Monitoring and characterisation of sand-mud sedimentation processes

Alan J S Cuthbertson,W John Mccarter,Olugbenga Ibikunle,Gerard Starrs

doi:10.1007/s10236-016-0958-7

Abstract

Estuaries and tidal inlets are often characterised by the presence of both cohesive and non-cohesive sediments. Knowledge of the sedimentation behaviour of sand-mud mixtures is therefore crucial to the understanding and prediction of the time-dependent structure (i.e. mixed or segregated), composition and erodibility of sediment bed deposits developing within these environments. In the current study, a series of settling column tests are conducted to investigate the hindered settling and initial bed consolidation phases of a range of sand-clay mixtures to determine the parametric conditions under which bed segregation occurs. A new, non-invasive, electrical resistivity measurement technique is employed to capture both temporal and spatial changes in the density, porosity and composition of the evolving sand-clay bed deposits, complimented by time-lapsed images of the sedimentation process within the column. The results show that the formation of segregated (sand-clay) bed layers with bed deposits is largely controlled by the initial fractional composition (i.e. relative sand and clay concentrations). Specifically, mixtures with low clay contents are shown to form well-defined (sand-clay) layer segregation within the resulting deposits, while higher clay contents result in more transitional segregation patterns or no layer segregation (for very high clay concentrations). The physical mechanisms under which these different segregation types can be generated are illustrated through predictions from an existing polydisperse hindered settling model. This model indicates that the degree of bed segregation, and time scale over which this occurs, correlates well with the difference in predicted hindered settling characteristics and upward displacements associated with the sand and clay fractions, respectively. In this regard, the new experimental dataset provides validation for the polydisperse model (for the first time), with the combined data and model predictions providing new insight into mixed (sand-clay) sedimentation processes.

Highlights

Accurate prediction of the transport and fate of cohesive and non-cohesive sediments within estuaries and coastal regions relies on physical understanding of the complex interplay between transportation processes such as flocculation, settling, deposition, erosion and consolidation (e.g. Grasso et al 2015)
Within the settling column arrangement employed in the current study, the typical sedimentation and bed formation processes displayed by different sand-clay mixtures are essentially one-dimensional, with the downward movement of sediments and the upward displacement of pore fluid (Torfs et al 1996)
The sedimentation behaviour of mixed sediment suspensions has been examined within parametric settling column experiments employing non-intrusive electrical resistivity profile measurements and time-lapsed imaging to characterise both the temporal and spatial evolution of the resulting bed deposits

Summary

Introduction

Accurate prediction of the transport and fate of cohesive (e.g. muds, silts) and non-cohesive (e.g. sand) sediments within estuaries and coastal regions relies on physical understanding of the complex interplay between transportation processes such as flocculation, settling, deposition, erosion and consolidation (e.g. Grasso et al 2015). Grasso et al 2015) In this respect, net sedimentation [i.e. an increase in bed level due to the difference in erosion and deposition rates, Winterwerp and van Kesteren (2004)] is most likely to occur within mixed sediment environments such as estuaries and tidal inlets under low energy conditions (i.e. in sheltered near-shore regions and/or at slack water) (te Slaa et al 2013). Archie 1942; Winsauer et al 1952; Atkins and Smith 1961), which is defined as the ratio of bulk resistivity ρbulk of the saturated rock to the resistivity of the saturating fluid ρf (i.e. pore water) (see Table 1) This formation factor F is related to the rock porosity φ through the relationship, F

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