共查询到20条相似文献，搜索用时 328 毫秒
1.
Bed load transport by bed form migration 总被引：1，自引：1，他引：0
A theoretically-based methodology is presented for the determination of bed load transport from high-resolution measurements of bed surface elevations for steady-state or developing dunes. The methodology is based on the general form of the Exner equation for sediment continuity and requires information on the distribution of sediment volume concentration as well as the migration velocity of bed layers. In order to determine layer speeds, a new method based on cross-correlation analysis of elevation slices is proposed. The methodology is tested using artificially-created data as well as data from a physical model and from a flume study of developing bed forms. The analyses show the applicability of the method to determine bed load transport without the need to introduce assumptions about the form of the migrating surface. It is shown that predicted transport rates match measured or theoretical transport rates for steadily moving bed forms of an arbitrary shape. The method can also be used to predict transport rates over deforming bed forms, with the reasons for potential deviations between predicted and measured or theoretical transport rates for deforming bed forms identified and discussed. It is further shown that a simplified bulk-surface approach, that is relatively straightforward to apply and in which it is assumed that bed-layer velocity is constant with depth, gives results that are comparable to analyses based on determined bed-layer velocity variation with depth. 相似文献
2.
三峡工程运用后,长江中游荆江河段持续冲刷,床沙与推移质、悬移质泥沙不断交换,从而造成该河段床沙发生不同程度的调整,对长江中下游河床演变及非平衡输沙机理的研究具有重要影响.在新水沙条件下,总结分析了沙波运动特性及床沙交换方式,引入Markov三态转移概率及非均匀沙隐暴系数,得到基于状态转移概率的沙质河段床沙级配调整的计算模型.研究结果表明:(1)20092014年,沙市站年内床沙中值粒径有先增大后减小的趋势,而监利站年内床沙中值粒径则先减小后增大,且荆江河段年际床沙中值粒径总体呈上升趋势,粗化程度约为6.9%~9.3%;(2)20092014年,沙市站床沙组成中粒径d<0.062 mm的泥沙所占比重不变,0.062 mm≤d<0.25 mm的泥沙所占比重逐年减少(累计减少11.4%),d≥0.25 mm的泥沙所占比重逐年增加(累计增加11.4%),而监利站床沙组成均存在波动性变化;(3)荆江河段床沙转换为推移质的概率随着泥沙粒径的增大而增大,床沙转换为悬移质的概率随着泥沙粒径的增大而减小,而推移质和悬移质转换为床沙的概率均随着泥沙粒径的增大而增大,河床发生冲刷粗化时泥沙输移的主要形式为悬移质(概率为81%~87%),而淤积细化时床沙补给主要来源于推移质(概率为8%~12%).通过验证,本文概率模型的计算结果与实测资料符合较好,能够应用于长江中游沙质河段年际床沙粗化及年内床沙级配调整过程预报,为进一步开展三峡工程下游非均匀悬移质泥沙沿程恢复机理的研究提供理论基础. 相似文献
3.
A combined numerical and experimental study of the propagation of an internal solitary wave (ISW) over a corrugated bed is
presented, in which the amplitude and the wavelength of the corrugated bed, together with the wave amplitude and wave speed
of the ISW, have been varied parametrically. Both ISWs of elevation and depression have been considered. The wave-induced
currents over the corrugated bed cause flow separation at the apex of the corrugations and a sequence of lee vortices forms
as a result. These vortices develop fully after the main wave has passed over the topographic feature, resulting in deformation
of the overlying pycnocline and, in some instances, significant vertical mixing. It is found that the intensity of the vortex
formation is dependent on both the amplitude and wavelength of the bottom topography. In the case of an ISW of depression,
the generation of vertically (upward)-propagating vortices is seen to result in entrainment of fluid from a bottom boundary
jet (Carr and Davies, Phys Fluids 18:016601, 2006), while, in the elevation case, a second mechanism is present to induce significant turbulent mixing in the water column.
It occurs when the bottom corrugations reach into, or are very near, the pycnocline at rest. Large waves of elevation that
are stable on approach to the corrugations exhibit evidence of a spatio-temporally developing shear instability as they interact
with the bottom corrugation. The shear instability takes the form of billows that have a vertical extent that can reach 50%
of the wave amplitude. 相似文献
4.
The particle size distribution of bed materials in the sandy river bed of alluvial rivers is important in the study of topics such as friction, river bed evolution, erosion, and siltation. It also can reflect the dependency relation between river bed sediment and flow intensity. In this paper, the critical pattern of sediment movement in the near-wall region of a sandy river bed was analyzed. According to the principle of momentum balance, the critical settling-rising condition of bed material in a sandy river bed was found to be instantaneous turbulent velocity equal to 2.7 times the sediment settling velocity in quiescent water. Based on a vertical instantaneous turbulent velocity with a Gaussian distribution, a theoretical relation for calculating the particle size distribution of bed materials in a sandy river bed without pre-known characteristic grain sizes was developed by solving a stochastic equation. The for-mula is verified using measured data, and the results show that the proposed formula was in accordance with the measured data. This study has theoretical significance and practical value for determining the bed material particle size distribution of the sandy bed of alluvial rivers. 相似文献
5.
Rumpa Chakraborty 《地球物理与天体物理流体动力学》2013,107(5):481-496
The two-dimensional problem of the generation of water waves due to instantaneous disturbances prescribed at the bed of a beach sloping at an arbitrary angle is studied here. It is formulated in terms of an initial-boundary-value problem for the velocity potential describing the motion in the fluid region assuming the linear theory. Using the Laplace transform in time and the Mellin transform in distance, the problem is reduced to solving a difference equation whose method of solution is of considerable importance in the literature. The form of the free surface is obtained in terms of a multiple infinite integral that is evaluated by the method of steepest-descent. For some prescribed forms of the disturbance at the bed of the beach, the free surface is depicted in a number of figures for different beach angles. It is observed that as the beach angle decreases, the maximum wave height increases, which is plausible. 相似文献
6.
Marcus Schulz Jan Priegnitz Jörg Klasmeier Stefan Heller Stefan Meinecke Michael Feibicke 《水文研究》2012,26(2):272-280
In several empirical and modelling studies on river hydraulics, dispersion was negatively correlated to surface roughness. In this study, it was aimed to investigate the influence of surface roughness on longitudinal dispersion under controlled conditions. In artificial flow channels with a length of 104 m, tracer experiments with variations in channel bed material were performed. By use of measured tracer breakthrough curves, average flow velocity, mean longitudinal dispersion, and mean longitudinal dispersivity were calculated. Longitudinal dispersion coefficients ranged from 0·018 m2 s?1 in channels with smooth bed surface up to 0·209 m2 s?1 in channels with coarse gravel as bed material. Longitudinal dispersion was linearly related to mean flow velocity. Accordingly, longitudinal dispersivities ranged between 0·152 ± 0·017 m in channels with smooth bed surface and 0·584 ± 0·015 m in identical channels with a coarse gravel substrate. Grain size and surface roughness of the channel bed were found to correlate positively to longitudinal dispersion. This finding contradicts several existing relations between surface roughness and dispersion. Future studies should include further variation in surface roughness to derive a better‐founded empirical equation forecasting longitudinal dispersion from surface roughness. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
7.
Flume experiments were carried out to study the turbulence and its impact on suspension and segregation of grain-sizes under unidirectional flow conditions over the sand-gravel mixture bed. The components of fluid velocity with fluctuations were measured vertically using 3-D Micro-acoustic Doppler velocimeter (ADV). The theoretical models for velocity and sediment suspension have been developed based on the concept of mixing length that includes the damping effect of turbulence due to sediment suspension in the flow over the sand-gravel mixture bed. Statistical analysis of segregation of grain-sizes along downstream of the bed has been performed using the principle of unsupervised learning or clustering problem. Exploratory data analysis suggests that there is a progressive downstream fining of sediment sizes with selective depositions of gravels, sand-gravels and sand materials along the stream, which may be segmented into three regions such as, the upstream, the transitional and the downstream respectively. This contribution is relevant to understand the direction of ancient rivers, the bed material character in the river form, sorting process and its role in controlling the sediment flux through landscape. 相似文献
8.
The present study proposes a model on vertical distribution of streamwise velocity in an open channel turbulent flow through a newly proposed mixing length, which is derived for both clear water and sediment-laden turbulent flows. The analysis is based on a theoretical consideration which explores the effect of density stratification on the streamwise velocity profile. The derivation of mixing length makes use of the diffusion equation where both the sediment diffusivity and momentum diffusivity are taken as a function of height from the channel bed. The damping factor present in the mixing length of sediment-fluid mixture contains velocity and concentration gradients. This factor is capable of describing the dip-phenomenon of velocity distribution. From the existing experimental data of velocity, the mixing length data are calculated. The pattern shows that mixing length increases from bed to the dip-position, having a larger value at dip-position and then decreases up to the water surface with a zero value thereat. The present model agrees well with these data sets and this behavior cannot be described by any other existing model. Finally, the proposed mixing length model is applied to find the velocity distribution in wide and narrow open channels. The derived velocity distribution is compared with laboratory channel data of velocity, and the comparison shows good agreement. 相似文献
9.
R. J. Bennett 《地球表面变化过程与地形》1976,1(2):131-150
This paper constructs a model of channel geometry composed of three subsections: a steady-state submodel, a gradedstate submodel, and a stochastic error submodel. With the aid of this representation of the morphology of channels, the at-a-point changes in geometry can be reproduced by a simple recursive equation of autoregressive, moving-average form which is derived from methods used in the statistical analysis of time series. A set of height loss data for three Japanese rivers derived from Yatsu's (1955) paper is used to determine the effect of bed material changes on adaptations within the graded-state submodel of the channel. The results of analyzing the autocorrelation function, spectrum, and adaptive parameter shifts within this model can be used to infer that significant changes in the amplitude of height change variation, and a shift to higher frequency oscillations of bed forms are associated with the shift in bed material conditions. 相似文献
10.
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
11.
Non-uniform sediment deposited in a confined, steep mountain channel can alter the bed surface composition. This study evaluates the contribution of geometric and resistance parameters to bed sta-bilization and the reduction in sediment transport. Flume experiments were done under various hydraulic conditions with non-uniform bed material and no sediment supply from upstream. Results indicate that flume channels respond in a sequence of coarsening and with the formation of bedform-roughness features such as rapids, cascades, and steps. A bedform development coefficient is introduced and is shown to increase (i.e. vertical sinuosity develops) in response to increasing shear stress during the organization process. The bedform development coefficient also is positively correlated with the critical Shields number and Manning's roughness coefficient, suggesting the evolution of flow resistance with increasing bedform development. The sediment transport rate decreases with increasing bed shear stress and bedform development, further illustrating the effect of bed stabilization. An empirical sedi-ment transport model for an equilibrium condition is proposed that uses the bedform development coefficient, relative particle submergence (i.e. the ratio of mean water depth and maximum sediment diameter), modified bed slope, and discharge. The model suggests bedform development can play a primary role in reducing sediment transport (increasing bed stabilization). The model is an extension of Lane's (1955) relation specifically adapted for mountain streams. These results explain the significance of bedform development in heightening flow resistance, stabilizing the bed, and reducing sediment transport in coarse, steep channels. 相似文献
12.
13.
Meinte Blaas Changming Dong Patrick Marchesiello James C. McWilliams Keith D. Stolzenbach 《Continental Shelf Research》2007
Suspended sediment-transport processes in Santa Monica and San Pedro Bay are analyzed using the sediment-transport capabilities of the Regional Oceanic Modeling System (roms). A one-month simulation for December 2001 has been carried out with a set of nested domains. The model inputs include tides, winds, surface waves, and idealized initial sediment conditions for sand and non-cohesive silt. Apart from the control run, the sensitivity of the results to surface waves, ripple roughness and bed armoring has been analyzed. From the control experiment, the horizontal transport of sand turns out to be limited to within a few km of the nearshore erosion zones. During high wave events, silt is transported over further distances and also partly offshelf in distinct plumes. The effectiveness of horizontal silt transport depends strongly on vertical mixing due to both surface wind stress and wave-enhanced bottom stress. High wave events coincident with strong winds (hence strong vertical mixing) are the most optimal conditions for sediment-transport. Excluding wave effects in the simulation shows that surface waves are the dominant factor in resuspending bed material on the Southern Californian shelves. The sensitivity experiments also show that the direct influence of additional ripple roughness on erosion and deposition is relatively weak. Switching off bed armoring locally results in increases of near-bottom concentrations by a factor of 20 for silt and a factor of 5 for sand as well as stronger spatial gradients in grain size. 相似文献
14.
Suspended load in flows on erodible bed 总被引：1，自引：0，他引：1
Steady state suspended-load of sediment transported in flow over erodible beds usually is treated by the advection-diffusion approach, though in recent years, it is being treated as a two-phase flow phenomenon incorporating kinetics of sediment particles. Among the advection-diffusion approaches, Rouse＇s equation is the well-known, although a number of researchers in later periods have attempted to improve it by modifying the mixing length concept taking into account other aspects. In this paper, the advection-diffusion approach and associated logarithmic law of flow velocity are revisited. It is concluded from the logarithmic law that the Reynolds shear stress is a linear function of height above the bed, which reduces to bed shear stress in the case of a long horizontal channel. As a consequence, it is shown that the volumetric concentration of sediment is best approximated by the sum of two power laws of height above the bed. An equation is derived for the suspended-load transport rate in terms of elementary functions. 相似文献
15.
A. D. Knighton 《Journal of Hydrology》1983,60(1-4):105-121
A simple model of channel geometry is presented which focuses on the way that a single form variable may be autocorrelated with upstream values of that variable and which is amenable to analysis using time series methods. Stream bed topography at the local scale is the variable of interest. Bed height series were obtained for four stream lengths located at different positions along a mountain stream to test the hypothesis that, as control variables change spatially, the form of the model will also change. The Box-Jenkins models describing the series are dominated by autoregressive terms and indicate a non-random variation of bed height. An increase in the order and parameter magnitude of the models is associated with an increase in discharge and a decrease in bed material size, suggesting that the structure of the model reflects the adjustability of the stream bed and, specifically, the extent of riffle-pool development, although channel gradients may be too high for a well-defined riffle-pool sequence to be developed. Changes in the structure of models obtained for bed height and other form variables may indicate the variable ability of a stream to adjust its channel morphology. 相似文献
16.
17.
Basil Gomez 《地球表面变化过程与地形》1983,8(1):41-54
The quantitative bedload transport data that are presently available confirm that the generalized bedload transport rate-stream power relationship is applicable to natural streams. However, the bedload transport rate is not solely dependent upon hydraulic parameters, but also upon the inter-relationship between bed material characteristics and flow properties. Segregation of the surficial bed material, as expressed through the development of an armour coat, limits the availability of transportable material. Under such circumstances observed bedload transport rates are less than the predicted values. The effect which the development of an armoured surface has upon the bedload transport rate is described with reference to bedload and bed material sampling in the Borgne d'Arolla, Valais, Switzerland. The data refer to two periods when the resumption of baseflow conditions following flood events which were of a sufficient magnitude to transport all but the coarsest (0·3–0·5 m) particles on the streambed, provided the opportunity for the bed to adjust to a comparatively stable flow regime. Observed and predicted bedload transport rate-stream power relationships are compared. The theoretical relationship does not adequately describe conditions in some gravel-bed channels, since it fails to take into account the effect which armouring may have upon the supply of transportable material. 相似文献
18.
The depth‐integrated momentum and kinetic energy equations contain velocity correlation terms that involve products of local deviations in velocity components about depth‐averaged values. Based on velocity data obtained from North Boulder Creek, Colorado, a simple scaling analysis suggests that certain of these terms, which normally can be neglected in the case of smooth channels, can be significant parts of the momentum and energy balances in steep, rough channels owing to the occurrence of non‐logarithmic velocity profiles. A linearized version of the kinetic energy equation suggests that, for flow accelerations over small‐amplitude bed forms, the energy of the mean motion is spatially partitioned between a form involving the depth‐averaged velocity and a form involving the deviatoric part of the velocity profile; this partitioning is associated with spatial variations in the uniformity of the vertical profile of the streamwise velocity. These points are consistent with published flume measurements involving flow over sand‐roughened dunes, and with published field measurements of flow over a gravel bar. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
19.
The role of water depth and bottom boundary layer turbulence upon lee-wave generation in sill regions is examined. Their effect
upon vertical mixing is also considered. Calculations are performed using a non-hydrostatic model in cross-section form with
a specified tidal forcing. Initial calculations in deeper water and a sill height such that the sill top is well removed from
the surrounding bed region showed that downstream lee-wave generation and associated mixing increased as bottom friction coefficient
k increased. This was associated with an increase in current shear across the sill. However, for a given k, increasing vertical eddy viscosity A
v reduced vertical shear in the across sill velocity, leading to a reduction in lee-wave amplitude and associated mixing. Subsequent
calculations using shallower water showed that for a given k and A
v, lee-wave generation was reduced due to the shallower water depth and changes in the bottom boundary layer. However, in this
case (unlike in the deepwater case), there is an appreciable bottom current. This gives rise to bottom mixing which in shallow
water extends to mid-depth and enhances the mid-water mixing that is found on the lee side of the sill. Final calculations
with deeper water but small sill height showed that lee waves could propagate over the sill, thereby reducing their contribution
to mixing. In this case, bottom mixing was the major source of mixing which was mainly confined to the near bed region, with
little mid-water mixing. 相似文献
20.
Active bed forms of three major classes are formed in Terek lower reaches during summer floods. They include ripples, dunes of the first order, and dunes of the second order (from smaller to larger), which commonly form an incomplete hierarchy. The morphology of the bed forms is essentially stochastic and can be adequately described by probability distribution functions of bed form characteristics for some narrow ranges of hydraulic flow characteristics. At the same time, the mean values of bed form morphometric characteristics (length, height, and asymmetry) show stable relationships with flow velocity and depth. The celerity of active dunes can be adequately described by a modified Snishchenko–Kopaliani formula. The channelforming sediments that move as bed forms in Terek lower reaches account for 7% of sediment transport rate of all channel-forming sediments, a value near the lower limit for rivers with sand alluvium. 相似文献