Ciencias de la Tierra y el Universo

The formation of sediment beds under different natural flow conditions is a common occurrence in many sedimentary environments such as estuarine and near-shore coastal marine environments. The structure and composition of these sediment beds are largely influenced by the combined action of settling, deposition and consolidation processes, as well as their subsequent entrainment and erodibility caused by changes in hydrodynamic conditions, i.e. change in energy level. In the context of coastal and estuarine waters, accurate prediction of the transport, interactions and fate of sand-mud mixtures is strongly associated with these sedimentation processes because, for instance, sediment beds in estuaries and tidal basins often consist of both sand (non-cohesive) and mud (cohesive).

Until recently, literature contains an overwhelming amount of work on the development and behaviour of bed deposits for monodisperse sediments in the marine environment (e.g. Winterwerp & Kesteren, 2004). More recently however, there have been

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considerable amount of laboratory tests that have revealed primarily, that, the rate or nature of settling, flocculation and consolidation processes characterise bed formation for polydisperse sediment (e.g. sand-mud mixtures) [Williamson, 1991; Torfs et al., 1996; Cuthbertson et al., 2008, 2010; Manning et al., 2011, Mehta, 2014]. For example, Cuthbertson et al. (2010) and Manning et al. (2011) revealed that processes controlling flocculation and settling rates (which, by extension, influence bed formation processes) can alter dramatically when cohesive and non-cohesive sediments are mixed. It is therefore clear from the literature (e.g. Torfs, 1994; Uncles et al., 1998; Whitehouse et al., 2000; van Ledden, 2003; and Amy et al., 2006; Le Hir, 2011), that sand and mud within these mixed sediment environments can be thoroughly mixed, may exhibit a horizontal gradient (e.g. resulting from gradients in the current or wave patterns), or can be layered in the bed, and witness the history of forcing events. In a situation where the sand and mud fractions have minimal interaction during the settling and deposition processes, then segregation of each fraction within the resulting bed deposits dominates, resulting in a well-sorted, layered bed structure (Torfs, 1994; Amy et al.,2006; and Manning et al., 2011).

Torfs et al. (1996) extensively described the results of various settling and consolidation experiments involving sand-mud mixtures based on many laboratory and field tests carried out by the Hydraulics Laboratory of the Katholieke University Leuven (KUL) and by HR Wallingford Ltd, with the aim to (i) investigate the nature of deposition of mixed sediments, (ii) follow the development of the density structure in time, (iii) study properties of the bed, and, (iv) look at different features of deposition and consolidation behaviour that occur in mixed sediments. However, effects of some parametric conditions such as ambient salinity (which has been identified to significantly alter the dynamics of sedimentation processes (Owen; 1970; Dyer, 1986; Winterwerp and van Kesteren, 2004; Mehta, 2014; etc.) were not fully considered and this is one of the areas the current study addresses. One of the observations of Torfs et al. (1996) was occurrence of segregation in mud-sand mixtures, they found that in some sand-mud mixtures heavier sand particles settled faster to the bottom of the column to form a sand rich base layer, and noticed this continued as long as the mud concentration is not high enough to form a continuous network structure to prevent this segregation. Figure 2.4 shows the size grading of the bottom and top layer (1 mm) of two of the experiments with Hong Kong mud and King’s Lynn Sand (C0 = 1 -3 g l-1, D50 sand = 230 μm) before and after the input of sand.

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Figure 2.4 Size grading of the top and bottom millimetre of the bed before and after a single input of sand. Left: no sand; Right: 66% added sand (From: Torfs et al., 1996)

It is obvious from the size grading in Figure 2.4 that clear segregation occurs between the top and the bottom of the bed for both tests, although strong segregation is seen in the test with added 66% sand, consisting entirely sand base layer. The segregation observed in the sand free experiments was attributed to strong, compact flocs that rapidly sank to the bottom of the bed. Torfs et al. (1996) therefore concluded that the occurrence of segregation may depend on factors, such as the type of mud (i.e. in terms of mineralogical and chemical composition, organic content and biological processes) and the initial input density, as segregation occurs for initial mud concentrations below the gelling point. They added that the degree of segregation is limited to a maximum sand content, which is a function of the mud type as well.

The mineral composition in combination with particle size distributions are important discriminators when consideration is being given to the mechanical behaviour of mixed sediments (especially in terms of textures and structure). For instance, the presence of clay minerals is a vital criterion for sediment mixtures to show cohesive behaviour, in other words, the cohesiveness or non-cohesiveness of mixed sediment matrix is largely dependent on the clay content within the matrix (van Ledden et al., 2014; te Slaa et al., 2013). A transition between non-cohesiveness to cohesiveness has been reported in mixed sediment bed at clay contents of 5-10% (van Ledden et al., 2014). From the sediment classification approach presented in the sand-silt-clay triangle (Figure 2.1), it is clear that network structures can be formed by solid fractions of sand, silt and clay which has been reported to be largely dependent on the overall porosity (te Slaa et al., 2013). For example, from the ternary diagram (Figure 2.1), the following network structures can be seen: (i) non-cohesive sand dominated; (ii) cohesive sand dominated; (iii) non-cohesive mixed; (iv) cohesive clay dominated; (v) cohesive silt dominated; and (vi) non-cohesive

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silt dominated. Transition to sand or sand–silt-dominated network structures for overall porosities of n = 40 % and n = 50 % are represented by the bold broken lines; while the horizontal bold broken line represents a clay content (i.e. 8 %) at which the transition between cohesive and non-cohesive behaviour can be initiated. Areas of sand- and silt- dominated network structures are respectively indicated by the shaded areas A and B (te Slaa et al., 2013 and Van Ledden et al., 2004)