Vertical Profiles Of Suspended Sediment Concentration Research Articles

Vertical Distributions of Suspended Sediment Concentrations in the Turbidity Maximum Zone of the Periodically and Partially Stratified Changjiang Estuary

The vertical distribution of suspended sediment concentration (SSC) plays a key role in the residual suspended sediment transport in the estuarine environments. A field measurement was carried out in the turbidity maximum zone (TMZ) of the Changjiang Estuary in 2012 to study the characteristics and formation mechanisms of SSC profile in the partially stratified estuary. Based on the observed data, we found that the vertical distribution of SSCs is mainly determined by the density stratification caused by the interactions between the river runoff and the saltwater intrusion. The vertical profiles of SSC were categorized into three types: (I) two-layer structure profile, (II) exponential profile, and (III) linear profile. Type I mainly occurred in condition of strong stratification, whereas types II and III mainly occurred in weak stratification and well-mixed conditions, respectively. We developed two new empirical equations to simulate the vertical profiles, which can accurately describe the observed exponential and linear profiles and reasonably/accurately describe the observed two-layer structure profiles. The two new equations can be easily used in other well-mixed or partially stratified estuaries given the surface and bottom SSC data, which can be obtained from many ways, for instance, the buoy and bottom tripod survey system, the remote-sensed images, and numerical models.

Numerical investigation of the factors influencing the vertical profiles of current, salinity, and SSC within a turbidity maximum zone

The flow-sediment interaction plays a considerable role on the vertical (internal) profiles of current, salinity and suspended sediment concentration (SSC) within a turbidity maximum zone (TMZ). Numerical modeling provides valuable insights into the complex estuarine physical processes. By combining numerical modeling with field observations, the influencing factors of fine sediment dynamics within the TMZ of Yangtze Estuary have been explored in this study. Firstly, during the neap tide, the measured data present that the current is too weak to break the density stratification, and the vertical flow structure is effectively altered. Secondly, a three-dimensional numerical model based on the Delft3D has been developed and a range of numerical sensitivity analyses were carried out to distinguish the dominant mechanisms and physical processes responsible for the phenomena observed from the measurement data. The numerical investigation highlights the following findings. (1) The vertical profile of currents within the TMZ is largely affected by saltwater intrusion, especially during lower currents when the baroclinic pressure gradient can significantly reshape the local vertical profiles of velocity. (2) The baroclinic effects are primarily determined by the stratification of salinity. (3) In addition to salinity, SSC also influences the local density stratification when its contribution to fluid density is comparable to that of salinity. (4) The settling velocity determines the overall sediment distribution and vertical profiles of the SSC in the water column. The SSC-dependent settling velocity (including the flocculation-induced acceleration and hindered settling deceleration phases) affects the longitudinal movement of the sediments. (5) The vertical profiles of current, salinity and SSC within the TMZ are highly associated with the turbulence determined by the model. The approach to modulate the vertical eddy viscosity in the model, based on the empirical dependency between Rig and Prt, may lead to a numerical instability in the stratified flow. In order to improve the stratification of SSC, additional turbulence damping effect is suggested to be implemented in the model.

Suspended Sediment Concentration Profiles From Synoptic Satellite Observations

A method is developed to estimate vertical suspended sediment concentration (SSC) profiles in Gulf of Kachchh, from the sediment concentration values derived from synoptic observations of Ocean Colour Monitor (OCM). Under the influence of currents, vertical SSC profiles are defined by power law being based on the frictional velocities and sediment settling velocities. For each water pixel in OCM, the vertical variations in SSC are determined from the SSC profiles calculated from power law using corresponding simulated frictional velocities and sediment size parameters. OCM derived SSC is considered to be summation of sediment concentration values from surface to first attenuation depth (hereafter OCM-depth) with an exponential decay of values with depth. Vertical variation profiles from surface to OCM-depth are scaled with respect to the corresponding OCM SSC values and the projected concentrations at the bottom are derived. The bottom concentrations derived is observed to vary chiefly with the tidal conditions prevailing within the gulf, when during the slack hours increase in bottom SSC values get enhanced owing to the settling of sediments from suspension.

Tide-induced vertical suspended sediment concentration profiles: phase lag and amplitude attenuation

In tidal environments, the response of suspended sediment concentration (SSC) to the current velocity is not instantaneous, the SSC lagging behind the velocity (phase lag), and the amplitude of SSC variation decreasing with height above the bed (amplitude attenuation). In order to quantitatively describe this phenomenon, a one-dimensional vertical advection–diffusion equation of SSC is derived analytically for uniform unsteady tidal flow by defining a concentration boundary condition using a constant vertical eddy diffusivity and sediment settling velocity. The solution, in simple and straightforward terms, shows that the vertical phase lag increases linearly with the height above the bed, while the amplitude of the SSC variation decreases exponentially with the height. The relationship between the SSC and the normalized current velocity can be represented by an ellipse or a line, depending on the phase lag. The lag of sediment movement or “diffusion/settling lag” is the mechanism generating the phase lag effect. Field observations used for validation show that the theoretically predicted and the observed curves of the vertical SSC phase lag and amplitude attenuation show reasonable agreement. The procedure proposed in this paper substantially simplifies the modeling of suspended matter transport in tidal flows.

Measuring and modelling longshore sediment transport

Field measurements of longshore sediment transport (LST) was undertaken on barred and non-barred beaches composed of fine, medium and coarse sands in Brazil, Denmark and Portugal. Measurements and predictions of vertical suspended sediment concentration profiles ( C-Profiles) and cross-shore hydrodynamic parameters were then combined in a new semi-empirical model for prediction of LST ( LT-MOD). Instantaneous LST predictions from LT-MOD and well-known bulk LST formulae were compared. Tests using LT-MOD to simulate measured changes in shoreline position in southern Brazil for periods of c. two years showed that LT-MOD gave more accurate predictions than existing bulk LST formulae. Results indicate that LT-MOD may have practical utility at sites where access to equipment is limited and where reliable estimates of LST are required over extended periods.

Comparing Calculated and Observed Vertical Suspended-Sediment Distributions from a Hudson River Estuary Turbidity Maximum

Suspended-sediment concentrations calculated using a vertical suspended-sediment distribution equation were compared to observations from a field study of the lower turbidity maximum of the Hudson River Estuary. At four stations, an instrumented tripod measured vertical profiles of suspended-sediment concentration, current velocity, salinity and temperature through a tidal cycle. Bed and suspended-sediment samples were also analysed to determine inorganic sediment size distributions. Velocities were as high as 1·3ms−1, with suspended-sediment concentrations up to 2000mgl−1. When a well-defined pycnocline existed, cross-isopycnal mixing was strongly damped (based on the gradient Richardson number). Suspended-sediment profiles were calculated with a stratification-modified Rouse equation, using (1) reference concentrations measured at 20cm above the bed, (2) estimates of shear velocity based on the quadratic stress law, and (3) a constant sediment settling velocity of 0·22cms−1. Differences between mean calculated and observed total suspended load for each station were −17, 7, 14 and 58%, respectively. An uncertainty analysis revealed that the two parameterizations most likely to account for differences of this magnitude were those used for settling velocity and stratification. Best results were found when substituting a power law relationship for settling velocity based on suspended-sediment concentration. This demonstrates the improvement which a power law formulation can provide over the commonly used constant wsparameterization in fine sediment environments.

Modelling Sand Resuspension by Waves over a Rippled Bed

With a two-dimensional k-ϵ model and equations to describe sediment entrainment, diffusion, convection and settling, wave-induced flow velocities and sediment concentration fields over rippled beds have been calculated. Results from the simulations are compared with time-averaged vertical suspended sediment concentration profiles, C-profiles, measured in a large wave flume using the autonomous multi-sensor instrument STABLE. The simulations make it possible to examine in detail the mechanisms of sediment entrainment, vortex formation and advection. The resulting animations of model output allow a quasi-3-D visualization of sediment resuspension processes and aid quantification of the resulting vertical suspended sediment concentration field due to regular waves, asymmetric waves and wave groups. The work contributes therefore, to knowledge about wave-sediment interactions and to understanding of sediment transport mechanics.

Suspended Sediment Concentration Profiles in Wave-Current Flows

Improved expressions to predict time-averaged and horizontally averaged vertical suspended sediment concentration profiles (C¯ profiles) in wave and current (w-c) conditions are required to advance our present ability to predict rates of noncohesive sediment transport in the marine environment. In the present paper, a new expression for predicting C¯ profiles has been calibrated and tested using measurements of waves, currents, and C¯ profiles obtained in a laboratory basin for a broad range of w-c conditions. Good agreement between observed and predicted C¯ profiles is demonstrated.

Field observations and predictions of bed shear stressesand vertical suspended sediment concentration profiles in wave-current conditions

During a study of sediment dynamics at an offshore field site adjacent to Middlekerke Bank, Belgium, high-frequency measurements of turbulence and vertical profiles of the time-averaged suspended sediment concentration, C̄, were obtained in the bottom 1.2 m of the water column above a rippled bed in a water depth of approximately 20 m using the autonomous multisensor instrument STABLE. During the experiment, a combination of large waves and strong currents resulted in the resuspension and transport of bottom sediments. Values for the physical roughness of the sea bed, k s , have been derived. Estimates of the bed shear stress attributable to currents in the presence of waves, τ ̄ c(tke) , and the peak wave-only bed shear stress, τ ̂ w , have been obtained using the turbulent kinetic energy ( tke) method and linear wave theory, respectively, and have been combined to obtain peak, τ ̂ wc , and time-averaged, τ ̄ wc , wave-current ( w– c) bed shear stress values for gain- and ripple-scale roughness using existing models. A new semi-empirical expression giving accurate prediction of measured vertical C̄ profiles for a wide range of w– c conditions has been derived. Using τ ̂ w , τ ̄ wc and k s values as input parameters to the expression, estimates of the dynamic in situ grain settling velocity, wave mixing coefficient and total diffusive bed shear stress that agree well with previous measurements and with theory have been obtained. Results indicate it may now be possible to predict vertical C̄ profiles, and hence suspended sediment transport rates, with knowledge of flow turbulence, wave orbital motion and C̄ measured accurately at only one location near the bed.

Modeling Mixed Sediment Suspended Load Profiles

Potential errors in calculated vertical suspended sediment concentration profiles in river flows carrying mixed sediments are investigated. These profiles are usually obtained through application of a standard ‘diffusion‐dispersion’ model; however, such models are normally formulated on the basis of a unique grain size for the sediment. Accordingly, in the case of sediment mixtures, the grain size distribution is usually divided into several size ranges (‘fractions’), which are then characterized by their respective geometric means. Numerical errors induced through this procedure are quantified in a parametric study. While depthaveraged concentrations can be over‐esimated by a factor of 1.2 when the concentration gradient is weak, they can be under‐estimated by a factor of 15 when the gradient is large. Estimates are found to be particularly sensitive when the chosen reference level is small and the fraction width is large. Fall velocity coefficients are introduced to correct the discrepancy between the a...