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1.
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   

2.
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   

3.
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   

4.
Two datasets of turbulence velocities collected over different bedform types under contrasting experimental conditions show similarity in terms of velocity‐intermittency characteristics and suggest a universality to the velocity‐intermittency structure for flow over bedforms. One dataset was obtained by sampling flow over static bedforms in different locations, and the other was based on a static position but mobile bedforms. A flow classification based on the velocity‐intermittency behaviour is shown to reveal some differences from that based on an analysis of Reynolds stresses, boundary layer correlation and turbulent kinetic energy. This may be attributed to the intermittency variable, which captures the local effect of individual turbulent flow structures. Locations in the wake region or the outer layer of the flow are both shown to have a velocity‐intermittency behaviour that departs from that for idealized wakes or outer layer flow because of the superposition of localized flow structures generated by bedforms. The combined effect of this yields a velocity‐intermittency structure unique to bedform flow. The use of a time series of a single velocity component highlights the potential power of our approach for field, numerical and laboratory studies. The further validation of the velocity‐intermittency method for non‐idealized flows undertaken here suggests that this technique can be used for flow classification purposes in geomorphology, hydraulics, meteorology and environmental fluid mechanics. © 2014 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

5.
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   

6.
This paper investigates by means of several large eddy simulations how the channel aspect ratio affects the transport and settling of suspended sediments. The numerical method is successfully validated using data of a physical experiment of fine sediment net deposition in an open channel flow. The channel aspect ratio, A, is known to be the determining factor for the development, strength and distribution of the turbulence‐driven secondary flow, and it is demonstrated that A influences the primary flow, turbulence quantities and the transport and fate of fine sediments. The secondary flow locally supports or hinders the falling of fine sediment particles in a turbulent flow, which results in a non‐uniform deposition of fine sediments over the cross‐section. While the channel aspect ratio has a large influence on the distribution of suspended sediments within the cross‐section, its effect on the cross‐sectional averaged deposition is negligibly small. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

7.
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   

8.
Experimental investigations have been done to analyze turbulent structures in curved sand bed channels with and without seepage. Measures of turbulent statistics such as time‐averaged near‐bed velocities, Reynolds stresses, thickness of roughness sublayer and shear velocities were found to increase with application of downward seepage. Turbulent kinetic energy and Reynolds normal stresses are increased in the streamwise direction under the action of downward seepage, causing bed particles to move rapidly. Analysis of bursting events shows that the relative contributions of all events (ejections, sweeps and interactions) increase throughout the boundary layer, and the thickness of the zone of dominance of sweep events, which are responsible for the bed material movement, increases in the case of downward seepage. The increased sediment transport rate due to downward seepage deforms the cross‐sectional geometry of the channel made of erodible boundaries, which is caused by an increase in flow turbulence and an associated decrease in turbulent kinetic energy dissipation and turbulent diffusion.  相似文献   

9.
Aggregation processes of fine sediments have rarely been integrated in numerical simulations of cohesive sediment transport in riverine systems. These processes, however, can significantly alter the hydrodynamic characteristics of suspended particulate matter (SPM), modifying the particle settling velocity, which is one of the most important parameters in modelling suspended sediment dynamics. The present paper presents data from field measurements and an approach to integrate particle aggregation in a hydrodynamic sediment transport model. The aggregation term used represents the interaction of multiple sediment classes (fractions) with corresponding multiple deposition behaviour. The k–ε–turbulence model was used to calculate the coefficient of vertical turbulent mixing needed for the two‐dimensional vertical‐plane simulations. The model has been applied to transport and deposition of tracer particles and natural SPM in a lake‐outlet lowland river (Spree River, Germany). The results of simulations were evaluated by comparison with field data obtained for two levels of river discharge. Experimental data for both discharge levels showed that under the prevailing uniform hydraulic conditions along the river reach, the settling velocity distribution did not change significantly downstream, whereas the amount of SPM declined. It was also shown that higher flow velocities (higher fluid shear) resulted in higher proportions of fast settling SPM fractions. We conclude that in accordance with the respective prevailing turbulence structures, typical aggregation mechanisms occur that continuously generate similar distribution patterns, including particles that settle toward the river bed and thus mainly contribute to the observed decline in the total SPM concentration. In order to determine time‐scales of aggregation and related mass fluxes between the settling velocity fractions, results of model simulations were fitted to experimental data for total SPM concentration and of settling velocity frequency distributions. The comparison with simulations for the case of non‐interacting fractions clearly demonstrated the practical significance of particle interaction for a more realistic modelling of cohesive sediment and contaminant transport. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

10.
A model is developed for predicting the settling velocity in suspensions of particles of two different sizes based on experimental data for the settling rate of two-size suspensions in various liquids using particles of equal density. In these experiments, the retarding effect of the smaller particles on the settling velocities of the larger ones is taken into account. The model considers Steinour’s fundamental equation and assumes a fixed arrangement of particles and constant velocity in a single-size susp...  相似文献   

11.
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   

12.
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13.
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   

14.
The relationship between turbulent fluid motions and sediment particle motions over mobile sand dunes was investigated by using a laser Doppler velocimeter and an acoustic backscatter system in laboratory experiments performed at the USDA-ARS-National Sedimentation Laboratory. Profiles of acoustic backscatter from particles and at-a-point turbulence data were collected while translating both measurement devices downstream at the speed of mobile dune bedforms. The resulting data set was used to examine the frequency （recurrence frequency） at which the fluctuating backscatter and fluid velocity signals exceeded magnitude thresholds based on the standard deviation （σ） of the local velocity and the magnitude the acoustic signal resulting from backscatter from suspended particles. The slope of the downstream and vertical velocity recurrence frequencies generally indicated a gradually increasing recurrence time with increasing elevation. The recurrence frequency for acoustic backscatter data was not strongly variable with elevation. The closest correspondence between the recurrence frequencies of sediment backscatter and vertical velocities at the 1σ magnitude threshold was in a region defined by X/L〈0.4 and y〈6 cm. The downstream velocity was most closely related to backscatter in a small region at 0.4〈X/L〈0.8 and less than 3-4 cm from the bed.  相似文献   

15.
Flume experiments were conducted on different bed stages across the ripple–dune transition. As flow velocity increases, an initially flat bed surface (made of fairly uniform sandy material) is gradually transformed into a two‐dimensional rippled bed. With further increase in velocity, two‐dimensional ripples are replaced by irregular, linguoid ripples. As the average velocity necessary for the ripple–dune transition to occur is imposed on the bed surface, these non‐equilibrium linguoid ripples are further transformed into larger, two‐dimensional dunes. For each of these stages across the transition, a concrete mould of the bed was created and the flow structure above each fixed bed surface investigated. An acoustic Doppler velocimeter was used to study the flow characteristics above each bed surface. Detailed profiles were used along a transect located in the middle of the channel. Results are presented in the form of spatially averaged profiles of various flow characteristics and of contour maps of flow fields (section view). They clearly illustrate some important distinctions in the flow structure above the different bedform types associated with different stages during the transition. Turbulence intensity and Reynolds stresses gradually increase throughout the transition. Two‐dimensional ripples present a fairly uniform spatial distribution of turbulent flow characteristics above the bed. Linguoid ripples induce three‐dimensional turbulence structure at greater heights above the bed surface and turbulence intensity tends to increase steadily with height above bed surface in the wake region. A very significant increase in turbulence intensity and momentum exchange occurs during the transition from linguoid ripples to dunes. The turbulent flow field properties above dunes are highly dependent on the position along and above the bed surface and these fields present a very high degree of spatial variability (when compared with the rippled beds). Further investigations under natural conditions emphasizing sediment transport mechanisms and rates during the transition should represent the next step of analysis, together with an emphasis on quadrant analysis. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

16.
This paper presents the predicted flow dynamics from the application of a Reynolds‐averaged Navier–Stokes model to a series of bifurcation geometries with morphologies measured during previous flume experiments. The topography of the bifurcations consists of either plane or bedform‐dominated beds which may or may not possess discordance between the two bifurcation distributaries. Numerical predictions are compared with experimental results to assess the ability of the numerical model to reproduce the division of flow into the bifurcation distributaries. The hydrodynamic model predicts: (1) diverting fluxes in the upstream channel which direct water into the distributaries; (2) super‐elevation of the free surface induced at the bifurcation edge by pressure differences; and (3) counter‐rotating secondary circulation cells which develop upstream of the apex of the bifurcation and move into the downstream channels, with water converging at the surface and diverging at the bed. When bedforms are not present, weak transversal fluxes characterize the upstream channel for almost its entire length, associated with clearly distinguishable secondary circulation cells, although these may be under‐estimated by the turbulence model used in the solution. In the bedform dominated case, the same hydrodynamic conditions were not observed, with the bifurcation influence restricted and depth scale secondary circulation cells not forming. The results also demonstrate the dominant effect bed discordance has upon flow division between the two distributaries. Finally, results indicate that in bedform dominated rivers. Consequently, we suggest that sand‐bed river bifurcations are more likely to have an influence that extends much further upstream and have a greater impact upon water distribution. This may contribute to observed morphological differences between sand‐bedded and gravel‐bedded braided river networks. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

17.
Entrainment of sediment particles from channel beds into the channel flow is influenced by the characteristics of the flow turbulence which produces stochastic shear stress fluctuations at the bed. Recent studies of the structure of turbulent flow has recognized the importance of bursting processes as important mechanisms for the transfer of momentum into the laminar boundary layer. Of these processes, the sweep event has been recognized as the most important bursting event for entrainment of sediment particles as it imposes forces in the direction of the flow resulting in movement of particles by rolling, sliding and occasionally saltating. Similarly, the ejection event has been recognized as important for sediment transport since these events maintain the sediment particles in suspension. In this study, the characteristics of bursting processes and, in particular, the sweep event were investigated in a flume with a rough bed. The instantaneous velocity fluctuations of the flow were measured in two-dimensions using a small electromagnetic velocity meter and the turbulent shear stresses were determined from these velocity fluctuations. It was found that the shear stress applied to the sediment particles on the bed resulting from sweep events depends on the magnitude of the turbulent shear stress and its probability distribution. A statistical analysis of the experimental data was undertaken and it was found necessary to apply a Box-Cox transformation to transform the data into a normally distributed sample. This enabled determination of the mean shear stress, angle of action and standard error of estimate for sweep and ejection events. These instantaneous shear stresses were found to be greater than the mean flow shear stress and for the sweep event to be approximately 40 percent greater near the channel bed. Results from this analysis suggest that the critical shear stress determined from Shield's diagram is not sufficient to predict the initiation of motion due to its use of the temporal mean shear stress. It is suggested that initiation of particle motion, but not continuous motion, can occur earlier than suggested by Shield's diagram due to the higher shear stresses imposed on the particles by the stochastic shear stresses resulting from turbulence within the flow.  相似文献   

18.
We deployed bottom-mounted quadrapod equipped with acoustic Doppler current profiler (ADCP), acoustic Doppler velocimeter (ADV), and optical backscatter sensor (OBS) over two semidiurnal tidal cycles along the western coast of the Yellow Sea, China. In combination with shipboard profiling of CTD and LISST-100, we resolved the temporal and spatial distributions of tidal currents, turbulent kinetic energy (TKE), suspended sediment concentration (SSC) and particle size distributions. During the observations, tidal-induced bottom shear stress was the main stirring factor. However, weak tidal flow during the ebb phase was accompanied by two large SSC and median size events. The interactions of seiche-induced oscillations with weak ebb flow induced multiple flow reversals and provided a source of turbulence production, which stripped up the benthic fluff layers (only several millimeters) around the Jiaozhou Bay mouth. Several different methods for inferring mean suspended sediment settling velocity agreed well under peak currents, including estimates using LISST-based Stokes’ settling law, and ADCP-based Rouse profiles, ADV-based inertial-dissipation balance and Reynolds flux. Suspended particles in the study site can be roughly classified into two types according to settling behavior: a smaller, denser class consistent with silt and clay and a larger, less dense class consistent with loosely aggregated flocs. In the present work, we prove that acoustic approaches are robust in simultaneously and non-intrusively estimating hydrodynamics, SSC and settling velocities, which is especially applicable for studying sediment dynamics in tidal environments with moderate concentration levels.  相似文献   

19.
An in‐house fully three‐dimensional general‐purpose finite element model is applied to solve the hydrodynamic structure in a periodic Kinoshita‐generated meandering channel. The numerical model solves the incompressible Reynolds‐averaged Navier–Stokes equations for mass and momentum, while solving the k ? ε equations for turbulence. The free surface is described by the rigid‐lid approximation (using measured water surface data) for flat (smooth‐bed) and self‐formed (rough‐bed) conditions. The model results are compared against experimental measurements in the ‘Kinoshita channel’, where three‐dimensional flow velocities and turbulence parameters were measured. This validation was carried out for the upstream‐valley meander bend orientation under smooth (flat bed) conditions. After validation, several simulations were carried out to predict the hydrodynamics in conditions where either it was not possible to perform measurements (e.g. applicability of the laboratory acoustic instruments) and to extrapolate the model to other planform configurations. For the flat smooth‐bed case, a symmetric (no skewness) planform configuration was modeled and compared to the upstream‐skewed case. For the self‐formed rough‐bed case, prediction of the hydrodynamics during the progression of bedforms was performed. It appears that the presence of bedforms on a bend has the following effects: (i) the natural secondary flow of the bend is disrupted by the presence of the bedforms, thus depending on the location of the dune, secondary flows might differ completely from the traditional orientation; (ii) an increment on both the bed and bank shear stresses is induced, having as much as 50% more fluvial erosion, and thus a potential increment on the migration rate of the bend. Implications on sediment transport and bend morphodynamics are also discussed in the paper. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

20.
Sheet flow hydrodynamics over a non-uniform sand bed channel   总被引：1，自引：0，他引：1  
The current study experimentally investigates the flow characteristics and temporal variations in the sheet flow profile of a non-uniform sand bed channel. Experiments were done to explore turbulent structures in the presence of a sheet flow layer with and without seepage. The turbulent events, such as stream wise velocity, Reynolds shear stresses, and turbulence intensities were found to be increasing and vertical velocity was found decreasing with a sheet layer. The presence of a sheet layer also effects the turbulent energy production and energy dissipation. All the turbulence parameters with and without a sheet layer have also been influenced by the presence of downward seepage. The rate of sheet flow movement is increased with seepage, owing to increased turbulence with seepage. The current study used wavelet analysis on temporally lagged spatial bed elevation profiles obtained from a set of laboratory experiments and synchronized the wavelet coefficients with bed elevation fluctuation at different spatial scales. A spatial cross correlation analysis at multiple scales, based on the wavelet coefficients, has been done on these bed elevation datasets to observe the effect of downward seepage on the dynamic behavior of sheet flow at different length scales. It is found that seepage increases average bed celerity and also increases the celerity of sheet flow of similar length scales. This increase in the celerity has been hypothesized as the increase of sheet flow movement as well as the increase in turbulent parameters with seepage, which destabilizes the bed particles resulting in a disruption in the continuous propagation pattern of the sheet flow. The increase of sheet flow celerity with seepage is confirmed from the saturation level of the wavelet power spectra of the bed elevation series. The presence of seepage also affects the non-uniformity of collective sheet material.  相似文献   

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