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1.
Single bed load particle impacts were experimentally investigated in supercritical open channel flow over a fixed planar bed of low relative roughness height simulating high‐gradient non‐alluvial mountain streams as well as hydraulic structures. Particle impact characteristics (impact velocity, impact angle, Stokes number, restitution and dynamic friction coefficients) were determined for a wide range of hydraulic parameters and particle properties. Particle impact velocity scaled with the particle velocity, and the vertical particle impact velocity increased with excess transport stage. Particle impact and rebound angles were low and decreased with transport stage. Analysis of the particle impacts with the bed revealed almost no viscous damping effects with high normal restitution coefficients exceeding unity. The normal and resultant Stokes numbers were high and above critical thresholds for viscous damping. These results are attributed to the coherent turbulent structures near the wall region, i.e. bursting motion with ejection and sweep events responsible for turbulence generation and particle transport. The tangential restitution coefficients were slightly below unity and the dynamic friction coefficients were lower than for alluvial bed data, revealing that only a small amount of horizontal energy was transferred to the bed. The abrasion prediction model formed by Sklar and Dietrich in 2004 was revised based on the new equations on vertical impact velocity and hop length covering various bed configurations. The abrasion coefficient kv was found to be vary around kv ~ 105 for hard materials (tensile strength ft > 1 MPa), one order of magnitude lower than the value assumed so far for Sklar and Dietrich's model. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

2.
The purpose of this paper is to examine the nature of particle saltation and movement over the beds of fixed roughness from flume experiments. A series of experiments are carried out to study the saltation of individual sand particles of different sizes over rough beds under different flow conditions. A 3‐D acoustic Doppler velocimeter is used to record the fluid velocity components; subsequently, under different flow conditions, the images of released sand particles are recorded using high‐speed video imaging technique. Systematic analysis is made with regard to the forces acting on the grains and the variation of their magnitudes along the saltation trajectories of the grains. Relations between the saltation parameters, flow intensity and bed roughness are developed. The distributions of the angle of orientations during a single saltation follows almost a Gaussian distribution. The shape of the Gaussian distribution depends on the particle size and bed roughness. Particle collisions with rough beds and the resulting coefficients of restitution are also discussed. A theoretical framework is developed to compute the mean particle velocity considering the spin in the energy balance equation. Results of the detailed analysis using the imaging technique are much better than in previously reported studies. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

3.
Three-dimensional saltating processes of multiple sediment particles   总被引：1，自引：1，他引：0  
The purpose of this study was to investigate the interacting mechanism between the saltating particles near a channel bed. A three-dimensional real-time flow visualization technique was developed to measure the interparticle collision behaviors during the saltating process. Based on the experimental data, the distribution of the collision points was found to be symmetric. This confirms the assumption that the projections of the collision points onto the reasonable plane are uniformly distributed. A three-dimensional saltating model was also developed. This model produced satisfactory results. The model is able to simulate the continuous saltating trajectories of several particles. The simulated dimensionless saltating height, longitudinal and vertical saltation velocity components were found to increase as the dimensionless particle diameter and the dimensionless flow transport capacity parameter increase, while the simulated lateral saltation velocity component varies inversely with the dimensionless flow transport capacity parameter. A regression equation for the bed load transport rate was also obtained.  相似文献   

4.
Depth profiles of particle streamwise velocity, concentration and bedload sediment transport rate were measured in a turbulent and supercritical water flow. One‐size 6 mm diameter spherical glass beads were transported at equilibrium in a two‐dimensional 10% steep channel with a mobile bed. Flows were filmed from the side by a high‐speed camera. Particle tracking algorithms made it possible to determine the position, velocity and trajectory of a very large number of particles. Approximately half of the sediment transport rate was composed by rolling grains, and the other half by saltation. This revealed a complex structure, with several concentration and flux peaks due to rolling, and one peak due to saltation. With an increase of the sediment transport rate, the depth structure remained the same at the water/granular interface, with peak value increases but with no shift in elevations. The saltation region expanded towards higher elevations with an increase of the particle velocity commensurate to the water velocity. The proportion of the sediment transport rate in saltation did not vary significantly. The particle streamwise velocity profiles exhibited three segments: an exponential decay in the bed, a linear increase where rolling and saltation co‐existed, and above this, a logarithmic‐like shape due to saltating particles. These results are comparable to profiles measured and modelled in dry granular free surface flows and in more intense bedload such as sheet flows. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

5.
A 3D Lagrangian model of the saltation of solid spherical particles on the bed of an open channel flow, accounting for turbulence-induced mechanisms, is proposed and employed as the key tool of the study. The differences between conventional 2D models and a proposed 3D saltation model are discussed and the advantages of the 3D model are highlighted. Particularly, the 3D model includes a special procedure allowing generation of 3D flow velocity fields. This procedure is based on the assumption that the spectra of streamwise, vertical and transverse velocity components are known at any distance from the bed. The 3D model was used to identify and quantify effects of turbulence on particle entrainment and saltation. The analysis of particle trajectories focused on their diffusive nature, clarifying: (i) the effect of particle mobility parameter; (ii) the effect of bed topography; and (iii) the effect of turbulence. Specifically, the results of numerical simulations describing the abovementioned effects on the change in time of the variance are presented. In addition, the change in time of the skewness and kurtosis, which are likely to reflect the turbulence influence on the spread of particles, are also shown. Two different diffusion regimes (local and intermediate) for each of the investigated flow conditions are confidently identified.  相似文献   

6.
A horizontal saltation layer of glass particles in air is investigated experimentally over a flat bed and also over a triangular ridge in a wind tunnel. Particle concentrations are measured by light scattering diffusion (LSD) and digital image processing, and velocities using particle image velocimetry (PIV). All the statistical moments of the particle concentration are determined such as mean concentration, root mean square concentration fluctuations, skewness and flatness coefficients. Over the flat bed, it is confirmed that the mean concentration decreases exponentially with height, the mean dispersion height being a significant length scale. It is shown that the concentration distribution follows quite well a lognormal distribution. Over the ridge, measurements were made at the top of the ridge and in the cavity region and are compared with measurements without the ridge. On the hill crest, particles are retarded, the saltation layer decreases in thickness and concentration is increased. Downwind of the ridge, particle flow behaves like a jet, in particular no particle return flow is observed. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

7.
Saltation of sediment particles is an important pattern of bedload transport.Based on force analysis for sediment particles,a Lagrangian model was proposed for the saltating motion of bedload in river flows,which was then solved with numerical method.Simulation results on the saltating trajectories neglecting particle rotation and turbulence effects compare fairly well with experimental observations.The mean values of the saltation parameters (saltation height,length and velocity) also agree well with the previous experimental data.Based on the numerical results,regression equations for the dimensionless saltation height,length and velocity were presented.Using the numerically achieved characteristics of the sediment saltation,we also obtained mathematical expression for the sediment transport rate.The studies in this paper are significant for its contribution to mechanism of the bedload motion and the computation of sediment transport rate.  相似文献   

8.
The movement of bedload in subcritical flow produces additional roughness as compared to flow in a rigid bed. The magnitude of this bed load roughness is proportional to the thickness of the sediment layer moving along the bed, the particle size and the sediment concentration. In a supercritical flow, however, further resistance is expected due to the momentum absorption by the high flow velocity. In this study the effect of sediment movement on the flow resistance in supercritical flow was experimentally investigated. The experiments included flows over smooth and rough beds carrying sediment of mean diameters D50=2.80, 5.42 and 7.06 mm in a rigid rectangular channel. The results show that the sediment transport may increase the friction factor by up to 90% and 60% in smooth and rough beds, respectively. Bedload extracts its momentum from the flow, which causes a reduction of near bed flow velocity and steeper velocity gradient near the bed resulting in an increase in shear velocity as well as in roughness height. The increase in friction factor is directly related to bedload concentration and particle size.  相似文献   

9.
Simulations using a mechanistic model of raindrop driven erosion in rain‐impacted flow were performed with particles travelling by suspension, raindrop induced saltation and flow driven saltation. Results generated by both a high intensity storm, and a less intense one, indicate that, because of the effect of flow depth on the delivery of raindrop energy to the bed, there is a decline in sediment concentration, and hence soil loss per unit area, with slope length when particles are transported by raindrop induced saltation. However, that decline is reversed when the critical velocities that lead to flow driven saltation are episodically exceeded during an event. The simulations were performed on smooth surfaces and a single drop size but the general relationships are likely to apply for rain made up of a wide range of drop size. Although runoff is not always produced uniformly, as a general rule, flow velocities increase with slope length so that, typically, the distance particles travel before being discharged during an event increase with slope length. The effect of slope length on soil loss per unit area is often considered to vary with slope length to a power greater than zero and less that 1·0. The simulations show that effect of slope length on sediment discharge is highly dependent on the variations in runoff response resulting from variations in rainfall duration‐intensity‐infiltration conditions rather than plot length per se. Consequently, predicting soil loss per unit area using slope length with positive powers close to zero when sheet erosion occurs may not be as effective as commonly expected. Erosion by rain‐impacted flow is a complex process and that complexity needs to be considered when analysing the results of experiments associated with rain‐impacted flow under both natural and artificial conditions. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

10.
The acceleration of saltating grains by overland flow causes momentum to be transferred from the flow to the grains, thereby increasing flow resistance and bed roughness. To assess the impact of saltating sediment on overland flow hydraulics, velocity profiles in transitional and turbulent flows on a fixed sand-covered bed were measured using hot-film anemometry. Five discharges were studied. At each discharge, three flows were measured: one free of sediment, one with a relatively low sediment load, and one with a relatively high sediment load. In these flows from 83 to 90 per cent of the sediment was travelling by saltation. As a result, in the sediment-laden flows the near-bed velocities were smaller and the velocity profiles steeper than those in the equivalent sediment-free flows. Sediment loads ranged up to 87·0 per cent of transport capacity and accounted for as much as 20·8 per cent of flow resistance (measured by the friction factor) and 89·7 per cent of bed roughness (measured by the ratio of the roughness length to median grain diameter). It is concluded that saltating sediment has a considerable impact on overland flow hydraulics, at least on fixed granular beds. Saltation is likely to have a relatively smaller effect on overland flow on natural hillslopes and agricultural fields where form and wave resistance dominate. Still, saltation is generally of greater significance in overland flow than in river flow, and for this reason its effect on overland flow hydraulics is deserving of further study. © 1998 John Wiley & Sons, Ltd.  相似文献   

11.
In gravel‐bed rivers, the microtopography of the bed is known to exert a significant influence on the generation of turbulent flow structures that owe their origin to fluid shear generated near the bed. Although field and laboratory measurements have indicated that flows over gravel beds contain a range of coherent flow structures, the kinematic and dynamic properties of these structures are still poorly understood. This paper describes a new experimental methodology to quantify simultaneously both the kinematic and dynamic characteristics of coherent flow structures based upon combined planar laser‐induced fluorescence and particle imaging velocimetry (PLIF‐PIV). The results confirm that the primary generative mechanism of coherent flow structures is at the bed, where merging hairpin vortices form around bed clasts and generate larger‐scale fluid motions that advect downstream. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

12.
The importance of particle-particle collisions in sediment saltation in the bed-load layer is analyzed herein by means of numerical simulation. The particle saltation theoretical/numerical model follows a Lagrangian approach, and addresses the motion of sediment particles in an open channel flow described by a logarithmic velocity profile. The model is validated with experimental data obtained from the literature. In order to evaluate the importance of the phenomenon, simulations with and without particle-particle collisions were carried out. Results for two different sediment concentrations are presented, namely 0.13% and 2.33%. For each concentration of particles, three different flow intensities were considered, and trajectories of two different particle sizes, within the sand range were computed. Changes in particle rotation, particle velocity, and angle of trajectory before and after particle-particle collisions appear to be relatively important at lower shear stresses, whereas they decrease in significance with increasing flow intensities. Analyses of the evolution in time of the second order moment of particle location suggest that inter-particle collisions introduce transverse diffusion in saltating particles in the span-wise direction.  相似文献   

13.
The effect of a step change in macro‐roughness on the saltation process under sediment supply limited conditions was examined in the atmospheric boundary layer. For an array of roughness elements of roughness density λ = 0.045 (λ = total element frontal area/total surface area of the array) the horizontal saltation flux was reduced by 90% (±7%) at a distance of ≈150 roughness element heights into the array. This matches the value predicted using an empirical design model and provides confidence that it can be effectively used to engineer roughness arrays to meet sand flux reduction targets. Measurements of the saltation flux characteristics in the vertical dimension, including: saltation layer decay (e‐folding) height and particle size, revealed that with increasing distance into the array, the rate of mass flux change with increasing height decreased notably, and (geometric) mean particle diameter decreased. The distribution of the saltation mass flux in the vertical remains exponential in form with increasing distance into the roughness array, and the e‐folding height increases as well as increasing at a greater rate as particle diameter diminishes. The increase in e‐folding height suggests the height of saltating particles is increasing along with their mean speed. This apparent increase in mean speed is likely due to the preferential removal, or sequestration, of the slower moving particles across the size spectrum, as they travel through the roughness array. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

14.
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   

15.
The saltation regime is very important for understanding the sediment transport mechanism. However,there is no consensus on a model for the saltation regime. This study answers several questions raised with respect to the Eulerian-Lagrangian modeling of sediment transport. The first question is why the previous saltation models that use different combinations of hydrodynamic forces yielded acceptable results? The second question is which shear lift model(i.e. a shear lift expression and its coefficient) is more appropriate? Another important question is which hydrodynamic forces have greater contributions to the saltation characteristics of a sediment particle? The last question is what are the contributions of the turbulence fluctuations as well as effects of using two-and three-dimensional(2 D and 3 D) models on the simulation results? In order to fairly answer these questions, a systematic study was done by considering different scenarios. The current study is the first attempt to clearly discuss these issues. A comprehensive 3 D saltation model for non-cohesive sediment was developed that includes all the hydrodynamic forces acting on the particle. The random nature of sediment transport was included using turbulent flow and bed-particle collision models. The eddy interaction model was applied to generate a3 D turbulent flow field. Bed-particle collisions were considered using the concept of a contact zone and a corresponding contact point. The validation of the model was done using the available experimental data for a wide range of sediment size(0.03 to 4.8 cm). For the first question, the results indicated that some of the hydrodynamic effects show opposing trends and some have negligible effects. With these opposing effects it is possible to adjust the coefficients of different models to achieve acceptable agreement with the same experimental data while omitting some aspects of the physics of the process. A suitable model for the shear lift force was developed by linking the lift coefficient to the drag coefficient and the contributions of the hydrodynamic forces and turbulence fluctuations as well as the consequences of using of 2 D and 3 D models were studied. The results indicate that the shear lift force and turbulent flow fluctuations are important factors for the saltation of both sand and gravel, and they cannot be ignored.  相似文献   

16.
Models for contaminant transport in streams commonly idealize transient storage as a well mixed but immobile system. These transient storage models capture rapid (near‐stream) hyporheic storage and transport, but do not account for large‐scale, stage‐dependent interaction with the alluvial aquifer. The objective of this research was to document transient storage of phosphorus (P) in coarse gravel alluvium potentially influenced by large‐scale, stage‐dependent preferential flow pathways (PFPs). Long‐term monitoring was performed at floodplain sites adjacent to the Barren Fork Creek and Honey Creek in northeastern Oklahoma. Based on results from subsurface electrical resistivity mapping which was correlated to hydraulic conductivity data, observation wells were installed both in higher hydraulic conductivity and lower hydraulic conductivity subsoils. Water levels in the wells were monitored over time, and water samples were obtained from the observation wells and the stream to document P concentrations at multiple times during high flow events. Contour plots indicating direction of flow were developed using water table elevation data. Contour plots of total P concentrations showed the alluvial aquifer acting as a transient storage zone, with P‐laden stream water heterogeneously entering the aquifer during the passage of a storm pulse, and subsequently re‐entering the stream during baseflow conditions. Some groundwater in the alluvial floodplains had total P concentrations that mirrored the streams' total P concentrations. A detailed analysis of P forms indicated that particulate P (i.e. P attached to particulates greater than 0·45 µm) was a significant portion of the P transport. This research suggests the need for more controlled studies on stage‐dependent transient storage in alluvial systems. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

17.
Results from a series of numerical simulations of two‐dimensional open‐channel flow, conducted using the computational fluid dynamics (CFD) code FLUENT, are compared with data quantifying the mean and turbulent characteristics of open‐channel flow over two contrasting gravel beds. Boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra‐grid‐scale roughness elements (e.g. particle clusters and small bedforms). Results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed (typically at y/h = 0·1–0·3). This is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed. Use of the random elevation model to represent supra‐grid‐scale roughness also allows a reduction in the height of the near‐bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness. Despite these benefits, the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution. Increasing the horizontal mesh resolution results in an increase in the near‐bed velocity gradient and turbulent kinetic energy, effectively roughening the bed. Varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles, but results in substantial changes to the shape of the turbulent kinetic energy profile. These findings have significant implications for the application of CFD within natural gravel‐bed channels, particularly with regard to issues of topographic data collection, roughness parameterization and the derivation of mesh‐independent solutions. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

18.
The transport of sand by the wind occurs predominantly by the process of saltation. Following the entrainment of sand by an above threshold wind, the saltation system is regulated by the mutual interaction of the atmospheric boundary‐layer, the sand cloud and the sand bed. Despite existing data on the spatial and temporal development of the sand transport system, very little is known about the development of the saltation system towards equilibrium. Results are presented from wind‐tunnel experiments that were designed to address the simultaneous spatial and temporal development of the saltation system, with and without artificial sand feed. The development of the saltation system was monitored over a streamwise length of 8 m during a period of 3600 s. Mass flux data were measured simultaneously at 1 m intervals by the downwind deployment of seven Aarhus sand traps. Wind velocity data were collected throughout the experiments. The downwind spatial development of the saltation system is manifested by an overshoot in mass flux and friction velocity prior to declining towards a quasi‐equilibrium. Mass flux overshoots at approximately 4 m downwind, in remarkable agreement with existing data of a comparable scale. Friction velocity overshoots at approximately 6 m downwind, a result not previously witnessed in saltation studies. The overshoot of mass flux prior to the overshoot in friction velocity is a spatial manifestation of the time lag between the entrainment of grains and the deceleration of the wind by the grains in transport. Temporally, the development of the saltation system is controlled by the availability of entrainable grains from the sand bed. Through time the saltation system develops from a transport‐limited to a supply‐limited system. The depletion of the sand bed through time limits the appropriateness of the assumption of ‘equilibrium’ for the universal prediction of mass flux. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

19.
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   

20.
P. I. A. Kinnell 《水文研究》2012,26(10):1449-1456
Sheet and interrill erosion areas are sources of soil material rich in nutrients and pollutants. The loss of soil, nutrients and other chemicals from these areas is a matter of concern both in terms of maintaining soil productivity and the health of offsite environments. Many experiments on rainfall erosion have shown enrichment of fine material, nutrients and other chemicals in the sediment discharged for sheet and interrill erosion areas, but often these results were obtained over short periods of time. A qualitative mechanistic model of raindrop‐induced saltation is used to illustrate how this transport mechanism influences the composition of sediment discharged by rain‐impacted flow. Initially, fast moving particles are enriched in the sediment discharge but, over time, during a rainfall event, slower moving particles become more represented. Raindrop‐induced saltation promotes the storage of material on the soil surface with a coarser composition than the original soil. Winnowing of material from this storage by the development of flow‐driven saltation during high‐intensity events can modify the composition of the sediment discharged later by raindrop‐induced saltation. Given stable soil particles, the composition of the sediment discharged at the steady state is the same as the original soil. Enrichment is a non‐steady‐state phenomenon and failure to recognize the transient nature of enrichment may lead to inappropriate interpretation of the implications of the results from short‐term experiments. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

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