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The rheological properties of fluid mud, a gelling suspension of predominantly colloidal clay particles in water, are usually obtained with a vane spindle for its presumed advantage of avoiding slip with the smooth surface of a standard cylindrical spindle. The vane configuration is presumed to be equivalent to a cylinder configuration by entrapping mud between its blades when it rotates. However, recent studies show that non-axisymmetric flow patterns are observed in vane configurations which make this assumption questionable. Therefore, the rheological study of fluid mud is needed to be re-assessed with the latest developments. In the present study, an in-house created serrated spindle, which approximates an infinite number of blades, is investigated to study possible slip and axisymmetric flow compared to vane and cylinder configurations. A wide-gap configuration is chosen since the estimated mean particle size of 10 μ m (of the finest clay aggregates) is quite coarse, while even in a standard narrow-gap configuration one cannot avoid that the yield surface is located within the gap (certainly at low rotation speeds). Therefore, the data are processed with the iterative method of Toorman (1994) valid for both configurations. Moreover, it allows a more practical handling of mud samples. The experimental results show a good reproducibility of the equilibrium flow curve and avoidance of the slip effect for the serrated configuration. In addition, numerical simulation results confirm that the flow of fluid mud with the vane configuration is not circular and deviates by 10%–20%. On the other hand, the flow of fluid mud with a serrated configuration is circular and deviates by less than 5%, matching the flow of fluid mud in the cylinder configuration. • Novel serrated spindle configuration for accurate rheological analysis of fluid mud. • Comparison of fluid mud flow properties in vane, cylinder and serrated configuration. • Investigation of circular flow with vane configuration using CFD-OpenFOAM. [ABSTRACT FROM AUTHOR]

• New multiphase model for sediment transport in free surface flows was developed. • μ(I) rheology was successfully employed to model the particle-particle interaction. • The newly developed model was validated and successfully used to model sediment transport in free surface flow including waves' condition. • k-ε turbulence model requires significant improvements in order to be suitable for multiphase flow simulations. Modeling of sediment transport in estuaries and coastal areas requires a lot of compromises to keep the computational costs within acceptable limits. Due to that, existing sediment transport models do not account for particle-scale physics, e.g. particle-particle interaction and turbulence modulation by sediment, which play a significant role, especially in the non-dilute regime. In the current study, a newly developed physics-based sediment transport model for free surface flows and its numerical implementation within the OpenFOAM framework is introduced. The new model is based on the multiphase mixture theory to account for interactions between sediment and water while tracking the free surface at the same time. A modified VOF equation for sediment-laden free surface flow was derived and implemented. The interphase momentum transfer is considered by solving an additional closure for the slip velocity which includes the effects of drag force, turbulent dispersion, and shear-induced diffusion. Dense granular flow rheology is used to supply the required closures for particle stresses. Additionally, suitable closures for the mixture and turbulent viscosities are introduced. The model was validated using experimental data and analytical solutions of five test cases of variable complexity. This includes pure sedimentation, laminar bedload transport, turbulent sheet flow, local scour due to a submerged jet, and wave-induced scour under a submarine pipeline. [ABSTRACT FROM AUTHOR]

Abstract The Floc Size Distributions (FSDs) of suspended fine-grained sediment flocs play a prime role to estimate their own fate and the transport of contaminates attached to the flocs. However, developing an efficient flocculation model that is capable of simulating continuous and multimodal FSDs is still a challenge. Recently, the population balance equation solved by the Quadrature-Based Method of Moments (QBMM) with lognormal kernel density functions has been developed to investigate the aggregation and breakage processes. It coincides with some recent observations which describe a measured FSD in coastal waters with a set of constituted lognormal distributions. The newly developed lognormal QBMM was tested with several ideal flocculation kinetic kernels, none of which, however, was used for interpreting cohesive sediment dynamics. Therefore, it raised our interest to evaluate the model performance for fine-grained sediments in shear turbulence dominated environments. In this study, additional validations against two kaolinite laboratory experiments were tested in the framework of the extended QBMM. It is hypothesized that these subordinate lognormal distributions share the same value of standard deviation. Different from the previous methods, the common standard deviation is determined empirically to reduce the number of tracers and better represent the FSDs. With sediment flocculation kinetics, the predicted FSDs reasonably reproduce the FSDs observed in both the mixing chamber and the settling column experiments. Despite the lacking of explicit descriptions of microbial effects at the current stage, this model has the potential to be implemented into large-scale particle transport models and deserves a more in-depth study in the future. Highlights • The Floc Size Distributions (FSDs) of cohesive sediments were modeled. • The FSDs were represented as a superposition of several constituted lognormal distributions. • The subordinate lognormal FSDs shared the same value of standard deviation. • The model was firstly applied in a mixing jar experiment as a 0-D calibration. • The model was also applied in a settling column experiment as a 1-D validation. [ABSTRACT FROM AUTHOR]

Highlights • The biomineral flocs in the Belgian coast were investigated during two contrasting periods. • The observed floc size distributions were decomposed to represent microflocs, macroflocs and megaflocs. • A simple flocculation model that can describe the biofilm growth of flocs was developed. • This model was successfully implemented in the open TELEMAC with five passive tracers. Abstract The Floc Size Distributions (FSDs) of biomineral suspended particles are of great importance to understand the dynamics of bio-mediated Suspended Particulate Matters (SPMs). Field observations were investigated at Station MOW1 in Belgian coastal waters (southern North Sea) during two typical periods with abundant and reduced biomass. In addition, the Shen et al. (2018) [Water Res. Vol 145, pp 473–486] multi-class population balance flocculation model was extended to address the occurrence of suspended microflocs, macroflocs and megaflocs during these contrasting periods. The microflocs are treated as elementary particles that constitute macroflocs or megaflocs. The FSD is represented by the size and mass fraction of each particle group, which corresponds to a temporal and spatial varying mass weighted settling velocity. The representative sizes of macroflocs and megaflocs are unfixed and migrated between classes mainly due to the effects of turbulent shear, differential settling and biofilm growth. The growth of an aggregate because of bio-activities is allotted to each elementary particle. It is further hypothesized that the growth kinetics of biomineral particles due to biofilm coating follows the logistic equation. This simple bio-flocculation model has been successfully coupled in the open source TELEMAC modeling system with five passive tracers in a quasi-1D vertical case. Within an intra-tide scale, the settling velocity (w s) is large during slack tides while it is small during maximum current velocities because of variations in turbulence intensities. Nonetheless, the w s may be largely underestimated when the biological effect is neglected. For a seasonal pattern, the w s is higher in biomass-rich periods in May than in biomass-poor periods in October. While the mean sizes of megaflocs are close during the two periods, the macroflocs during algae bloom periods are more abundant with a larger mean size. This study enhances our knowledge on the dynamics of SPMs, especially the biophysical influences on the fate and transport of estuarine aggregates. [ABSTRACT FROM AUTHOR]

In this study, the main object is to investigate the performance of a few new physics-based process models by implementation into a numerical model for the simulation of the flow and morphodynamics in the Western Scheldt estuary. In order to deal with the complexity within the research domain, and improve the prediction accuracy, a 2D depth-averaged model has been set up as realistic as possible, i.e. including two-way hydrodynamic-sediment transport coupling, mixed sand-mud sediment transport (bedload transport as well as suspended load in the water column) and a dynamic non-uniform bed composition. A newly developed bottom friction law, based on a generalised mixing-length (GML) theory, is implemented, with which the new bed shear stress closure is constructed as the superposition of the turbulent and the laminar contribution. It allows the simulation of all turbulence conditions (fully developed turbulence, from hydraulic rough to hydraulic smooth, transient and laminar), and the drying and wetting of intertidal flats can now be modelled without specifying an inundation threshold. The benefit is that intertidal morphodynamics can now be modelled with great detail for the first time. Erosion and deposition in these areas can now be estimated with much higher accuracy, as well as their contribution to the overall net fluxes. Furthermore, Krone's deposition law has been adapted to sand-mud mixtures, and the critical stresses for deposition are computed from suspension capacity theory, instead of being tuned. The model has been calibrated and results show considerable differences in sediment fluxes, compared to a traditional approach and the analysis also reveals that the concentration effects play a very important role. The new bottom friction law with concentration effects can considerably alter the total sediment flux in the estuary not only in terms of magnitude but also in terms of erosion and deposition patterns. [ABSTRACT FROM AUTHOR]

Multimodal particle size distributions (PSDs) of fine-grained cohesive sediments are common in marine and coastal environments. The curve-fitting software in this study decomposed such multimodal PSDs into subordinate log-normal PSDs. Four modal peaks, consisting of four-level ordered structures of primary particles, flocculi, microflocs, and macroflocs, were identified and found to alternately rise and sink in a flow-varying tidal cycle due to shear-dependent flocculation. The four modal PSD could be simplified further into two discrete size groups of flocculi and flocs. This allowed the development of a two-class population balance equation (TCPBE) model with flocculi and flocs to simulate flocculation involving multimodal PSDs. The one-dimensional vertical (1-DV) TCPBE model further incorporated the Navier-Stokes equation with the k- ε turbulence closure and the sediment mass balance equations. Multimodal flocculation as well as turbulent flow and sediment transport in a flow-varying tidal cycle could be simulated well using the proposed model. The 1-DV TCPBE was concluded to be the simplest model that is capable of simulating multimodal flocculation in the turbulent flow field of marine and coastal zones. [ABSTRACT FROM AUTHOR]

The article focuses on the vertical mixing in the unstable layer of sediment-loaded open-channel flow. It states that the time between opposite fluctuations is short to set up a steady velocity. Field data for integral timescales are also presented, as well as the particle turbulent fluctuations. It emphasizes that exponential closures are only useful for shallow channel flows, and adds that a Rousean concentration profile should be used for approximation of deep-water scales.

Abstract: Bimodal flocculation of marine and estuarine sediments describes the aggregation and breakage process in which dense microflocs and floppy macroflocs change their relative mass fraction and develop a bimodal floc size distribution. To simulate bimodal flocculation of such sediments, a Two-Class Population Balance Equation (TCPBE), which includes both size-fixed microflocs and size-varying macroflocs, was developed. The new TCPBE was tested by a model-data fitting analysis with experimental data from 1-D column tests, in comparison with the simple Single-Class PBE (SCPBE) and the elaborate Multi-Class PBE (MCPBE). Results showed that the TCPBE was the simplest model that is capable of simulating the major aspects of the bimodal flocculation of marine and estuarine sediments. Therefore, the TCPBE can be implemented in a large-scale multi-dimensional flocculation model with least computational cost and used as a prototypic model for researchers to investigate complicated cohesive sediment transport in marine and estuarine environments. Incorporating additional biological and physicochemical aspects into the TCPBE flocculation process is straight-forward also. [Copyright &y& Elsevier]

Abstract: In this study, the performance of the horizontal large eddy simulation module, developed at the University of Leuven (HLES-KULeuven module) is assessed. A comparison between different subgrid scale models has been carried out. The study is concerned with the non-rotating and unstratified flows. The results of the simulation for an oscillatory backward facing (BFS) flow are presented in case of an expanding flume based on a one-length scale approach and a two-length scale approach. Three subgrid scale (SGS) models have been tested: Smagorinsky SGS model (Smagorinsky, J., (1963). General circulation experiments with the primitive equations, I. the basic experiments. Monthly Weather Review, 91(3), 99–164), Uittenbogaard SGS model (Uittenbogaard, R.E., and van Vossen, B., (2004). Subgrid-scale model for quasi-2D turbulence in shallow water. Shallow Flows. Jirka and Uijttewaal (Eds.), Taylor & Francis Group, London, ISBN 90 5809 700 5) and a proposed two-length scale approach. The first two models are considered to be a one-length scale models. A simulation without a subgrid scale model for the horizontal mixing has also been conducted. In all simulations, a quadratic friction model parameterizes the dissipation produced by the 3D-subdepth scale turbulence. The two-length scale concept uses a newly mixing length formulation for the quasi-2D turbulence and doesn''t depend on the filter width in contrast to the one-length scale approach, in which the mixing length is function of the filter width. The outputs of the HLES-KULeuven module have been compared with the experimental data taken from Stelling, G.S., and Wang, L.X., (1984). Experiments and computations on separating flow in an expanding flume. Dept. Civil Engineering, Delft University of Technology, Report 2–84.). The two-length scale approach has been validated with experimental data from SERC Flood Channel Facility at HR Wallingford. In general, there is a qualitative agreement with the experimental data. It has also been found that the two-length scale approach produces more elongated and less isotropic vortex than the one-length scale models. [Copyright &y& Elsevier]

Eulerian equations for the vertical flux and momentum of suspended particles in dilute sediment-laden open-channel flow in equilibrium have been derived using the two-fluid approach. Reynolds averaging has been applied in order to allow validation of individual terms with experimental data. Consideration of the various terms of the vertical momentum balance with experimental flume data indicates that the drag force has to be separated into a mean and a turbulent contribution with different timescales, respectively, the (gravitational) particle timescale and the integral turbulence timescale. The resulting formulation further provides a new theoretical closure for the turbulent Schmidt number. [ABSTRACT FROM AUTHOR]

In this paper, the cross-shore propagation and shoaling of bound infragravity (IG) waves have been investigated. By establishing a conceptual 1D model utilizing a constant slope in the shoaling zone, a semi-analytical solution for bound IG waves could be obtained. Through the use of this semi-analytical solution, a collection of bichromatic conditions have been simulated, thus confirming the dependence of the forced long wave shoaling rate on a normalized bed slope β. β was found to be the dominant factor controlling the shoaling of bound long waves, yet when the bichromatic wave steepness become more significant (in this study around 0.10), the mean primary wave frequencies come into play. A detailed analysis confirmed an over-estimation of the equilibrium solution by Longuet-Higgins and Stewart (1960) (LHS60), which was derived based on a flat bottom in the first place. This over-estimation was found to be the most significant for primary short waves with a small wave steepness in the offshore region. A correction to the equilibrium solution of LHS60 has been derived, and its validity has been verified through a comparison to a high-resolution laboratory experiment under bichromatic wave conditions. Furthermore, the phase shift between the short wave groups and the IG waves has been discussed. At the innermost of the shoaling zone, the bound IG waves phases are still found to be strongly governed by the local forcing of short wave groups, as has been pointed out by Baldock (2006). For the near-resonant case, the bound IG waves are anti-phase to the short wave groups, and a larger phase shift in the shoaling zone might be the result of superposition of incoming free IG waves. • Cross-shore shoaling of bound infragravity (IG) waves on slopes studied. • The dependence of bound IG wave shoaling on the normalized bed slope confirmed. • An approximate correction to the original equilibrium solution (LHS60) proposed. • The new shoaling formulation is verified by comparing to laboratory data. [ABSTRACT FROM AUTHOR]

Discusses research on the equation for the hindered settling velocity of flocs in dense cohesive sediment suspensions. Implication of the derivation of the equation for the hindered settling velocity; Comments on the validation made on the study.

To manage coastal and estuarine waters, it is critical to accurately predict the movements of cohesive and non-cohesive sediments. There are well-established methods to estimate the behavior of non-cohesive sediments; however, without extensive knowledge on flocculation processes it remains difficult to predict the behavior of cohesive sediments. Flocculation is one of the main processes (e.g., erosion, deposition, settling, consolidation and flocculation) in cohesive sediment dynamics. The study of flocculation is an interdisciplinary work since it relates to various physical (e.g., transport, settling and deposition), chemical (e.g., contaminant uptake and transformation) and biological (e.g., community structure activities and metabolism) activities. Nevertheless, a widely-accepted flocculation model that can quantitative simulating the Floc Size Distributions (FSDs) for a relatively large study domain has not yet been fully developed. In this study, a multi-class population balance flocculation model was developed to address the occurrence of suspended microflocs, macroflocs and megaflocs in Belgian coastal waters (southern North Sea). The floc size distributions were represented by the size and mass fraction of each particle group. The representative sizes of macroflocs and megaflocs are unfixed and migrated between classes mainly due to the effects of turbulent shear, differential settling and biofilm growth. Specifically, the growth of an aggregate because of biofilm attachment and extracellular polymeric substance glue is averaged to each elementary particle, with its growth rate response to various bio-activities. This simple bio-flocculation model has been successfully coupled in the open source TELEMAC modeling system with five passive tracers in a quasi-1D vertical case. It was validated by observation data at the station MOW1 close to Zeebrugge harbor during both peak algae bloom and low biomass periods. It shows that when the biomass is abundant the predictions of the mean settling velocities are largely underestimated when the biological effect is neglected. This model will enhance our knowledge of the dynamics of suspended particulate matters, especially the biophysical influences on the fate and transport of estuarine aggregates. [ABSTRACT FROM AUTHOR]

The means by which mud is conditioned for use in laboratory experiments affects the behaviour of the mud during the experiments, and hence the results and reproducibility. This study discusses the impact of different mud conditioning techniques and procedures using multiple series of short-term consolidation tests. The mud used originates from the Zeebrugge docks of the Port of Antwerp-Bruges, Belgium. For all experiments the initial density of the mud was around 1.08 g ⋅ cm−3, which is below its gel density. This has the advantage that the comparison can be limited to the behaviour during the typical initial settling phases, allowing experiments to be restricted in time to seven days. Each series of experiments is repeated multiple times using two separate batches of mud. The objective is to quantify the repeatability across these different series of experiments, and to identify the ideal conditioning procedure for optimal reproducibility for future laboratory experiments using mud. Distinct differences in settling curves are observed which confirm the intended influence of each mixing technique. A conditioning procedure based on a combination of axial and radial mixing yields the most control over settling behaviour during the initial stages of the sedimentation process and therefore results in the best repeatability of settling behaviour. It is recommended that such a mud conditioning procedure is further used for experiments with mud to allow for a better uniformity in research where the behaviour of mud is critical. • The way mud is conditioned influences its behaviour during sedimentation. • Today, there is no generally known uniform method for conditioning mud. • Experimental fluid dynamics with mud needs better reproducibility of mud behaviour. • A combination of shear and axial mixing yields the highest level of reproducibility. [ABSTRACT FROM AUTHOR]

A dynamic two-dimensional depth-averaged (2DH) parameterization for flocculation of cohesive sediments is proposed based on the kinetic model by Winterwerp (J Hydraul Res 36:309–326, 1998). The aim is to achieve a realistic representation of the suspended sediment field by accounting for flocculation, also taking into consideration its dependence on advection, turbulent diffusion, and turbulent shear. This formulation is evaluated in a sand-mud model of the Belgian Coast and the Western Scheldt. Results indicate that it can reproduce known sediment transport patterns: modelled floc size and suspended sediment concentrations are in the range of measurements. When evaluating the model results spatially, the extent and shape of the coastal sediment plumes are similar to the observed suspended particle matter (SPM) maps from the PROBA-V satellite. Therefore, the use of the presently proposed flocculation model has added value to improve sediment transport calculations in coastal areas. [ABSTRACT FROM AUTHOR]

The presented model allows for fast simulations of the near-field behaviour of overflow dredging plumes. Overflow dredging plumes occur when dredging vessels employ a dropshaft release system to discharge the excess sea water, which is pumped into the trailing suction hopper dredger (TSHD) along with the dredged sediments. The fine sediment fraction in the loaded water-sediment mixture does not fully settle before it reaches the overflow shaft. By consequence, the released water contains a fine sediment fraction of time-varying concentration. The sediment grain size is in the range of clays, silt and fine sand; the sediment concentration varies roughly between 10 and 200 g/l in most cases, peaking at even higher value with short duration. In order to assess the environmental impact of the increased turbidity caused by this release, plume dispersion predictions are often carried out. These predictions are usually executed with a large-scale model covering a complete coastal zone, bay, or estuary. A source term of fine sediments is implemented in the hydrodynamic model to simulate the fine sediment dispersion. The large-scale model mesh resolution and governing equations, however, do not allow to simulate the near-field plume behaviour in the vicinity of the ship hull and propellers. Moreover, in the near-field, these plumes are under influence of buoyancy forces and air bubbles. The initial distribution of sediments is therefore unknown and has to be based on crude assumptions at present. The initial (vertical) distribution of the sediment source is indeed of great influence on the final far-field plume dispersion results. In order to study this near-field behaviour, a highly-detailed computationally fluid dynamics (CFD) model was developed. This model contains a realistic geometry of a dredging vessel, buoyancy effects, air bubbles and propeller action, and was validated earlier by comparing with field measurements. A CFD model requires significant simulation times, which is not available in all situations. For example, to allow correct representation of overflow plume dispersion in a real-time forecasting model, a fast assessment of the near-field behaviour is needed. For this reason, a semi-analytical parameter model has been developed that reproduces the near-field sediment dispersion obtained with the CFD model in a relatively accurate way. In this paper, this so-called grey-box model is presented. [ABSTRACT FROM AUTHOR]

A correction to the article "Vertical Mixing in the Fully Developed Turbulent Layer of Sediment-Laden Open-Channel Flow" in the September 2008 issue is presented.

Two-phase Large-Eddy Simulations (LES) of turbidity plumes in a crossflow are presented, representative for mixtures of water and fine sediment particles released through an overflow pipe of a dredging vessel. The model was tested based on experimental data of vertical plumes in a still environment as well as of plumes in crossflow. Simulations include the effect of the wake of a schematised hull shape on the dispersion and turbulent structure of the plume, as a schematisation of plumes released from dredging vessels. Criteria for a minimum fraction of resolved turbulent kinetic energy were used to evaluate the so-called completeness of the LES simulations. It is shown that while the grid resolution is a factor three to six lower compared to earlier Direct Numerical Simulation (DNS) simulations, comparable results on turbulent structures and turbulent kinetic energy can be obtained. Results of mean trajectory and plume dispersion show good agreement with experimental data. The different types of turbulent structures found in our experiments as well as in literature are reproduced by the model. It is shown that a release point shortly upstream of the schematised hull’s stern causes the upper fringes of the plume to be torn off in cases with a relatively strong crossflow, which enhances mixing and hampers the containment of the material transported by a plume. [ABSTRACT FROM AUTHOR]

New methods have been developed to extract turbulent fluxes of momentum and fine sediments from Acoustic Doppler Velocimeter (ADV) data. The methods were validated with turbidity plume experiments. The ADV's backscatter amplitude signal was used to determine the sediment concentration and its turbulent fluctuations. However, different kinds of noise are found in the backscatter amplitude and the velocity signals, which are polluting the turbulent fluxes results. Therefore, spectral noise correction methods have been developed, which allow more accurate quantification of turbulent velocity and sediment concentration fluctuations. The techniques are applied to two benchmark cases of a vertical sediment-laden jet. Reynolds stresses, turbulent intensity of velocity and sediment concentration as well as turbulent sediment fluxes are shown to agree well with two-fluid plume measurements reported in the literature. The methods presented in this paper can be applied to the processing of measurements in cohesive sediment plumes, turbidity currents or mixing layers in the absence of, or in stable, flocculation. [ABSTRACT FROM PUBLISHER]

This work presents the experimental and numerical results of towing experiments with a 0.16 m chord surface-piercing hydrofoil in muddy environments. Various test conditions were examined to understand the fluid flow behaviour and forces on the hydrofoil. The experiments were conducted in a flume tank where the hydrofoil was towed at a steady velocity at different drift angles and Under Keel Clearances (UKCs) with respect to the mud layer. A 6DoF load cell measured the hydrodynamic forces and moments acting on the hydrofoil. Pressure sensors placed at mid-span recorded the pressure variations on the surface of the object while moving. The presence of the natural mud significantly affected the measured forces for the cases of negative UKCs. The viscoplastic properties of the natural mud display an additional force when the hydrofoil is fully or partially submerged in the mud layer. The experimental measurements were compared for different conditions with CFD simulations. The numerical results presented in this work demonstrated a fair prediction of the forces and pressure variations along the surface of the body for the whole velocity range in single and two interface conditions. • Natural mud significantly influences the overall hydrodynamic forces acting on floating objects. • Yield stress displays an initial rigidity in the mud layer, which consequently affects the reacting forces. • CFD tools can fairly predict the averaged forces acting on the body in muddy environments. • Experimental measurements can be used as validation cases for more complex mud representations. [ABSTRACT FROM AUTHOR]

Abstract: Divalent cations have been reported to develop bridges between anionic polyelectrolytes and negatively-charged colloidal particles, thereby enhancing particle flocculation. However, results from this study of kaolinite suspensions dosed with various anionic polyacrylamides (PAMs) reveal that Ca2+ and Mg2+ can lead to colloid stabilization under some conditions. To explain the opposite but coexisting processes of flocculation and stabilization with divalent cations, a conceptual flocculation model with (1) particle-binding divalent cationic bridges between PAM molecules and kaolinite particles and (2) polymer-binding divalent cationic bridges between PAM molecules is proposed. The particle-binding bridges enhanced flocculation and aggregated kaolinite particles in large, easily-settleable flocs whereas the polymer-binding bridges increased steric stabilization by developing polymer layers covering the kaolinite surface. Both the particle-binding and polymer-binding divalent cationic bridges coexist in anionic PAM- and kaolinite-containing suspensions and thus induce the counteracting processes of particle flocculation and stabilization. Therefore, anionic polyelectrolytes in divalent cation-enriched aqueous solutions can sometimes lead to the stabilization of colloidal particles due to the polymer-binding divalent cationic bridges. [Copyright &y& Elsevier]

Abstract Estuarine and coastal regions are often characterized by a high variability of suspended sediment concentrations in their waters, which influences dredging projects, contaminant transport, aquaculture and fisheries. Although various three-dimensional open source software are available to model the hydrodynamics of coastal water with a sediment module, the prediction of the fate and transport of cohesive sediments is still far from satisfied due to the lack of an efficient and robust flocculation model to estimate the floc settling velocity and the deposition rate. Single-class and sometimes two-class flocculation models are oversimplified and fail to examine complicated floc size distributions, while quadrature-based or multi-class based flocculation models may be too complicated to be coupled with large scale estuarine or ocean models. Therefore, a three-class population balance model was developed to track the sizes and number concentrations of microflocs, macroflocs and megaflocs, respectively. With the assumption of a fixed size of microflocs and megaflocs, only four tracers are needed when coupled with the open-source TELEMAC system. It enables better settling flux estimates and better addresses the occurrence and concentration of larger megaflocs. This tri-modal flocculation model was validated with two experimental data sets: (1) 1-D settling column tests with the Ems mud and (2) in-situ measurements at the WZ Buoy station on the Belgian coast. Results show that the flocculation properties of cohesive sediments can be reasonably simulated in both environments. It is also found that the number of macroflocs created, when a larger macrofloc breaks up, is a statistical mean value and may not be an integer when applying the model in the field. Graphical abstract Image Highlights • The sizes of flocs are dynamically altered because of particle aggregation and breakage. • A three-class population balance flocculation model was developed. • This flocculation model has been successfully implemented in the open source TELEMAC. • Four tracer variables, representing the floc size distributions, were included in TELEMAC. • The model was validated with settling column tests and field measurements. [ABSTRACT FROM AUTHOR]

An annular flume combined with a microscopic system is used to explore local erosion mechanisms of transparent cohesive beds (yield stress fluids) under shear-induced flowing liquid. The observations at a microscopic scale for erosion flume experiments are the originality of this work. Our first results reveal that the deformation and erosion dynamics of bed material mainly depends on its miscible state with the flume fluid, its structural organization being associated with mechanical properties and the measurement section. Shear localization within the bed is also observed. Moreover, the erosion rate and the local bed shear stress can vary with time even for an imposed constant global hydrodynamic flow as they result from the coupling between the fluid flow and cohesive bed conditions at the bottom. We also try to link the local resistance to erosion to the local cohesive strength estimated from rheometric microscopic data for the same sample conditions with homogeneous properties. [ABSTRACT FROM AUTHOR]

The quantification and monitoring of sediment dynamics in estuaries has received plenty of attention in recent years due to both economic and environmental interests. Numerical models have been widely applied for predicting the morphodynamic evolution of these areas. However, the degree of uncertainty of these models is often very high due to the lack of measurements necessary for a complete model calibration and validation. In this paper, different methods have been integrated to overcome these limitations. The hydrodynamics and suspended sediment transport in the IJzer estuary were estimated by means of field data and with the use of a two-dimensional hydrodynamic model coupled to a morphodynamic model. Tidal pumping was recognized as the main driving mechanism of sediments inside the estuary. Erosion-deposition areas calculated with the numerical model were compared to the locations and volumes of dredged material. Model results and measurements were generally in good agreement. Additional support for the morphodynamic model results came from airborne hyperspectral images acquired in 2001 and 2005. [ABSTRACT FROM AUTHOR]

[Display omitted] The quasi-Monte Carlo (QMC) method was enhanced to solve the population balance model (PBM) including aggregation and fragmentation processes for simulating the temporal evolutions of characteristic sizes and floc size distributions (FSDs) of cohesive sediments. Ideal cases with analytical solutions were firstly adopted to validate this QMC model to illustrate selected pure aggregation, pure fragmentation, and combined aggregation and fragmentation systems. Two available laboratory data sets, one with suspended kaolinite and the other with a mixture of kaolinite and montmorillonite, were further used to monitor the FSDs of cohesive sediments in controlled shear conditions. The model results show reasonable agreements with both analytical solutions and laboratory experiments. Moreover, different QMC schemes were tested and compared with the standard Monte Carlo scheme and a Latin Hypercube Sampling scheme to optimize the model performance. It shows that all QMC schemes perform better in both accuracy and time consumption than standard Monte Carlo scheme. In particular, compared with other schemes, the QMC scheme using Halton sequence requires the least particle numbers in the simulated system to reach reasonable accuracy. In the sensitivity tests, we also show that the fractal dimension and the fragmentation distribution function have large impacts on the predicted FSDs. This study indicates a great advance in employing QMC schemes to solve PBM for simulating the flocculation of cohesive sediments. [ABSTRACT FROM AUTHOR]

PROBA-V (Project for On-Board Autonomy-Vegetation) is a global vegetation monitoring satellite. The spectral quality of the data and the coverage of PROBA-V over coastal waters provide opportunities to expand its use to other applications. This study tests PROBA-V data for the retrieval of turbidity in the North Sea region. In the first step, clouds were masked and an atmospheric correction, using an adapted version of iCOR, was performed. The resulted water leaving radiance reflectance was validated against AERONET-OC stations, yielding a coefficient of determination of 0.884 in the RED band. Next, turbidity values were retrieved using the RED band. The PROBA-V retrieved turbidity data was compared with turbidity data from CEFAS Smartbuoys and ad-hoc measurement campaigns. This resulted in a coefficient of determination of 0.69. Finally, a time series of 1.5 year of PROBA-V derived turbidity data was plotted over MODIS data to check consistencies in both datasets. Seasonal dynamics were noted with high turbidity in autumn and winter and low values in spring and summer. For low values, PROBA-V and MODIS yielded similar results, but while MODIS seems to saturate around 50 FNU, PROBA-V can reach values up till almost 80 FNU. [ABSTRACT FROM AUTHOR]

Abstract: Mudflats are important for coastal zone ecosystems by providing wildlife habitat and by acting as natural sea defenses that serve to dissipate tidal and wave energy. Geomorphological models of these intertidal flats and estuaries require site-specific bio-physicochemical sediment parameters as input. Hyperspectral remote sensing can be used as a tool providing synoptic maps of these properties. However, the interpretation of hyperspectral remotely sensed images over mudflats is only possible if the appropriate bio-geophysical or empirical models for information extraction are available. Therefore, the objective of this paper was to model the effects of varying sediment properties on the reflectance in laboratory conditions. The methodology consisted of (i) hyperspectral measurements of sediment mixtures with varying physical and biological characteristics in laboratory conditions, (ii) determination and quantification of specific absorption features and (iii) regression between the absorption features and physical parameters. In laboratory conditions, quantification of clay in dry sediment, moisture in unsaturated sediment and chlorophyll a in sediment mixtures was achieved with coefficients of determination (r 2) of 0.98, 0.90 and of 0.96 using the scaled band area of absorption features at 2204, 1450 and 673nm, respectively. Additionally, the water absorption at 1190nm was identified as suitable to predict moisture content in very wet sediment and preliminary results showed the potential of hyperspectral signals to assess the effect of bioturbation on sediment properties. Future work will consider the applicability of this methodology in field situations to relate bio-physical sediment parameters to the hyperspectral signal. [Copyright &y& Elsevier]