transport rates and bedform dynamics. © 2020 John Wiley & Sons, Ltd.  相似文献   

3.
This paper presents results of a field study designed to examine the structure of flow over mobile and fixed bedforms in a natural stream and to compare the results with findings of previous laboratory studies within the framework of double time–space averaging approach. Measurements of turbulence were obtained in a small river in Illinois, USA, over a fine spatial grid of sampling points above a mobile sandy bedform and its artificially moulded replica. Flow structure over the artificial bedform is similar to that observed in laboratory studies, but is markedly different from the flow structure over natural bedforms. These differences are most pronounced in the roughness sublayer, whereas flow in the logarithmic layer over natural and artificial sand waves is fairly similar and exhibits spatial uniformity. The double time–space averaged distributions of turbulence statistics conform to the multilayer model of flow structure over bedforms. Mean velocity distributions indicate neither classical flow recirculation nor substantial reduction of velocities in the lee of bedform crests. However, vertical patterns of turbulence statistics over depth suggest that stacked wakes similar to those observed in laboratory studies exist above the bedforms. Thus, despite the absence of flow separation, wake development seems to be induced by the systematic influence of upstream bedforms on the vertical structure of turbulence. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

4.
This study, using an experimental approach, focuses on the effect of downward seepage on a threshold alluvial channel morphology and corresponding turbulent flow characteristics. In all the experiments, we observed that the streamwise time‐averaged velocities and Reynolds shear stresses were increased under the influence of downward seepage. Scales of eddy length and eddy turnover time were significantly increased with the application of downward seepage, leading to sediment transport and initiation of bedforms along the channel length. As the amount of seepage discharge increased, eddy length and turnover time were further increased, causing the development of larger bedforms. It was revealed that the geometry of bedforms was linked with the size of eddies. In this work, statistics of bedform dynamics are presented in terms of multi‐scalar bedforms in the presence of seepage. These multi‐scalar ubiquitous bedforms cast a potential impact on flow turbulence as well as stream bed morphology in channels. We used wavelet to analyse temporally lagged spatial bed elevation profiles that were obtained from a set of laboratory experiments and synchronized the wavelet coefficients with bed elevation fluctuations at different length scales. A spatial cross‐correlation analysis, based on the wavelet coefficients, was performed on these bed elevation datasets to observe the effect of downward seepage on the dynamic behaviour of bedforms at different length scales. It was found that celerity of bedforms reduced with increase in seepage percentage. Bedform celerity was best approximated by a probability density function such as Rayleigh distribution under varying downward seepage. Further, statistical analysis of physical parameters of bedforms ascertained that the reduction in bedform celerity was a result of increased bedform size. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

5.
Laboratory observations and computational results for the response of bedform fields to rapid variations in discharge are compared and discussed. The simple case considered here begins with a relatively low discharge over a flat bed on which bedforms are initiated, followed by a short high‐flow period with double the original discharge, during which the morphology of the bedforms adjusts, followed in turn by a relatively long period of the original low discharge. For the grain size and hydraulic conditions selected, the Froude number remains subcritical during the experiment, and sediment moves predominantly as bedload. Observations show rapid development of quasi‐two‐dimensional bedforms during the initial period of low flow with increasing wavelength and height over the initial low‐flow period. When the flow increases, the bedforms rapidly increase in wavelength and height, as expected from other empirical results. When the flow decreases back to the original discharge, the height of the bedforms quickly decreases in response, but the wavelength decreases much more slowly. Computational results using an unsteady two‐dimensional flow model coupled to a disequilibrium bedload transport model for the same conditions simulate the formation and initial growth of the bedforms fairly accurately and also predict an increase in dimensions during the high‐flow period. However, the computational model predicts a much slower rate of wavelength increase, and also performs less accurately during the final low‐flow period, where the wavelength remains essentially constant, rather than decreasing. In addition, the numerical results show less variability in bedform wavelength and height than the measured values; the bedform shape is also somewhat different. Based on observations, these discrepancies may result from the simplified model for sediment particle step lengths used in the computational approach. Experiments show that the particle step length varies spatially and temporally over the bedforms during the evolution process. Assuming a constant value for the step length neglects the role of flow alterations in the bedload sediment‐transport process, which appears to result in predicted bedform wavelength changes smaller than those observed. However, observations also suggest that three‐dimensional effects play at least some role in the decrease of bedform wavelength, so incorporating better models for particle hop lengths alone may not be sufficient to improve model predictions. Published in 2011. This article is a US Government work and is in the public domain in the USA.  相似文献   

6.
The effect exerted by the seabed morphology on the flow is commonly expressed by the hydraulic roughness, a fundamental parameter in the understanding and simulation of hydro- and sediment dynamics in coastal areas. This study quantifies the hydraulic roughness of large compound bedforms throughout a tidal cycle and investigates its relationship to averaged bedform dimensions. Consecutive measurements with an acoustic Doppler current profiler and a multibeam echosounder were carried out in the Jade tidal channel (North Sea, Germany) along large compound bedforms comprising ebb-oriented primary bedforms with superimposed smaller secondary bedforms. Spatially averaged velocity profiles produced log-linear relationships which were used to calculate roughness lengths. During the flood phase, the velocity profiles were best described by a single log-linear fit related to the roughness created by the secondary bedforms. During the ebb phase, the velocity profiles were segmented, showing the existence of at least two boundary layers: a lower one scaling with the superimposed secondary bedforms and an upper one scaling with the ebb-oriented primary bedforms. The drag induced by the primary bedform during the ebb phase is suggested to be related to flow expansion, separation, and recirculation on the downstream side of the bedform. Three existing formulas were tested to predict roughness lengths from the local bedform dimensions. All three predicted the right order of magnitude for the average roughness length but failed to predict its variation over the tidal cycle.  相似文献   

7.
One explanation for bedform patterns is self‐organization in which the pattern emerges because of interactions among the bedforms themselves. Models, remote images, field studies and lab experiments have identified bedform interactions that involve whole bedforms, only bedform defects, or that are remote interactions between bedforms. It is proposed that bedform interactions form a spectrum from constructive to regenerative in pattern development. Constructive interactions, including merging, lateral linking, cannibalization, and remote transfer of sediment, push the system toward fewer, larger, more widely spaced bedforms. Regenerative interactions, including bedform splitting, defect creation and calving, push the system back toward a more initial state. Other interactions, including off‐center collision, defect migration, and bedform and defect repulsion, cause pattern change, but may not be strongly constructive or regenerative. Although bedform interactions are ubiquitous to any field of bedforms, their dynamics, flow‐field modification, and impact upon measurable pattern parameters are yet poorly understood. Most bedform interactions span bedform types and fluids, supporting the hypothesis that pattern emerges from dynamics at the bedform level in a hierarchy that includes lower levels of bedform‐flow and grain–fluid interactions. Bedform interactions alone, however, cannot account for the rich diversity of bedform patterns in nature. It is proposed that field diversity arises because of boundary conditions, which are the environmental variables within which a field evolves. Conceptually, boundary conditions modify the shape of the attractor toward which a field evolves, possibly by altering the type and frequency of bedform interactions. Boundary conditions are broadly similar within system types, but are unique for each bedform field so that no two are ever exactly alike. Although aeolian and fluvial systems share some types of boundary conditions, flow depth is a unique boundary condition in shallow fluvial systems. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

8.
The purpose of the present study is to investigate experimentally the development of bedforms in a configuration where the sediment supply is limited. The experimental setup is a rectangular closed duct combining an innovative system to control the rate of sediment supply Qin , and a digitizing system to measure in real time the 3D bedform topography. We carried out different sets of experiments with two sediment sizes (100 µm and 500 µm) varying both the sediment supply and the water flow rate to obtain a total of 46 different configurations. After a transient phase, steady sub‐centimeter bedforms of various shapes have been observed: barchans dunes, straight transverse dune, linguoid transverse dunes and bedload sheets. Height, spacing, migration speed, and mean bed elevation of the equilibrium bedforms were measured. For a given flow rate, two regimes were identified with fine sediment: (i) a monotonic increasing regime where the equilibrium bedform height and velocity increase with the sediment supply rate Qin and (ii) an invariant regime for which both parameters are almost independent of Qin. For coarse sediment, only the first regime is observed. We interpret the saturation of height and velocity for fine sediment bedforms as the transition from a supply‐limited regime to a transport‐limited regime in which the bedload flux has reached its maximum value under the prevailing flow conditions. We also demonstrate that all experiments can be rescaled if the migration speed and height of the bedforms are, respectively, divided and multiplied by the cube of the shear velocity. This normalization is independent of grain size and of bedform morphology. These experimental results provide a new quantification of the factors controlling equilibrium height and migration speed of bedforms in supply‐limited conditions against which theoretical and numerical models can be tested.  相似文献   

9.
Drumlins are subglacial bedforms streamlined in the direction of ice flow. Common in deglaciated landscapes, they have been widely studied providing rich information on their internal geology, size, shape, and spacing. In contrast with bedform investigations elsewhere in geomorphology (aeolian and fluvial dunes and ripples for example) most drumlin studies derive observations from relict, and thus static features. This has made it difficult to gain information and insights about their evolution over time, which likely hampers our understanding of the process(es) of drumlin formation. Here we take a morphological approach, studying drumlin size and spacing metrics. Unlike previous studies which have focussed on databases derived from entire ice sheet beds, we adopt a space‐for‐time substitution approach using individual drumlin flow‐sets distributed in space as proxies for different development times/periods. Framed and assisted by insights from aeolian and fluvial geomorphology, we use our metric data to explore possible scenarios of drumlin growth, evolution and interaction. We study the metrics of the size and spacing of 36 222 drumlins, distributed amongst 71 flow‐sets, left behind by the former British‐Irish Ice Sheet, and ask whether behaviour common to other bedform phenomena can be derived through statistical analysis. Through characterizing and analysing the shape of the probability distribution functions of size and spacing metrics for each flow‐set we argue that drumlins grow, and potentially migrate, as they evolve leading to pattern coarsening. Furthermore, our findings add support to the notion that no upper limit to drumlin size exists, and to the idea that perpetual coarsening could occur if given sufficient time. We propose that the framework of process and patterning commonly applied to non‐glacial bedforms is potentially powerful for understanding drumlin formation and for deciphering glacial landscapes. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

10.
Experiments are conducted in a laboratory flume on the propagation of a surface wave against unidirectional flow with a sediment bed. This article presents the spatial variation of bedforms induced by the wave-blocking phenomenon by a suitably tuned uniform fluid flow and a counter-propagating wave. The occurrence of wave-blocking is confirmed by finding a critical wave frequency in a particular flow discharge in which the waves are effectively blocked and is established using the linear dispersion relation. The purpose of this work is to identify wave-blocking and its influence on the development of bedforms over the sediment bed. Interestingly bedform signatures are observed at a transition of bedforms in three zones, with asymmetric ripples having a steeper slope downstream face induced by the incoming current, followed by flat sand bars beneath the wave-blocking zone and more symmetric ripples below the wave-dominated region at the downstream. This phenomenon suggests that the sediment bed is segmented into three different regions of bed geometry along the flow. The deviations of mean flows, Reynolds stresses, turbulent kinetic energy, and power spectral density due to the wave-blocking phenomenon are presented along the non-uniform flow over sediment bed. The bottom shear stress, bed roughness and stochastic nature of the bedform features are also discussed. The results are of relevance to engineers and geoscientists concerned with contemporary process as well as those interested in the interpretation of palaeoenvironmental conditions from fossil bedforms. © 2019 John Wiley & Sons, Ltd.  相似文献   

11.
Biogeochemical and ecological transformations in hyporheic zones are dependent on the timing of hyporheic exchange. We show through linked modeling of open channel turbulent flow, groundwater flow, and solute transport that the residence time distributions of solutes advected by hyporheic flow induced by current–bedform interaction follow power-laws. This tailing behavior of solutes exiting the sediments is explained by the presence of multiple path lengths coupled with very large variability in Darcy flow velocity, both occurring without heterogeneity in sediment permeability. Hyporheic exchange through bedforms will result in short-time fractal scaling of stream water chemistry.  相似文献   

12.
On the basis of experiments carried out in flume with a wavy bed with vegetation cover, flow velocity, turbulence intensities and Reynolds stress distributions are investigated. The wavy bed was similar to dune in this study. The fixed artificial dunes were constructed over the bed and artificial vegetation put over them in a laboratory flume. An Acoustic Doppler Velocimeter and spatially-averaged method were applied to determine turbulent flow components and shear velocity. Results were compared with a gravel bedform. It was observed that vegetation cover influences considerably the flow structure and displays clearly the flow separation and reattachment point. The law of the wall was not valid within the vegetation cover, but it was fitted well to the zone above the vegetation cover within the inner layer. For a wavy bed having the same dimensions, shear velocity and friction factor over vegetation cover are 1.7 and 2.6 times of those for the gravel bedform, respectively. The results of laboratory study were compared with those of river study.  相似文献   

13.
This paper summarizes measurements of velocity along three reaches of a small mountain channel with step–pool bedforms. A one‐dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph. Measurement cross‐sections were located upstream from a bed‐step, at the step lip, downstream from the step, and in a uniform‐gradient run. Data analyses focused on characteristics of velocity profiles, and on correlations between velocity characteristics and the potential control variables bedform type, reach gradient and flow depth. To test the hypothesis that velocity characteristics are related to channel bedform types, ANOVA and ANCOVA tests were performed for the average velocity and coefficient of variation of point velocity data. Results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge. Velocity became more variable as stage increased, particularly at low‐gradient reaches with less variable bed roughness. Velocity profiles suggest that locations immediately downstream from bed‐steps are dominated by wake turbulence from mid‐profile shear layers. Locations immediately upstream from steps, at step lips, and in runs are dominated by bed‐generated turbulence. Adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation, whereas favourable pressure gradients at steps are suppressing turbulence. The bed‐generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake‐generated turbulence and form drag of step–pool bedforms. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

14.
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15.
The aim of this experimental study is to investigate the interaction between turbulent flow and a gravelbed that mimics the actual roughness structures of a natural bed and its implication on sediment transport.In particular,the response of the Reynolds stresses and the role of intermittency to the bed roughness is the primary focus of the current study.To this end,the flow field,measured with an Acoustice Doppler Velocimeter (ADV),is thoroughly examined,considering the conditional Reynolds shea...  相似文献   

16.
The outline and trend of 6566 subglacial bedforms in the New York Drumlin Field have been digitized from digital elevation data. A spatial predictive model has been used to extend values of bedform elongation over an area measuring 200 km × 110 km. The resulting surface is used in conjunction with depth‐to‐bedrock data and an assumed duration of ice residence to test three proposed controls on bedform elongation. Upon comparison, the resulting display of morphometry is best explained by differences in ice velocity across the field of study. The existence of multiple zones of fast‐moving ice located along the southern margin of the Laurentide Ice Sheet is implied by the observed patterns of bedform elongation and orientation. We present two interpretations that are consistent with the observations. First, enhanced basal sliding caused by decreasing effective pressure near a calving margin is suggested as a possible mechanism by which localized fast ice flow is initiated and maintained. Second, topographically controlled ice streams likely occupied the fjord‐like troughs of the Appalachian Upland northern rim. Contrary to previous understanding of the Laurentide southern margin in New York State, the resulting palaeoglaciological reconstruction illustrates a dynamic mosaic of ice stream and/or outlet glacier activity. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

17.
Streams and rivers, particularly smaller ones, often do not maintain steady flow rates for long enough to reach equilibrium conditions for sediment transport and bed topography. In particular, streams in small watersheds may be subject to rapidly changing hydrographs, and relict bedforms from previous high flows can cause further disequilibrium that complicates the prediction of sediment transport rates. In order to advance the understanding of how bedforms respond to rapid changes in flow rate,...  相似文献   

18.
Suspended sediment is conventionally regarded as that sediment transported by a fluid that it is fine enough for turbulent eddies to outweigh settling of the particles through the fluid. Early work in the fluvial field attributed suspension to turbulence, and led to the notion of a critical threshold for maintaining sediment in suspension. However, research on both turbulence structures and the interactions between suspended sediment and bedforms in rivers has shown a more complex story and, although there appear to have been no studies of the impact of bedforms on aeolian suspended sediment concentrations, turbulent flow structures and transport rates of saltating particles have been shown to be affected. This research indicates that suspended sediment neither travels with the same velocity as the flow in which it is suspended, nor is it likely to remain in suspension in perpetuity, even under conditions of steady flow or in unsteady flow the where dimensionless critical threshold is permanently exceeded. Rather, like bedload, it travels in a series of hops, and is repeatedly deposited on the bed where it remains until it is re‐entrained. Is there, therefore, a qualitative difference between suspended and saltating sediment, or is it just a quantitative difference in the size of the jump length and the frequency of re‐entrainment? It is our contention that the distinction of suspension as a separate class of sediment transport is both arbitrary and an unhelpful anthropocentric artefact. If we recognize that sediment transport is a continuum and applies to any fluid medium rather than split into different “processes” based on arbitrary thresholds and fluids, then recognizing the continuity will enable development of an holistic approach sediment transport, and thus sediment‐transport models that are likely to be viable across a wider range of conditions than hitherto. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

19.
《国际泥沙研究》2020,35(6):666-678
The current paper investigates the flow and turbulence characteristics over dune bedforms by means of laboratory experiments, where spatially dense and temporally high frequency velocity measurements were done. Although similar studies are available in the literature, the focus and novelty of the current study is to assess the influence of surface roughness of the dune bedforms on the nearbed flow. For direct comparison, two different surface roughness heights over idealized, fixed-shaped, high-angled dune bedforms were tested; one with a hydraulically-smooth surface, and the other with a fully-rough surface. Spatial variation of time-averaged flow as well as turbulence statistics were examined, which was complemented by streamline plots and spectral analyses. The results are interpreted from sediment entrainment and sediment transport points of view. The results show that increased dune surface roughness reduces the nearbed flow velocity, but increases the flow velocities at upper regions. The upward directed flow near the dune crests becomes stronger in the case of smooth surface, while the re-attachment point moves further downstream compared to the rough wall case. It is concluded that the roughness of the dune surface affects the nearbed flow and turbulence characteristics qualitatively and quantitatively, which is shown to have direct consequences on sediment entrainment characteristics.  相似文献   

20.
Early‐stage aeolian bedforms, or protodunes, are elemental in the continuum of dune development and act as essential precursors to mature dunes. Despite this, we know very little about the processes and feedback mechanisms that shape these nascent bedforms. Whilst theory and conceptual models have offered some explanation for protodune existence and development, until now, we have lacked the technical capability to measure such small bedforms in aeolian settings. Here, we employ terrestrial laser scanning to measure morphological change at the high frequency and spatial resolution required to gain new insights into protodune behaviour. On a 0.06 m high protodune, we observe vertical growth of the crest by 0.005 m in two hours. Our direct measurements of sand transport on the protodune account for such growth, with a reduction in time‐averaged sediment flux of 18% observed over the crestal region. Detailed measurements of form also establish key points of morphological change on the protodune. The position on the stoss slope where erosion switches to deposition is found at a point 0.07 m upwind of the crest. This finding supports recent models that explain vertical dune growth through an upwind shift of this switching point. Observations also show characteristic changes in the asymmetric cross‐section of the protodune. Flow‐form feedbacks result in a steepening of the lee slope and a decline in lower stoss slope steepness (by 3°), constituting a reshaping of protodune form towards more mature dune morphology. The approaches and findings applied here, (a) demonstrate an ability to quantify processes at requisite spatial and temporal scales for monitoring early‐stage dune evolution, (b) highlight the crucial role of form‐flow feedbacks in enabling early‐stage bedform growth, alluding to a fluctuation in feedbacks that require better representation in dune models, and (c) provide a new stimulus for advancing understanding of aeolian bedforms. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

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