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• 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: 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]

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]

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]

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 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]

[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]

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]