Formulas for Sediment Porosity and Settling Velocity

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.     

Fluctuations of Suspended Sediment Concentration and Turbulent Sediment Fluxes in an Open-Channel Flow

A complex problem of turbulent-sediment interactions in an open-channel flow is approached experimentally, using specially designed field experiments in an irrigation canal. The experimental design included synchronous measurements of instantaneous three-dimensional (3D) velocities and suspended sediment concentration using acoustic Doppler velocimeters (ADV) and a water sampling system. Various statistical measures of sediment concentration fluctuations, turbulent sediment fluxes, and diffusion coefficients for fluid momentum and sediment are considered. Statistics, fractal behavior, and contributions of bursting events to vertical fluxes of fluid momentum and sediment are evaluated using quadrant analysis. It has been found that both turbulence and sediment events are organized in fractal clusters which introduce additional characteristic time and spatial scales into the problem and should be further explored. It is also shown that Barenblatt’s theory of sediment-laden flows appears to be a good approximation of experimental data.     

Sediment Flocs: Settling Velocity, Flocculation Factor, and Optical Backscatter

Improved relationships for the flocculation factor and floc settling velocity are proposed. They differ from the existing formulas by accounting for the effect of floc porosity on the hydrodynamic drag and for the fractal properties of the floc projection on the plane. These relationships predict that the fractal dimension d influences the floc settling velocity Ws only if d ≥ 2. A flocculation effect on optical backscatter is also considered, and a fractal-based relationship for quantifying this effect is proposed. This relationship serves as a basis for a simple optical backscatter sensor (OBS) based technique for estimating some fractal-related parameters of sediment flocs involved in the proposed relationships for the settling velocity and flocculation factor. The technique is tested using data from three field sites. The potential effect of particle size on backscatter efficiency is highlighted, and an approach for accounting for this effect is suggested.     

Settling Velocity Grading of Particle Bound PAHs: Case of Wet Weather Flows within Combined Sewer Systems

The elaboration, management, and design of the sedimentation devices require an improved knowledge not only on the suspended solids (SS) settling velocity but also on the distributions of these particulate pollutants by category of SS settling velocity. Nevertheless, there is currently a lack of available data regarding the settling velocity grading of particle bound pollutants. As a consequence, this study investigating the settling velocity grading of particle bound polycyclic aromatic hydrocarbons (PAHs) in wet weather flows within combined sewers was launched. The pursed objectives were to compare the SS and PAH settling velocity grading curves and hence to assess whether or not PAHs were associated with a specific settling velocity category. Investigations were carried out for three storms at two sampling sites within the Parisian combined sewer. Results indicate that most of PAHs seem to be preferentially associated with particles with a settling velocity >0.3?mm?s?1.     

VICAS—An Operating Protocol to Measure the Distributions of Suspended Solid Settling Velocities within Urban Drainage Samples

To meet the need for an operating protocol to measure the distributions of suspended solid settling velocities within urban drainage, the CEREVE research laboratory has developed and validated the VICAS protocol. This procedure makes it possible, through the use of a compact, inexpensive and easy-to-operate settling column, to measure the settling velocity of suspended solids conveyed by either storm water or combined sewage, immediately after collecting a reduced volume sample (4.5 L), and without any pretreatment steps. Two measurement data processing methods were developed and may be easily implemented by means of a series of macros written in Excel. Tests were also conducted for the purpose of evaluating the level of measurement uncertainty specific to the protocol.     

Sphere Drag and Settling Velocity Revisited

Sphere drag data from throughout the twentieth century are available in tabular form. However, much of the data arose from experiments in small diameter cylindrical vessels, where the results might have been influenced by the wall effect. Wall effect corrections developed by others were applied to 178 of the 480 data points collected. This corrected data set is believed to be free of the influence of wall effects. Existing drag and settling velocity correlations were compared to this data set. In addition, new correlations of the same forms were developed using the corrected data. Two new correlations of sphere terminal velocity are proposed, one applicable for all Reynolds numbers less than 2×105, and the other designed to predict settling velocities with exceptional accuracy for terminal Reynolds numbers less than 4,000, a region that contains almost all applications of interest in environmental engineering. The trial and error solution for settling velocity using the Fair and Geyer equation for drag should be retired in favor of the direct calculation available from these new correlations.     

Experimental Study of Fine Sand Particle Settling in Turbulent Open Channel Flows over Rough Porous Beds

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.     

Lateral Variations in Suspended Sediment Concentration over Dunes

A series of laboratory flume experiments were conducted to characterize lateral variations of suspended sediment over dunes. Forty experimental runs were made using flow depths of 0.328 and 0.127?m in a 1.2-m-wide channel. The Froude number was 0.47 and the median diameter of the bed material was 0.52?mm. Point samples of suspended sediment and depth-integrated values calculated using acoustic backscatter data were collected simultaneously at two lateral positions at spacings of 0.41, 0.20, 0.10, and 0.05?m. Root mean square differences between the paired samples increased from 30 to 94% and from 20 to 45% of the mean concentration for point samples and depth-integrated samples, respectively, as the lateral spacing between samples was increased. Lateral variations in the concentration of suspended sediment should be considered when designing sediment sampling strategies over dune beds.     

Monitoring Suspended Sediment Dynamics Using MBES

This technical note describes use of a multibeam echosounder (MBES) to quantify the dynamics of suspended sediment in a large open channel. A methodology is detailed that uses the backscatter magnitude from the MBES water-column data to adjust the magnitude of sonar returns for the various sonar settings, spatially and temporally average the data to account for the random nature of acoustic backscatter from the suspended sediment, and calibrate the processed data with direct samples. A case study of flow at the confluence of the Rio Paraná and Rio Paraguay, Argentina, where there is a distinct turbidity difference along the mixing interface of the two flows, is used to demonstrate the unique capabilities of MBES to quantify sediment concentrations and dynamics within the water column.     

Modeling Nonuniform Suspended Sediment Transport in Alluvial Rivers

Problems and difficulties in modeling sediment transport in alluvial rivers arise when one uses the theory of equilibrium transport of uniform sediment to simulate riverbed variation. A two-dimensional mathematical model for nonuniform suspended sediment transport is presented to simulate riverbed deformation. Through dividing sediment mixture into several size groups in which the sediment particles are thought to be uniform, the nonuniformity and the exchange between suspended sediment and bed material are considered. The change of concentration along the flow direction, size redistribution, and cross-sectional bed variation can then be described reasonably well by the model. In simulating the flow field with big dry-wet flats, moving boundary problems are solved very well by introducing a so-called finite-slot technique. Verification with laboratory data shows that the model has a good ability to simulate channel bed variations. Last, the model was applied to a real alluvial river system. Variables such as water level, sediment concentration, suspended sediment size distribution, and riverbed variation were obtained with encouraging results.     

Gravitational Settling Velocity Regimes for Heterodisperse Urban Drainage Particulate Matter

Gravitational settling regimes, most commonly discrete (Type I) or flocculent (Type II) can be hindered (Type III) in unmaintained urban drainage appurtenances. In coastal zones or in areas with deicing salt applications, high salinity may influence particulate matter (PM) settling. This study examines settling of PM with a heterodisperse particle size distribution (PSD); typical of urban runoff. For Type I settling at low concentration ( ～ 0.1?g/L), Newton’s and logarithmic matching models each predict measured settling velocity (Vs) for PM from 2–4,750?μm. For Type I Vs in runoff, PM diameter dominates the influence of PM density, fluid temperature and salinity parameter ranges. Integrating Type I Vs across a common heterodisperse PSD for regulatory testing, a physically validated computational fluid dynamics model of a baffled hydrodynamic separator (HS) illustrates the influence of Vs parameters as PM diameter?PM?density>temperature ≈ salinity. Event-based PM separation by a screened HS loaded by unsteady flows and heterodisperse PSDs is predicted by overflow rate, and integrating measured (or validated Newton’s Law) Vs, flow and PSD data. For Type II settling, Vs is a first-order function of PM concentration (1, 2, and 3 g/L); as well as time and settling depth. Type III Vs is modeled as a power law function of PM concentration. Across the PSD tested, salinity up to 30 ppt does not have a significant (at p = 0.05) role on Type I Vs. However, salinity is significant (at p = 0.05) in Type II settling; as well as for Type III settling below 120 g/L. For all regimes, PM concentration, PSD heterodispersivity, and time dominate salinity or temperature influences.     

In Situ Measurements of Settling Velocity near Baimao Shoal in Changjiang Estuary

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.     

Influence of Coherent Flow Structures on the Dynamics of Suspended Sediment Transport in Open-Channel Flow

The influence of suspended sediments on coherent flow structures has been studied by simultaneously measuring the longitudinal and vertical components of the instantaneous velocity vector and the instantaneous suspended particle concentration with an acoustic particle flux profiler. The measurements were carried out in clear water and in particle-laden open-channel flows. In both cases, they clearly show the predominance of ejection and sweep phases that are part of a burst cycle. The analysis further demonstrates the importance of the ejection and sweep phases in sediment resuspension and transport. Ejections pick up the sediment at the bed and carry it up through the water column close to the surface. It is shown that ejections and sweeps are in near equality in the near-bottom layer, whereas ejections clearly dominate in the remaining water column. The implications of these results for sediment transport dynamics are discussed.     

Investigation of Twin Vortices near the Interface in Turbulent Compound Open-Channel Flows Using DNS Data

The direct numerical simulation of turbulent flows in a compound open channel is described. Mean flows and turbulence structures are provided, and are compared with numerical and measured data available in the literature. The simulated results show that twin vortices are generated near the interface of the main channel and the floodplain and that their maximum magnitude is about 5% of the bulk streamwise velocity. Near the interface, the simulated wall shear stress reaches a maximum, contrary to experimental data. A quadrant analysis shows that both sweeps and ejections become the main contributor to the production of Reynolds shear stresses near the interface. Through the conditional quadrant analysis, it is demonstrated how the directional tendency of dominant coherent structures determines the production of Reynolds shear stress and the pattern of twin vortices near the interface. In addition, the time-dependent characteristics of three-dimensional vortical structures in a compound open-channel flow were investigated using direct numerical simulation (DNS) data.     

Deposition Properties of Fine Sediment

This paper describes depositional properties of the fine portion (D<0.075?mm) of the surficial bed sediments taken from the Upper St. Clair River. The experiments were conducted using the circular flume facility at the Canada Center for Inland Waters in Burlington, Canada. Tests were conducted under the same initial conditions, which involved mechanical mixing following the application of a very high shear stress that resulted in a solids suspension of 200 g/L. Deposition was allowed to occur under five different shear stresses. The deposition rate and the floc size were monitored using a laser based Malvern particle size analyzer. The results were used to estimate the settling velocity as a function of applied shear stress and lapsed time for the selected shear stress.     

Effects of Inlet Position and Baffle Configuration on Hydraulic Performance of Primary Settling Tanks

Circulation regions always exist in the settling tanks. These regions reduce the tank’s performance and decrease effective volume of the tank. Recirculation zones would also result in short-circuiting and high flow mixing problems. Inlet position would have an effect on these, too. Use of good baffle configuration may increase performance of settling tanks. One method for comparison performance of different tanks with each other is to use flow through curves (FTCs). In this note using FTCs, effects of inlet position and baffle configuration on the hydraulic performance of primary settling tanks are studied. The best position of baffle is also determined.     

Analysis of Suspended Sediment Transport in Open-Channel Flows: Kinetic-Model-Based Simulation

This paper presents a comprehensive analysis of suspended sediment transport in open channels under various flow conditions through a kinetic-model-based simulation. The kinetic model, accounting for both sediment-turbulence and sediment-sediment interactions, successfully represents experimentally observed diffusion and transport characteristics of suspended sediments with different densities and sizes. Without tuning any model coefficients, the nonmonotonic concentration distribution and the noticeable lag velocity with a negative value close to the wall are reasonably reproduced. Examination of flow conditions typical of suspension dominated rivers shows that the conventional method may overestimate or underestimate unit suspended-sediment discharge, depending on the Rouse number, sediment size, as well as shear velocity. The error may be less than 20% for dp<0.5?mm and might exceed 60% for dp>1.0?mm under typical flow conditions where shear velocity ranges from 1.0?to?12.5?cm/s and flow depth ranges from 1.0?to?5.0?m.     

Diagonal Cartesian Method for the Numerical Simulation of Flow and Suspended Sediment Transport over Complex Boundaries

There is increasing demand for simulation tools of flow and suspended sediment transport over complex boundaries in hydraulic engineering. The diagonal Cartesian method, which approximates complex boundaries using both Cartesian grid lines and diagonal lines segments, is presented in the paper to simulate the complex boundaries of two-dimensional shallow-water turbulence equations and nonequilibrium suspended sediment transport equation. The method, which utilizes cell-centered nodes on a nonstaggered grid, uses boundary velocity information at the wall boundary to avoid the specification of water level. An enlarged finite-difference method is introduced for momentum and suspended sediment equations on the complex boundary. This paper describes an application of the diagonal Cartesian method to calculate the tidal current and suspended sediment concentration of Quanzhou Bay in the Fujian province of China. The results show that the method predicts the flow and suspended sediment concentration well, and the calculations agree well with the measurement.     

Settling Characteristics of Calcareous Sand

This paper provides quantitative comparisons and relationships for the fall velocities and drag coefficients of 998 calcareous sand grains collected on Oahu, Hawaii. Laboratory analyses of the grains provide the three orthogonal dimensions, fall velocity, and density, from which the nominal diameter, drag coefficient, Reynolds number, and shape factor are evaluated for each grain. The fall velocity and drag coefficient expressed, respectively, as functions of nominal diameter and Reynolds number show strong correlation over a wide range of Corey shape factors. The present relationships are validated with published data based on quartz grains. Analysis of the data by flow regime shows that particle shape has a stronger influence on the settling characteristics when turbulent boundary layers develop around the grains.