Vertical Distribution Of Suspended Sediment Research Articles

Analytical Eddy Viscosity Model for Turbulent Wave Boundary Layers: Application to Suspended Sediment Concentrations over Wave Ripples

Turbulence related to flow oscillations near the seabed, in the wave bottom boundary layer (WBBL), is the phenomenon responsible for the suspension and transport of sediments. The vertical distribution of turbulent eddy viscosity within the WBBL is a key parameter that determines the vertical distribution of suspended sediments. For practical coastal engineering applications, the most used method to parameterize turbulence consists in specifying the shape of the one-dimensional-vertical (1DV) profile of eddy viscosity. Different empirical models have been proposed for the vertical variation of eddy viscosity in the WBBL. In this study, we consider the exponential-type profile, which was validated and calibrated by direct numerical simulation (DNS) and experimental data for turbulent channel and open-channel flows, respectively. This model is generalized to the WBBL, and the period-averaged eddy viscosity is calibrated by a two-equation baseline (BSL) k-ω model for different conditions. This model, together with a β-function (where β is the inverse of the turbulent Schmidt number), is used in modeling suspended sediment concentration (SSC) profiles over wave ripples, where field and laboratory measurements of SSC show two kinds of concentration profiles depending on grain particles size. Our study shows that the convection–diffusion equation, for SSC in WBBLs over sand ripples with an upward convection term, reverts to the classical advection–diffusion equation (ADE) with an “apparent” sediment diffusivity εs*=α εs related to the sediment diffusivity εs by an additional parameter α associated with the convective sediment entrainment process over sand ripples, which is defined by two equations. In the first, α depends on the relative importance of upward convection related to coherent vortex shedding and downward settling of sediments. When the convective transfer is very small, above low-steepness ripples, α≈1. In the second, α depends on the relative importance of coherent vortex shedding and random turbulence. When random turbulence is more important than coherent vortex shedding, α≈1, and “apparent” sediment diffusivity reverts to the classical sediment diffusivity εs*≈ εs. Comparisons with experimental data show that the proposed method allows a good description of both SSC for fine and coarse sand and “apparent” sediment diffusivity εs*.

Non-Fickian transport models for characterising the sediment suspension in unsteady flows

Non-Fickian transport of suspended sediment has been observed at field and laboratory scales. Such as turbulent bursting, resulting in complex dynamics for the sediment particles movement. The erosion and deposition of sediment have an impact on the hydraulic engineering and environment. This study makes an attempt to develop the variable-order fractional advection-diffusion equation (VOFADE) and variable-order Hausdorff fractal derivative advection-diffusion equation (VOHADE) models to describe the vertical distribution of suspended sediment in unsteady turbulent flows. From a classical viewpoint, the distribution of the concentration in sediment-laden flows is determined based on Fick?s first law. However, the vertical diffusion of suspended particles exhibits the non-locality/space scale dependency and history memory/time scale dependency properties due to turbulent bursting. Moreover, previous literatures have indicated that turbulence structure changes with the water depth. Hereby, we employ the space-dependent VOFADE and VOHADE models to describe the vertical diffusion of suspended sediment in unsteady flows, and further test its applicability with the experimental data. Numerical simulation results confirm that the VOFADE and VOHADE models give a better agreement with the experimental data and can well characterise the space-dependent anomalous transport. Hence, the models proposed by this study may help to provide a powerful mathematical physical model in the quantification of suspended sediment transport.

Formation of Oil-Particle-Aggregates: Numerical Model Formulation and Calibration

When oil spills occur in turbid waters, the oil droplets and mineral grains can combine to form oil-particle aggregates (OPAs). The formation of OPAs impacts the vertical transport of both the oil and the mineral grains; especially increasing deposition of oil to the seabed. Though the coastal oceans can be very turbid, to date, few numerical ocean models have accounted for aggregation processes that form OPAs. However, interactions between oil and mineral aggregates may be represented using techniques developed to account for sediment aggregation. As part of Consortium for Simulation of Oil Microbial Interactions in the Ocean (CSOMIO), we modified an existing, population dynamics-based sediment flocculation model to develop OPAMOD, a module that accounts for the formation of OPAs. A zero-dimensional model using OPAMOD is shown to be capable of reproducing the size distribution of aggregates from existing laboratory experimental results. Also using the zero-dimensional model, sensitivity tests were performed on two model parameters, the fractal dimension and collision efficiency. Results showed that fractal dimension played a role in the OPA size distribution by influencing the effective particle density, which modified the number concentration of flocs for a given mass concentration. However, the modeled particle characteristics and oil sequestration were relatively insensitive to collision efficiency. To explore OPA formation for an outer continental shelf site, two simulations were conducted using a one-dimensional (vertical) implementation of the model. One scenario had high sediment concentration near the seabed to mimic storm-induced resuspension. The other scenario represented river plume sediment delivery by having high sediment concentration in surface waters. Results showed that OPA formation was sensitive to the vertical distribution of suspended sediment, with the river plume scenario creating more OPA, and sequestering more oil within OPA than the storm resuspension scenario. OPAMOD was developed within the Coupled Ocean-Atmosphere-Wave-and-Sediment Transport (COAWST) modeling system, therefore the methods and parameterizations from this study are transferrable to a three-dimensional coupled oil-sediment-microbial model developed by CSOMIO within the COAWST framework.

Field observations of turbulence, sediment suspension, and transport under breaking tidal bores

Turbulence dynamics and sediment suspension, under intense tidal bore conditions are investigated using field observations from the Qiantang Estuary, China. During the passage of the bore the tidal current bottom generated turbulence is augmented by turbulence injection from the surface as part of the breaking processes of both bore front and the bore associated secondary waves. These processes lead to enhanced vertical mixing and vertical distribution of the advected sediments. The latter contributes to increased sediment transport in the direction of the bore propagation that potentially modifies the normal tidally-driven estuarine sediment dynamics and morphology. The results show that turbulence intensity increases with increased tidal currents and oscillating motions associated with the breaking processes. At the early stage of the tidal bore, the vertical mixing of suspended sediment is more convective than diffusive due to the elevated length scales associated with bore and breaking secondary wave generated turbulence. Turbulent momentum and vertical sediment flux are determined by intermittent, large magnitude turbulent events (i.e., turbulent bursting). At early period of the tidal bore (~ 40s), inward and outward interactions contribute most to the Reynolds stress and vertical sediment flux, in contrast to later periods when sweep and ejection become more important. Negative vertical sediment flux (i.e., toward the bed) is observed, indicating that the sediment buoyancy acts as a source of turbulent kinetic energy, possibly due to the modification of vertical distribution of suspended sediment by the bore. These observations, in flow conditions under intense tidal bore breaking, reveal different sediment dynamics from traditional boundary process by injecting breaking induced turbulence which modifies the distribution and transport of the suspended sediment.

The vertical distribution of suspended sediment and phosphorus in a channel with ice cover.

Ice cover is a common channel phenomenon in some cold regions. But most of the researchers focused on the velocity distribution; the suspended sediment and phosphorus distribution in an ice-covered channel need more attention. Six different discharge treatments were carried out in this paper; and vertical velocity, suspended sediment, and phosphorus concentration were measured. The results showed, based on the convection-diffusion equation of sediment, the analytical solution of sediment vertical distribution given, and the coefficient of determination (R2) between the measured data and predicted data ranging from 0.9519 to 0.9777 and the root mean square error (RMSE) between the measured data and predicted data ranging from 0.2959 to 0.5892, that the analytical solution has a good simulation result. Particulate phosphorus concentration (Cp) is smaller in the top layer and larger in the bottom layer, and the dissolved phosphorus concentration (Cd) is larger in the middle and small in both sides; partition coefficient (Kd) is uniform in the channel which is assumed as a constant in previous studies. This result is helpful to know the retention law of suspended sediment and phosphorus in an ice-covered channel.

Vertical Distribution of Suspended Sediments above Dense Plants in Water Flow

Plants in natural water flow can improve water quality by adhering and absorbing the fine suspended sediments. Dense plants usually form an additional permeable bottom boundary for the water flow over it. In the flow layer above dense plants, the flow velocity generally presents a zero-plane-displacement and roughness-height double modified semi-logarithmic profile. In addition, the second order shear turbulent moment (or the Reynolds stress) are different from that found in non-vegetated flow. As a result, the turbulent momentum diffusivity of flow and thus the diffusivity of sediment will shift, which will cause the vertical profile of suspended sediment and the corresponding Rouse formula deform. A set of physical experiments with three different diameters of fine suspended sediments was conducted in an indoor water flume. These experiments investigated a new distribution pattern of suspended sediment and the correspondingly deformed Rouse formula in the flow layer over the dense plants. Experimental results showed that above the dense plants, the shear turbulent momentum of flow presented a plant-height modified negative linear profile, which has been proposed by a previous study, and the vertical distribution of fine suspended sediments presented an equilibrium pattern. Based on the plant-modified profiles of flow velocity and the shear turbulent momentum a new zero-plane and plant-height double modified Rouse formula were analytically derived. This double-parameter modified Rouse formula agrees well with the measured profile of suspended sediment concentration experimentally observed in the present study. By adjusting the Prandtl–Schmidt number, i.e., the ratio of sediment diffusivity to flow diffusivity, the double-parameter modified Rouse formula can be applied to submerged dense plant occupied flow.

Dynamics of sediment transport and stratification in Changjiang River Estuary, China

Suspended sediment transport and stratification in the entire Changjiang River Estuary were examined using the field data of sea-surface level, currents, salinity, and suspended sediment concentration (SSC). The data were simultaneously observed at one-hour intervals and at six vertical layers at 23 stations covering two spring tidal cycles in May, 2004. The results showed that ebbing tides had a longer tidal duration and stronger tidal currents than flooding tides, due to a fresh water discharge. The tidal range reached about 4 m and the peak tidal current of about 2.9 m/s occurred in the north passage in the observed period. A stratification caused by salinity and suspended sediment appeared and varied with tides indicated by the overall Richardson number. The potential energy anomaly was larger when SSC was factored in to the water density. A stratification ratio (Sr), which is the ratio of potential energy required to induce complete mixing to the overall potential energy, was proposed to measure the degree of sediment stratification. Sr values indicated that mixing the stratified turbid water took as much as 0.25% of the overall potential energy at a particular field station. A turbidity maximum zone (TMZ) was observed in the north passage, where the salt-fresh-water interface and stratification appeared, due to the advective and tidal transport component. The main components controlling the vertical-averaged net sediment transport were the Eulerian velocity, Stokes drift and tidal pumping. Salinity intrusion creates the stratification in the north passage, which depresses the turbulence and re-suspension of sediment. The stratification is further enhanced by the vertical distribution of suspended sediment, which results in a TMZ and induces sediment deposition/trapping in the channel. Hence, the SSC was essential to be considered in the study of estuarine stratification, particularly in the highly turbid estuaries.

The effects of saltwater intrusion on suspended sediment movement in estuary

A three dimensional model of simplified riverine estuary was established with numerical simulation to investigate the distribution of estuarine salinity and suspended sediment under different flow rates. The spatial distribution of suspended sediment and deposition in the simulation were inspected.(1)There are two types of vertical distribution of suspended sediment, one increasing with water depth all the way and the other beginning with increasing and then followed by decreasing.(2)The head of salt wedge where the turbidity maxima is formed contributes to a large number of sediment deposition on bed;(3)With the increase of flow rates, saltwater intrusion is weakened, the corresponding turbidity maximum zone and the location of rapid sedimentation both moved toward the sea.(4)The estuarine circulation dominates the major processes of the turbidity and deposition at the salt front.

Fractal derivative model for the transport of the suspended sediment in unsteady flows

This paper makes an attempt to develop a Hausdorff fractal derivative model for describing the vertical distribution of suspended sediment in unsteady flow. The index of Hausdorff fractal derivative depends on the spatial location and the temporal moment in sediment transport. We also derive the approximate solution of the Hausdorff fractal derivative advection-dispersion equation model for the suspended sediment concentration distribution, to simulate the dynamics procedure of suspended concentration. Numerical simulation results verify that the Hausdorff fractal derivative model provides a good agreement with the experimental data, which implies that the Hausdorff fractal derivative model can serve as a candidate to describe the vertical distribution of suspended sediment concentration in unsteady flow.

Combining echo and hydrodynamic measurements for estimating non uniform sand transport under waves and currents

We present recently obtained measurements in the Atlantic coast of Uruguay, with a 1 MHz ADCP (Sentinel V20, Teledyne RDI, USA). The ADCP was deployed at 16 m depth for 4 months using a light weight structure deployed from a small boat. During the deployment, 3.6 m significant high, 12 s peak period waves, and 0.8 m/s currents were recorded. The vertical distribution of suspended sediment in this environment is controlled by the turbulence level induced by both currents and waves. Assuming a representative sediment size and adapting the Rouse-Vanoni profile showed to give results that did not agree with the concentration profiles inverted from the recorded echo profiles. Therefore, a method that combined variations of the sediment size distribution with the turbulence level was developed showing very good agreement with the information from the echo profile. In the developed method, the turbulent sediment diffusivity was computed from the mean current and wave measurements. However, this showed limitations for conditions with strong waves and weak currents. As an alternative, direct measurements of the turbulent fluctuations from the instantaneous beam velocity recordings were used to compute the turbulent diffusivity. In addition, different turbulent diffusivity distributions for combined current-wave conditions were evaluated.

Empirical model for estimating vertical concentration profiles of re-suspended, sediment-associated contaminants

Vertical distribution processes of sediment contaminants in water were studied by flume experiments. Experimental results show that settling velocity of sediment particles and turbulence characteristics are the major hydrodynamic factors impacting distribution of pollutants, especially near the bottom where particle diameter is similar in size to vortex structure. Sediment distribution was uniform along the distance, while contaminant distribution slightly lagged behind the sediment. The smaller the initial sediment concentration was, the more time it took to achieve a uniform concentration distribution for suspended sediment. A contaminants transportation equation was established depending on mass conservation equations. Two mathematical estimation models of pollutant distribution in the overlying water considering adsorption and desorption were devised based on vertical distribution of suspended sediment: equilibrium partition model and dynamic micro-diffusion model. The ratio of time scale between the sediment movement and sorption can be used as the index of the models. When this ratio was large, the equilibrium assumption was reasonable, but when it was small, it might require dynamic micro-diffusion model.

Hydrodynamic interaction in suspended sediment distribution of open channel turbulent flow

In the present study, a theoretical model is proposed to predict the vertical distribution of suspended sediment in open channel turbulent flow. We derive our mathematical model based on the six prominent hydrodynamic mechanisms, which are upward sediment flux due to the turbulent diffusion, downward gravitational settling of the sediment, hindered settling phenomenon, secondary current, fluid induced lift force on the suspended particles and the gradient of Reynolds normal stress of the suspended sediment. The importance of the hydrodynamics is described by the changes of suspended sediment concentration profile in terms of the particle-flow interaction caused by those mechanisms. We also address the significance of the co-existence of those mechanisms for estimating the suspended sediment concentration profile. The present model agrees satisfactorily with a wide spectrum of experimental data in available literature. Unlike the previous researchers, we select a broad range of previously published models of vertical sediment concentration distribution for comparison analysis, and the proposed model shows better prediction accuracy which is confirmed by error analysis.

One-Dimensional Modeling of Suspended Sediment Dynamics in Dam Reservoirs

AbstractContinuous sedimentation of fine sediments in dam reservoirs triggers considerable economic and ecological impacts worldwide. The definition of appropriate reservoir management procedures requires accurate tools for predicting the sediment dynamics both in the reservoir and downstream reaches. In this article, a one-dimensional numerical model aiming to reproduce suspended sediment dynamics is presented. To take into account water intake locations and vertical distribution of suspended sediments in areas of larger water depths close to dams, an additional module is developed. The model is applied to a reach of the French Upper Rhone River, including two reservoirs. The calibrated model provides accurate flux estimates and reproduces erosion and deposition patterns observed in the reservoir for several scenarios. The additional module allows to improve the estimation of the sand load released by low-level outlets. Eventually, the model could be used either to provide insights on the outcome of past...

English

A 1DH modeling is implemented to study a particular case of transport in suspension along a channel corresponding to a 2DV problem. In order to include the vertical dimension, a model named α-β is used. This latter was developed to describe the vertical distribution of suspended sediment in a flow corresponding to an unsteady and/or non-uniform state, and including deposition and resuspension phenomena (SANCHEZ, 2013). Results show that in some regions of the studied domain, the α-β model describing the vertical distribution of suspended sediment, can be simplified because its two parameters (α and β) remain constant.

Controls on suspended sediment concentration profiles in the shallow and turbid Yangtze Estuary

The vertical distribution of suspended sediment in continental shelf waters has significant implications for water quality, aquatic ecology, and sediment transport. Nevertheless, there have been relatively few systematic field studies aimed at determining whether a general vertical trend in suspended sediment concentration (SSC) exists, or how any such vertical trend can be expressed mathematically. In this study, based on 402 individual SSC profiles measured hourly during 16 dual-tide voyages at 8 stations in the outer Yangtze Estuary, we found that SSC followed an average depth profile that was smoothly concave-up. This average profile, and the relationship between SSC and height above the seabed, was exponential (R2>0.99). In comparison, the traditional Rouse equation based on hydrodynamics describes the measured average SSC profile poorly, in that the greater the distance from the near-bed height (where the reference SSC is taken), the greater the deviation of the simulated SSC from the measured SSC. However, in this study, a new approach was developed to overcome this flaw in the Rouse equation, which divides the equation into two parts. One part uses a reference SSC from a near-bed level, and an upwards decreasing coefficient, in the same way as the conventional Rouse equation. Conversely, the other part uses a reference SSC from at the water surface, and a downwards decreasing coefficient. Our modified hydrodynamics-based equation expresses the measured SSC well, with an R2 between the simulated and measured SSCs exceeding 0.99. According to the average SSC profiles, the near-bed SSC was 2.8 times greater than the SSC at the water surface (close to the value of 3 suggested for modeling studies in the absence of empirical data), and the depth-averaged SSC was 1.8 times greater than the SSC at the water surface. We also found that the average SSC profile showed a uniform (from water surface to seabed) decline in SSC (25%) in response to the dramatic decline in the suspended sediment load supplied by the Yangtze River over the past three decades. However, the shape and vertical ratio of the average SSC profile have changed little, which suggests that the general form of the SSC profile is determined mainly by local hydrodynamics and sediment properties, and not by fluvial sediment supply.

Calculation Method about Vertical Distribution of Saturated Sediment Concentration Based on Exchange Equilibrium

Diffusion theory is the leading one which is used to study the vertical distribution of sediment concentration. And diffusion coefficient is a key parameter to determine the vertical distribution of suspended sediment. First of all, the calculation methods are introduced based on the momentum transfer theory and fluctuating velocity. According to the sediment equation of exchange equilibrium in vertical, the new expression is obtained for sediment diffusion coefficient and the vertical distribution of sediment concentration. By the flume experimental data and field data in natural river, the difference is analyzed among the different expressions.

Experimental study on suspended sediment concentration and its vertical distribution under spilling breaking wave actions in silty coast

In this paper, flume experiments are focused on sediment transport inside and outside the surf zone. According to the energy dissipation balance principle of sediment-laden flow and the similarity between energy dissipation of spilling breaking wave and hydraulic jump, formulas are proposed to predict time averaged suspended sediment concentration under both non-breaking and breaking waves. Assuming that the sediment diffusion coefficient, which is related with energy dissipation, is proportional to water depth, formulas are proposed to predict close-to-bed suspended sediment concentration and vertical distribution of suspended sediment under spilling breaking waves, and the prediction shows a good agreement with the measurement.

A model of the vertical distribution of suspended sediment in the bottom layer of a natural water body

The paper suggests a stationary model of the vertical distribution of the concentration of suspended sediment in the bottom layer of a natural water body with a flat bottom. The model explains the concentration distribution, formed jointly by the settling of suspended particles and turbulent diffusion. The flow is assumed geostrophic above the bottom-influenced layer, while in the bottom layer the effect of turbulent diffusion by the large-scale turbulence constituent is assumed to dominate over the diffusion effect caused by the small-scale turbulence constituent. It is shown that for the characteristic diffusion length scale of the eddies much smaller than the height of the Ekman bottom boundary layer the model results in an analytic expression for the vertical distribution of the concentration of suspended sediment, which includes also the case with the presence of the lutocline. The model outcome is compared with the results of a laboratory experiment with sand-injected flume flow.

Effects of some parameters on numerical simulation of coastal bed morphology

PurposeThe aim is to investigate in detail the sensitivity of sediment transport and bed morphology with respect to some parameters including bed slope, non‐hydrostatic pressure term, sand grain size, temperature, salinity and lower boundary conditions for suspended sand concentration on a regional scale through numerical simulations based on a mathematical model.Design/methodology/approachThe numerical model consists of a 3D hydrodynamic code amended by a sediment transport module. At the same time, the influence of wave action has been taken into account. The model is applied to the Sylt‐Romo tidal bay covering approximately 20 × 30 km2 spanned by about 2.7 × 106 active grid points with the constant wind and wave fields.FindingsThe computed results of seven different cases over 150 h show that the effect of bed slope correction is very strong, especially in case of largely changeable bathymetry and depends on the horizontal grid resolution. Sand grain size strongly influences the vertical distribution of suspended sediment and then sedimentation. The impact of sea water temperature is relatively clear despite being less powerful than two former parameters. Non‐hydrostatic pressure perturbations of the flow field and the kind of the lower boundary condition as well as salinity are negligible allowing for considerable savings of CPU time when the numerical simulation is carried out for a large area and for a very long‐time period.Originality/valueThe results of the study demonstrate that the geometrical factor of coastal bed and the range of sand particle size on the bottom contribute to the tendency of bed evolution in some measure. Additionally, the increase of temperature of sea water due to global warming may also make a considerable change to the mechanism of sediment transport and sedimentation in future. Therefore, the human intervention in the process of natural evolution is possible through the behaviour to the nature. At the same time, this is also interesting and useful information and it can consolidate the idea for coastal engineering projects.

Quantitative simulative method of sediment resuspension in Lake Taihu

由于风浪对浅水湖泊底泥的强烈扰动作用, 底泥再悬浮的生态效应研究成为目前国际研究热点之一。但方法的缺乏极大地限制了有关底泥再悬浮环境效应的深入研究。本研究应用一种新的沉积物再悬浮装置, 模拟了太湖南部长兜港水域不同风浪影响下悬浮物的垂向分布, 获得水柱总悬浮物量(T)与扰动频率(n)的定量关系:T=19.77×exp(n/2.61)-71.7。结合现场实测风情下的水柱总悬浮物量, 建立了室内水动力条件、水柱总悬浮物量和现场风情的定量关系。研究2004年10月至2005年10月太湖全年代表性小风、中风及大风与再悬浮装置的扰动频率对应关系, 估算了小风、中风和大风常规风情引起的总悬浮物量分别为159、230、425g/m²。结果表明, 模拟获得的太湖悬浮物垂向分布规律与现场状况较为接近, 是目前较为适用的浅水水体沉积物再悬浮模拟方法。;Concerns on ecological effects of sediment resuspension are on the rise because the sediments in shallow lakes are frequently disturbed by winds. However, few methods could be suitable for supporting its related study. In this paper, Y-shape apparatus for simulating sediment resuspension was used to study vertical distribution of suspended sediments in water column and their total suspended amounts driving by different wind speed in Chang-dougang Harbor, southern Lake Taihu. The results showed that the relationship between the total suspended amount of sediments in water column (T) and the motor rotation frequency(n) could be expressed byT=19.77 exp(n/2.61)-71.7. Based on the total suspended amounts in water column under field wind conditions, quantitative relationships were achieved among the simulated hydrodynamic conditions, the total suspended amounts of sediments and the field wind conditions. According to the correspondent frequency of motor rotation to field wind speed in Lake Taihu from October 2004 to October 2005, the total suspended amounts of sediments were estimated at 159, 230 and 425 g/m² for field light, moderate and strong winds, respectively. The estimated amounts, together with the vertical distribution of suspended sediments in water column, were consistent with those observed under field conditions. The Y-shape apparatus was thus recommended for use in studying the ecological effects of sediment resuspension in shallow aqueous system.