Jeremy Spearman

Cohesive Sediment Flocculation and the Application to Settling Flux Modelling

This chapter provides an outline of the flocculation process; flocculation measurement methods and the importance of data; floc settling behaviour; examples of cohesive sediment depositional model approaches, including ways of parameterising flocculation.

A review of the physical impacts of sediment dispersion from aggregate dredging

The disturbance and subsequent dispersion of sediment arising from aggregate dredging results in increases in suspended sediment concentrations and, potentially, settlement of fine sediment or sand onto the bed, which may both cause adverse effects on local ecology. This subject is one area which has seen much research over many years and this paper sets out to synthesise some basic general conclusions for use when assessing the significance of planned operations. The literature detailing the dispersion of fine sediment plumes, and the longer term dispersion of sand released through the dredging process, is scrutinised, and in some cases re-evaluated, and used to identify an evidence-based footprint of potential impact.

Chapter 28 On the significance of mud transport algorithms for the modelling of intertidal flats

Abstract As part of the Defra funded Estuaries Processes Research Project (EstProc), a number of mud transport algorithms have been developed. These algorithms relate to a range of processes, such as the combined shear from waves and currents, the effect of biology on erosion, the nature of the threshold of deposition, and the effect of shear and concentration on flocculation and settling velocity. This paper highlights the applicability, relevance, and sensitivity issues surrounding these algorithms in addition with other important considerations including the choice of 2D or 3D models, and the nature of the 3D vertical resolution. The suspended sediment (mud) transport for a typical, but hypothetical mud flat scenario is modelled using the flow and mud transport modules of TELEMAC-2D and the fully coupled TELEMAC-3D mud transport model. The study concludes that the effect of biology appears to have the maximum impact on the prediction of erosion on mud flats, but that use of a formula for deposition without a deposition threshold, in combination with the choice of settling velocity, can also have a similarly large impact on the prediction of erosion. Less important, but still significant, were the approaches used to calculate the combined bed shear stress due to waves and currents and numerical considerations, such as choice of 2D or 3D model, and the number of layers included in the latter.

On the hindered settling of sand-mud suspensions

Hindered settling, the process by which the settling of sediment particles becomes impeded due to the proximity of other sediment particles, can be an important process for the coastal modeller, especially in highly muddy environments. It is also a significant process in other disciplines such as chemical engineering, the modelling of debris flow, the study of turbidites, piping of slurries and the understanding of processes occurring within a dredger hopper. This study first examines the hindered settling behaviour of monodisperse suspensions in order to create a framework for polydisperse hindered settling that works for both non-cohesive and cohesive suspensions. The Richardson–Zaki equation is adapted to make it compatible with the changes with viscosity that occur near the point at which suspensions become solid. The modified monodisperse settling equation is then compared to data for hindered settling of cohesive suspensions and shown to be consistent with the transition between hindered settling and the initial permeability phase of consolidation. Based on the monodisperse framework developed initially, this paper proposes a hindered settling model for sand/mud mixtures which is based on a modification of the Masliyah (1979) and Lockett and Bassoon (1979) hindered settling equation. The model is shown to reproduce the hindered settling of a variety of different sediment mixtures whilst reducing the extent of empiricism often associated with the modelling of polydisperse hindered settling of mud/sand mixtures.

An examination of the rheology of flocculated clay suspensions

A dense cohesive sediment suspension, sometimes referred to as fluid mud, is a thixotropic fluid with a true yield stress. Current rheological formulations struggle to reconcile the structural dynamics of cohesive sediment suspensions with the equilibrium behaviour of these suspensions across the range of concentrations and shear. This paper is concerned with establishing a rheological framework for the range of sediment concentrations from the yield point to Newtonian flow. The shear stress equation is based on floc fractal theory, put forward by Mills and Snabre (1988). This results in a Casson-like rheology equation. Additional structural dynamics is then added, using a theory on the self-similarity of clay suspensions proposed by Coussot (1995), giving an equation which has the ability to match the equilibrium and time-dependent viscous rheology of a wide range of suspensions of different concentration and mineralogy.