Experimental and analytical study on mixed sediment (sand-mud) transport processes

Abstract

Estuaries and tidal inlets are often characterised by the co-existence of cohesive (i.e. mud: clay and silt, D<65 um) and non-cohesive (i.e. sand, D>65 um) sediments in different fractional concentrations. Knowledge of the dynamic sedimentation behaviour of sand mud mixtures is therefore crucial to the physical understanding and prediction of the time dependent structure (i.e. mixed or segregated), composition and erodibility of sediment bed deposits developing within these sedimentary environments.

The current study develops and applies a new, non-invasive electrical resistivity measurement technique (ERMT) to capture both temporal and spatial changes in density, porosity and composition of the evolving sand-clay bed deposits, complemented by timelapsed images of the sedimentation processes. A series of settling column tests are then conducted to investigate spatial and temporal variations in sediment bed structure and composition resulting from differential settling of a range of sand-clay mixtures over three different parametric conditions (i.e. sediment composition, initial mixture concentration and ambient pore fluid salinity). Further experiments on erosion and deposition of mixed-sediment beds are conducted with benthic annular flume. Therefore, discussion, analysis and critical reflection on the current experimental results and findings have provided new insight into mixed (sand-clay) sedimentation and erosion processes.

The results show that the formation of segregated (sand-clay) bed layers within 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 mechanism under which these different segregation types can be generated are illustrated through predictions from an existing polydisperse hindered settling model of Cuthbertson et al. (2008). 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. From the erosion experiments, a negative correlation between the proportion of cohesive sediment and the inception of erosion is further established. Specifically, a 5% clay fractional content within sand-clay sediment bed is identified as the critical cohesive fractional content that delineates the non-cohesive and cohesive bed erosion regimes.