Erik Toorman

The characterisation of cohesive sediment properties

Abstract This paper describes apparatus, techniques and methods used by participants in MAST G6M project 4 (Cohesive Sediments) for determining cohesive sediment properties. This comparison of methods aims to stimulate a more general discussion on standardisation of techniques which will lead to characterisation of muds in terms of physical parameters. Such characterisation would allow inter-comparison of muds from different sources. Methods are given for sediment properties of grain size distribution, settling velocity and rheological parameters and for water-bed exchange properties of permeability, effective stress and critical shear stress for erosion and deposition. Accuracy and repeatability are discussed. A typical range of values is indicated for each of these parameters.

Modelling the thixotropic behaviour of dense cohesive sediment suspensions

A dense cohesive sediment suspension, which contains primarily clay particles, is a thixotropic non-ideal Bingham fluid with a true yield stress. Its time-dependent rheological behaviour can be described by the structural kinetics theory in which the yield stress is taken as a measure for the structural parameter. This theory allows the construction of a more general equation of state, which is independent of the rate equation. The applicability of the model is demonstrated by examples of the prediction of constant structure curves and of transient behaviour. The thixotropy model is incorporated into a Navier-Stokes solver to stimulate the flow behaviour in a Couette viscometer.

Settling and consolidation of mud/sand mixtures

Abstract The formation of a cohesive sediment bed is a combination of settling and consolidation processes. These processes strongly influence the structure, properties and erodibility of the sediment bed. At low values of the bed shear stress, suspended sediments, individual particles or flocs, deposit onto the bed. Subsequently, during the consolidation process, the flocs and aggregates rearrange themselves to form a denser structure. The pore water, initially supporting the particles, is being expelled. In an extensive set of laboratory experiments the influence of sand on the settling and the consolidation of mud has been studied. The deposition of mud/sand mixtures has been closely followed in settling column experiments for different types of mud at different sand contents. During the experiments settling rates, density profiles and pore water pressures have been measured. The presence of a sand fraction in the initial suspension has a large impact on the bed formation processes. The results show that the heavier sand particles settle faster and form a separate layer at the bottom of the column as long as the mud does not form a continuous network structure that prevents this segregation. A continuous mud matrix is formed at low sediment supply rates or at high initial suspension concentrations, when the concentration of the mud fraction in the mixture exceeds the gel point density. The settling rates of the mud/sand suspension increase with increasing sand content. The local bed densities within the consolidating bed increase with increasing sand content of the initial suspension, even for the sand-free top layer. Sand addition also speeds up the consolidation process and permeability increases. However, these effects seem to be limited to a maximum sand content above which no additional effect is found. Using the experimental results, guidelines for modelling the settling and consolidation of mud/sand mixtures have been formulated.

A two-class population balance equation yielding bimodal flocculation of marine or estuarine sediments

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.

Modelling of turbulent flow with suspended cohesive sediment

Traditional (cohesive) sediment transport models contain several simplifications which are no longer justifiable when sediment concentrations or stratification effects become significant. This paper gives a rather technical overview of various modifications to a sediment transport model with k-ɛ turbulence closure, used as research tool, in order to improve the physics described by the model. The attention is focussed on the modelling of sediment-turbulence interactions.

Competition between kaolinite flocculation and stabilization in divalent cation solutions dosed with anionic polyacrylamides

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 Ca(2+) and Mg(2+) 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.

An analytical solution for the velocity and shear rate distribution of non-ideal Bingham fluids in concentric cylinder viscometers

An analytical solution is presented for the calculation of the flow field in a concentric cylinder viscometer of non-ideal Bingham-fluids, described by the Worrall-Tuliani rheological model. The obtained shear rate distribution is a function of the a priori unknown rheological parameters. It is shown that by applying an iterative procedure experimental data can be processed in order to obtain the proper shear rate correction and the four rheological parameters of the Worrall-Tuliani model as well as the yield surface radius. A comparison with Kriegers correction method is made. Rheometrical data for dense cohesive sediment suspensions have been reviewed in the light of this new method. For these suspensions velocity profiles over the gap are computed and the shear layer thicknesses were found to be comparable to visual observations. It can be concluded that at low rotation speeds the actually sheared layer is too narrow to fullfill the gap width requirement for granular suspensions and slip appears to be unavoidable, even when the material is sheared within itself. The only way to obtain meaningfull measurements in a concentric cylinder viscometer at low shear rates seems to be by increasing the radii of the viscometer. Some dimensioning criteria are presented.

Numerical simulation of cohesive sediment transport: intercomparison of several numerical models

Five different numerical models are used to reproduce estuarine cohesive sediment transport and intercomparisons are made of the resulting predictions. Comparison with test cases have shown that the numerical treatment of cohesive sediment is very sensitive to model parameters and formulations, and requires good calibration. Some of the specific processes that have been developed through the MAST.3-COSINUS European project are used here to improve the ability of numerical models to reproduce the sediment behaviour in real estuaries. Despite this progress, it is shown that numerical models results still have many limitations, and their results should always be interpreted with great care.

A hindered settling model for the prediction of settling and consolidation of cohesive sediment

The prediction of the settling and consolidation behavior of mixtures of cohesive and non-cohesive sediment is possible using a numerical model that solves the solids mass balance for each fraction. Consistency with the hindered settling theory as well as classical soil mechanics is preserved. Semi-empirical relationships for settling rate and effective stress as a function of density are required as constitutive equations. A simple procedure, using settling column experiment data, for calibration is proposed. The possibility of distinguishing different kinds of particles allows prediction of layered deposits.

Multimodal particle size distributions of fine-grained sediments: mathematical modeling and field investigation

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.