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We derive a simple theoretical model for estimating the change in the magnitude of the particle slip velocity and the deflection in the trajectory of a particle induced by the lift force acting on a freely rotating particle settling under gravity in a shear flow. The lift force on the particle in small, intermediate, and large particle Reynolds number, is studied. We have found that the magnitude of the slip velocity is always reduced by the lift force, implying reduced particle settling velocities. The reduction in the slip velocity and the angle of deflection are the greatest when the shear is the greatest and the particle Reynolds number is the smallest. However, these effects decrease as the particle Reynolds number becomes larger. The derived equations are applied to evaluate the effect of the lift force on particle transport within turbidity currents, on the entrainment of small particles embedded in the lower boundary of a shear flow, and on sediment-laden open-channel flow. [ABSTRACT FROM AUTHOR]

This paper applies dimensional analysis to propose an alternative model for estimating the effective density of flocs (Δρf). The model takes into account the effective density of the primary particles, in addition to the sizes of the floc and primary particles, and does not consider the concept of self-similarity. The model contains three dimensionless products and two empirical parameters (αf and βf), which were calibrated by using data available in the literature. Values of αf=0.7 and βf=0.8 were obtained. The average value of the primary particle size (Dp) for the data used in the analysis, inferred from the new model, was found to vary from 0.05 μm to 100 μm with a mean value of 2.5 μm. Good comparisons were obtained in comparing the estimated floc-settling velocity on the basis of the proposed model for effective floc density with the measured value. [ABSTRACT FROM AUTHOR]

At the South Branch of the Changjiang Estuary near Baimao Shoal, two in situ approaches were used to estimate the settling velocity, ws, of suspended, fine-grained sediments. The first approach was used when the current was less than 1.5 m/s and was based on measurements from an optical backscatter sensor (OBS-3A) and a laser in situ scattering and transmissometer (LISST-100, Type C). A modification, using the measured ratio of volume concentration for each floc size class to the total volume concentration as a weighting factor. To improve a previously published approach, a better algorthim was implemented to estimate ws. Results of the modified approach (0.4 to 4.6 mm/s) are about twice those of the original approach, which assumes that all sizes of flocs have the same floc density. The second approach used the Rouse equation to estimate the depth-averaged ws when the current was strong and nearly steady around maximum ebb. Results from the second approach show a much greater depth-averaged ws (4-8.5 mm/s). This is attributed to the large bed shear stresses (between 3 and 3.6 Pa) bringing large sediments into the water column. [ABSTRACT FROM AUTHOR]

Hydrodynamic separators are proprietary underground devices designed to remove floatable debris (e.g., leaves, trash, oil) and to remove suspended solids from storm-water runoff by sedimentation. They are designed for storm-water treatment in urban areas to meet tight space constraints. Limited data on the suspended solids removal performance of installed devices are available, and existing data are questionable because of the problems associated with assessment by monitoring. The objectives of our research are to: (1) investigate the feasibility and practicality of field testing to assess the performance of hydrodynamic separators as underground storm-water treatment devices; (2) evaluate the effects of sediment size and storm-water discharge on the performance of six devices from different manufacturers; and (3) develop a universal approach for predicting the performance of a device for any given application. In the field tests, a controlled and reproducible synthetic storm event containing sediment of a well defined size distribution and concentration was fed to a precleaned device. The captured sediment was then removed, dried, sieved, and weighed. To assess the performance of the devices, suspended sediment removal efficiency was related to a Péclet number, which accounts for two major processes that control performance: (1) settling of particles; and (2) turbulent diffusion or mixing of particles. After analyzing the data, all devices showed similar behavior, therefore, a three-parameter performance function was proposed for all devices. Performance functions were developed from the result of the field tests and parallel testing of two other full-scale devices in the laboratory. The performance functions can be used to determine the efficiency of the tested devices and to improve the selection and sizing of hydrodynamic separators and the assessment of their overall performance after installation. [ABSTRACT FROM AUTHOR]

A recent companion paper reported an experimental protocol used to analyze sediment properties. This protocol identified for both freshwater and marine sediments a surface layer with specific dynamic properties (critical erosion shear stresses in the range 0.025–0.05 N m-2) and a second layer with critical erosion shear stresses about ten times larger. The present study compares these former results with recent work which extended the applicability domain of the Shields diagram to very fine particles. The surface layer is shown to consist in fine and unconsolidated sediments that behave like noncohesive material whereas the second layer is characterized as being cohesive. The surface layer is mainly representative of recent deposits of suspended particles. This points out the existence of a fluffy layer of fine sized particles resting near the bed, with specific erosion characteristics, which has to be considered separately when studying sediment properties. [ABSTRACT FROM AUTHOR]

Results are presented from laboratory studies investigating the behavior of fine sand particles within turbulent open channel flow conditions flowing over rough, porous beds. A particle tracking technique was employed to record and analyze sand particle motion within the flow, while mean and fluctuating flow velocities were measured by an acoustic Doppler velocimeter probe. Measured particle settling rates show a strong influence from flow turbulence, being generally enhanced in the near-bed and intermediate flow regions and retarded in the outer flow region, compared to their fall velocity in still water conditions. Experiments also reveal the relative degree of settling enhancement to increase with decreasing particle size. Correlation between particle and small-scale fluid motions is demonstrated through a quadrant analysis technique, with higher-order events for the two phases found to be dominated by ejections and sweeps associated with the bursting process. Particle interactions with large-scale turbulent flow structures, revealed through flow visualization with a moving frame of reference, are found to result in particle accumulation in peripheral trajectories on the downflow side of local eddy structures. Analytical and theoretical considerations suggest that both these turbulence scales provide preferential transportation mechanisms that will account for the enhanced sand particle settling rates observed. [ABSTRACT FROM AUTHOR]

Several existing formulas for the initial porosity and settling velocity of sediment have been tested by using extensive data collected from different countries and regions, and modified to achieve better reliability or convenience in use. [ABSTRACT FROM AUTHOR]

The propagation dynamics of the turbid density currents of the Imha Reservoir is investigated using the FLOW-3D computational fluid dynamics code. The renormalization group (RNG) turbulence scheme in a Reynolds-averaged Navier-Stokes (RANS) framework was applied for the field investigation. A new particle dynamics algorithm was developed and coupled with the FLOW-3D model to simulate the settling of sediment particles, and the model was tested with field measurements during Typhoon Ewiniar. The coupled model correctly predicted the spatial and temporal evolution of intrusive density currents with plunging flows at a depth of 20 m and interflows propagating downstream at about . The simulation results with sediment settling demonstrated that the ratio of transported to initial sediment concentration () varies as a function of particle size and sediment concentration. Density currents can be classified into three regimes: (1) a suspended regime (), where interflows will transport most of the sediment inflows from the reservoir inlet to Imha Dam; (2) a transition regime (), where deposition rates alter the flow dynamics; and (3) a deposition regime (), where sediment particles rapidly settle out without forming a density current. For the suspended regime, river inflows will define three different types of density currents in the reservoir: (1) interflows when ; (2) multiple intrusions when ; and (3) plunging underflows when . [ABSTRACT FROM AUTHOR]