Motion Of Sediment Particles Research Articles

Threshold of Motion of Natural Sediment Particles in Oscillatory Flows

The mechanism of the threshold of sediment motion under wave action has been investigated by examining the dynamic forces acting on the sediment particle. The force analysis has led to a range of nondimensional parameters of clear physical significance, which are then applied to the analysis of a range of existing experimental data. The introduction of some new nondimensional parameters has resulted in significant improvement in the definition of the threshold of motion condition. Several empirical equations involving different nondimensional parameters have been derived, providing a mathematical description of the threshold condition of the sediment motion. The accuracy of these empirical equations is examined in the context of scientific and engineering applications and a simple equation is recommended for its accuracy and ease of use.

Stability Analysis Of DredgingThe Flow Sediment Regiment Upstream A Dam

The overall aim of this study was to investigate stability of flow-sediment regime upstream from a dam, to develop and calibrate a one-dimensional flow-sediment transport numerical model to deal with the many river-reservoir sedimentation problems including river reservoir dredging upstream from a dam. The basic physical principles of the conservation of mass and momentum are used to describe the fluid flow. The conservation of mass and semi-empirical equations governing sediment particle movement are adopted to establish the interaction between the sediment movement and fluid flow. The resulting mathematical formulation is highly non-linear and complex. It is impractical, if not impossible, to solve them analytically. Therefore the three governing equations of water continuity, sediment continuity, and momentum were solved numerically. The three governing equations were solved in an approximate linear form as well as in the more complete non-linear form. Also, by ignoring certain terms, the sediment continuity equation was uncoupled from the other two. Algorithms were developed for linear or non-linear and coupled or uncoupled solutions.

Three-Dimensional Modeling of Sediment Transport in a Narrow 90° Channel Bend

A three-dimensional numerical model was used for calculating the velocity and bed level changes over time in a 90° bended channel. The numerical model solved the Reynolds-averaged Navier-Stokes equations in three dimensions to compute the water flow and used the finite-volume method as the discretization scheme. The k-ε model predicted the turbulence, and the SIMPLE method computed the pressure. The suspended sediment transport was calculated by solving the convection diffusion equation and the bed load transport quantity was determined with an empirical formula. The model was enhanced with relations for the movement of sediment particles on steep side slopes in river bends. Located on a transversally sloping bed, a sediment particle has a lower critical shear stress than on a flat bed. Also, the direction of its movement deviates from the direction of the shear stress near the bed. These phenomenona are considered to play an important role in the morphodynamic process in sharp channel bends. The calculated velocities as well as the bed changes over time were compared with data from a physical model study and good agreement was found.

INFLUENCE OF SEDIMENT GRAIN SIZE ON DEBRIS FLOW

Results obtained from both of flume data and theories suggest that equilibrium bed slope in flow over an erodible bed are determined uniquely by sediment discharge rate when the movements of sediment particles are laminar and thus no suspended transportation take place. This means that the static friction force is dominant in debris flow and that sediment concentration is determined by shear stress balance on the bed surface as seen in our previous studies. On the other hand, if part of sediment particles in debris flow body is transported in suspension, sediment concentration will be larger and the equilibrium bed slope will decrease. These facts are supported by Egashira et al.'s flume data and others' experimental data.The present study discusses experimentally an influence of flow scales on an equilibrium bed slope and flow structure, based on experimental data and emphasizes that the equilibrium bed slope decreases with increasing of flow scale if part of debris flow body is turbulent.

LAGLANGIAN COUPLING FOR SOLID-LIQUID TWO PHASE FLOW BY DEM-MPS METHOD

The Lagrangian model of solid-liquid two phase flow is proposed by combining the Moving Particle Semi-implicit (=MPS) method with the Distinct Element Method (=DEM). The governing equations of two fluid model is discretized by the MPS method, with the model of interparticle collision in solid phase based on the DEM. The relationships of present model with the previous Lagrangian model of a motion of single sediment particle and the non-Newtonian fluid model are discussed. The performance of the present model is investigated by simulating the dropping lump of mud into still water. By comparing the solutions of three models, such as the present model; the two fluid model by the MPS method without the DEM; and the MPS method with the passively moving solids module, the effect of cohesion of mud is reproduced well by the present model.

Discrete Particle Model for Analyzing Bedform Development

A discrete element model is used to simulate the emergence and evolution in time of wind ripples with the use of simple rules for the motion of individual sediment particles. It is shown, by analyzing the probabilities for the particles to be entrained and deposited over the bed implied by the selected rules, that the same model can be used to simulate the development of underwater bedforms. The results of the model show that the particle-transport processes implied by the simple rules used in the model can represent many different complex aspects of the physics of bedforms. The results of the model are compared with experimental results on underwater ripples obtained by the authors, and a surprisingly good agreement is found in terms of the main dynamical properties of the bedforms, such as growth rate of the amplitude, evolution of the steepness, growth saturation, and bedform celerity. Even though the simulated bedforms do not reproduce the exact geometry of their physical counterparts, the richness of the strongly nonlinear processes that this simple model is able to simulate makes this kind of approach useful to explain many of the observed features of underwater bedforms, which are otherwise very difficult to address theoretically.

Theoretical and probabilistic analyses of incipient motion of sediment particles

Initiation of sediment particles by the flow can be classified by three different modes: rolling, sliding, and lifting. In this paper, threshold conditions of each mode obtained from force and momentum balances are investigated. Theoretical analysis of these conditions leads to curves similar to Shields diagram, resulting that the rolling and lifting thresholds yield the minimum and the maximum values of dimensionless critical shear stresses, respectively. A probabilistic analysis of the same problem is carried out under the assumption that the velocity approaching the particle has a normal distribution. The results revealed that about 4% of the particles on the bed are lifted when the critical bed shear stress by Shields is applied. Entrainment probability and saltating probability are also estimated, and they are compared with existing results.

Sediment Pickup on Streamwise Sloping Beds

This paper presents an experimental investigation on noncohesive sediment pickup under a unidirectional steady-uniform stream flow on streamwise steeply sloping (down slope and adverse) sedimentary beds. The characteristic parameters affecting the sediment pickup, identified based on the physical reasoning and dimensional analysis of the sediment particle movement under stream flow, are the transport-stage parameter, particle parameter, and geometric standard deviation of sediment particles. A large number of experiments (426 runs) were carried out in two long rectangular ducts (closed-conduit flow) with nine types of sediments (six uniform and three nonuniform sediments), having a variation of bed slope from −15° (adverse slope) to 25° (down slope). In an open channel flow (laboratory flume study), the uniform flow is a difficult, if not impossible, proposition for a steeply sloping channel and is impossible to obtain in an adversely sloping channel. To avoid this problem, the tests were conducted with a closed-conduit flow. Measurements included flow discharge and sediment pickup rate. The bed shear stress for a particular run was computed considering side wall correction. The experimental data were used to determine the equation of sediment pickup function through a regression analysis. The equation is adequate to estimate sediment pickup not only on horizontal and mild slopes but also on steep and adverse slopes.

An application of image processing in the study of sediment motion

The influence of turbulence on the entrainment of sediment particles from a plane mobile bed was investigated using image processing techniques to record sediment particle motion in an experimental flume while simultaneously monitoring the turbulence characteristics of the flow. The importance of the image processing techniques applied arise from the use of the collected images for determination of the area of sediment entrained and the instantaneous particle velocity. A subtraction technique was used to derive the difference between consecutive images, from which the number of particles entrained in an increment of time could be determined. A cross correlation was applied to find if a relation between the number of entrained particles and the instantaneous shear stress in sweep events existed. A significant correlation with was found for this relation. From the results obtained, the exceedance probability of the area entrained was found to correlate with the exceedance probability of instantaneous shear stress in a sweep event. Image processing was found to be a useful technique for the analysis of experimental data and fundamental to this study aimed at investigating the entrainment mechanisms for sediment particles.

Computer Vision Technique for Tracking Bed Load Movement

An advanced image analysis system, called Khoros, was used to investigate the bed load movement of sediment particles in a laboratory flume. Incipient flow conditions prevailed throughout the experiments. Painted glass balls of identical diameter and density were used to simulate the sediment particles. They were uniformly placed on top of a tightly packed flat porous bed. Experiments were performed with two distinct surface packing configurations. A video camera was used to monitor their motion within a specified area of view. The resulting video record was converted to digital images using a frame grabber. These digital images were downloaded to a workstation for analysis. The outcome of this analysis provided quantitative information about the frequency of the entrainment of the glass beads, their displacement distance, and the mode of their motion. Such information, when used in conjunction with laser Doppler velocimeter measurements of the fluid velocity, can elucidate the physical mechanisms that are responsible for the entrainment of sediment. During the analysis of the tests, it was observed that the displacement of the beads was sporadic and occurred typically by rolling. The glass beads moved predominately along the flow direction, while on some occasions they were displaced in the transverse direction. For the two packing density tests that were examined, the minimum traveling distance in the longitudinal direction was found to be equal to one bead diameter and the maximum was equal to 10 bead diameters. In the transverse direction, the maximum particle traveling distance was equal to four bead diameters. Finally, it is shown that the existing imaging workspace can be used to accurately identify the displacements of small particles, which are typically encountered near incipient flow conditions and are not easily detectable with the bare eye. The imaging method described here is dynamic in nature and may prove to be a valuable tool for studying two-phase flows, as well as for visualizing flow structures taking place near the boundary in turbulent flows.

Drift Velocity Concept for Sediment-Laden Flows

The vertical distribution of suspended sediment concentration in open channel flows is usually described by the advection-diffusion equation. Hence, only the mechanisms of advection and diffusion affect the movement of sediment particles. However, recent publications show that the equation could not describe fully the sediment concentration distribution in open channels. A new model for particle migration in sediment-laden turbulent flow is presented. The model assumes there is an additional mechanism influencing the particle movement in sediment-laden flows. A new mechanism, which is called the drift, acts independently of advection and turbulent diffusion. Based on all three mechanisms, a new equation for the concentration distribution is derived. The equation describes the vertical distribution of sediment concentration more accurately than the advection-diffusion equation.

Experiment on Flow-Velocity Profile and Sediment Motion in Transition Process from Saltation to Sheetflow

Experiment on the flow-velocity profile and the motion of sediment particle has been conducted in open channel under the various hydraulic conditions. The transition region from saltation to sheetflow was focused from the viewpoint of the difference of the characteristics of flow-velocity profile. The flow-velocity profiles of saltation dominant flow show clear difference form that of sheetfiow dominant flow. These difference was discussed based no the difference in the characteristics of the sediment-particle motion between two fiow modes.

Interaction between Transported-Sediment Particle and Bed-Material Particles

Lagrangian simulation is one of the most effective tools for analyzing the detail mechanism of sediment transport. In the previous Lagrangian simulation, the motion of sediment particle was traced on the fixed bed in which bed material particles were irregularly arranged. While, in this study, bed-material particles are treated as granular material by using Distinct Element Method, or DEM, to describe particle/particle interaction. The velocity profile of the moving particles and the existing-height probability of sediment particle, or sediment concentration, are calculated based on the simulated motion of sediment particles by DEM to consider the physics of particle/particle interaction especially in the neighborhood of bottom movable boundary.

Sediment Transport in Sheetflow Regime under Oscillatory Flow

Sediment transport in sheetflow regime under the oscillatory flow is investigated by both experimental and numerical approaches. Firstly, motion of sediment particle is traced by video film analysis, and the change of the velocity profile and concentration profile of sediment particles during half period of oscillation are displayed. Sediment concentration is estimated on the basis of the correlation between brightness of the image and the concentration. Secondary, the detail of sediment motion is investigated numerically by using distinct element method. The simulated velocity profile and the concentration profile of sediment particle shows the good agreement with experimental results. The simulated snapshots of sediment particles bring important information to understand the mechanism of sediment motion.

Sediment Movement Modes and Transport Capacity in Open Channel with Rigid Bed

The occurrences of three kinds of sediment transport mode are explained on the basis of constitutive relations for debris flows; shear flow or individual movement of sediment particles on rigid bed, partial plug flow and shear flow on movable bed. Formulas of velocity profile' and sediment transport capacity in each mode are derived from momentum equation and the constitutive relations. Such theoretical results are certified with flume data.

The motion of sediment‐water mixtures during intense bedload transport: computer simulations

ABSTRACTA new model, which couples fluid and particle dynamics, has been developed to study the motion of the sediment‐water mixture during intense bedload transport, including the velocity profiles of both sediment and water, the roughness length of an upper plane bed and the thickness of moving sediment layers. Standard mixing length theory is used to model the motion of water above the boundary between the overlying water and the sediment‐water mixture. The turbulent flow within the moving sediment layers is described by a shear stress model, in which the effective viscosity of the flowing water is proportional to the velocity difference between the fluid and the sediment. The particle dynamics method, in which the equations of motion of each of many particles are solved directly, is applied to model the movement of sediment particles. The particle‐fluid interaction is expressed by a velocity‐squared fluid drag force exerted on each sediment particle. Both computer simulation results and theoretical analysis have shown that the velocities of both sediment and fluid during intense sediment transport decrease exponentially with depth in the top layers of a fast‐moving sediment—water mixture. The thickness of the moving sediment layers, obtained from the computer simulation results, is proportional to the shear stress, which agrees with previous experimental observations.

Initial motion of sediment under waves and wave-current combined motions

Recently, one of the authors proposed a very convenient formula to evaluate the friction coefficient induced by wave-current combined motion, which spans all flow regimes including the transitional regime and requires no iteration. This formula is applied to an analysis of existing experimental data regarding initial motion of sediment particles under pure waves and wave-current combined motion. It was found that most of the data analyzed here were located in the transitional regime. Thus, the full-range friction coefficient formula can be conveniently applied as compared with previous implicit formulae and graphical friction laws. The formula of general motion under pure wave motion can be modified to compute the critical water depth of sediment movement. Some discontinuities in the former work in the transitional regime now can be overcome in the present study.

Bioturbation: A facilitator of contaminant transport in bed sediment

AbstractThe biological activity of benthic organisms (=bioturbation) in bed sediments can be significantly more important than physicochemical processes in the movement of sediment particles and interstitial constituents within the sediment column and to the overlying water. Bioturbation‐driven transport processes may be several orders of magnitude more rapid than molecular‐driven processes for some constituents, i.e., particle reactive compounds like PCB, when considered on a chemical pore water concentration gradient basis. Benthic organisms affect particle redistribution and solute transport by burrowing, ingestion/excretion, tube‐building and biodeposition. In most benthic environments, whether freshwater, estuarine or marine, numbers of organisms and rates of sediment turnover are highest in the oxygenated zone above the redox boundary, generally the top 2–5 cm of the sediment column. However, a variety of organisms penetrate deeper into the sediment and may also be important in transport of both dissolved and particle reactive constituents. When considering capping of contaminated sediment in place or capping of contaminated dredge spoils, the effects of bioturbation on the mobility of the contaminants must first be considered before establishing engineering design criteria. Capping of contaminated bed sediments or dredge spoils can elevate the bioturbated zone above the contaminated bed sediment and thus potentially reduce the rate of contaminant transport by several orders of magnitude.

An energy approach to non-breaking wave-induced motion of bottom sediment particles

A general equation, which applies to incipient motion, general surface motion and disappearance of ripples under non-breaking water waves, is derived using an energy approach. Lower and upper limits of ripple development are derived, showing that existing criteria are not entirely sufficient. An explicit formula for the dissipation factor as a function of D/A (grain diameter near-bed wave amplitude) is also obtained. The formula has been used to evaluate the variation of f e with increasing flow characteristics for constant grain diameter. Finally, values of the sand grain roughness height, k s, in relation to grain diameter are given for different types of bottom motion.

Sediment Transport on a Gentle Slope due to Waves

A criterion for initiation of sediment movement on a gentle slope is established by balancing the forces acting on a sediment particle lying on the slope. A relationship for the instantaneous rate of bedload transport on a gentle slope is then derived from a simple and quasi-steady analysis of the motion of sediment particles. A gentle slope produces net movement of sediment even in pure oscillatory flow. The theory is in reasonable agreement with experiments conducted on a horizontal bed, corresponding to zero slope. The relationship derived for the net rate of bedload transport is applied to the problem of infilling of relatively small and symmetric trenches caused by wave action. The analysis includes the influence of the disposal distance of dredge spoil on the infilling. The model qualitatively explains observed infilling of test pits for San Francisco’s ocean outfall and smoothing of ice gouges due to summer storms in the Southern Beaufort Sea north of Alaska, for which limited data is available.