Bed formation concept

Two major mechanisms are responsible for bed formation, namely: sedimentation of particles or flocculated particles and consolidation (Ross, 1988; Mehta, 2014). Sedimentation can be defined as the process by which particles, or aggregates of particles, under the influence of gravitational force leave suspension and settle to form a bed deposit. Consolidation in a fully saturated environment is a process which results from the deformation of the bed deposit particle framework under an applied stress. The applied stress could either be as a result of net negative buoyancy i.e. self-weight or imposed overburden loading (Dyer, 1986; Ross, 1988; Mehta, 2014).

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In the context of bed formation by sedimentation and consolidation, when a column of suspended sediment settles in still water under gravity, the sequence of the complete processes is (Dyer, 1986; Mehta, 2014): flocculation, settling, deposition and consolidation (Figure 2.11). Depending on factors such as sediment types, composition and concentration, the sequence may be altered, for example, the first three processes can occur simultaneously and flocculation can be omitted completely in predominantly non- cohesive sediments (this is discussed further in Chapter 5). A general description and graphical model of the bed formation processes (Figure 2.11) was given by Imai (1981). Due to low submerged weight that makes flocculated sediments to settle more slowly than coarse or non-cohesive sediment of the same size as flocs, the model therefore describes the three general stages that flocculated sediments undergo to form bed deposits:

Figure 2.11 Imai (1981) description and graphical model of bed formation process (From Mehta, 2014)

Flocculation stage

Aggregation in particular is the defining process in this stage. The process of aggregation (flocs building-up) and breaking-up is called flocculation. Particles aggregation results when two particles collide and stick together and the rate of aggregation is driven by frequency of collisions, the efficiency of the collisions in getting the particles to stick together and the particle concentration. Particle collisions are initiated by Brownian motion of particles, turbulence within the suspending liquid and differential settling of the suspended particles (Van Leussen, 1994; Winterwerp and van Kesteren, 2004; Mehta, 2014). At this stage, floc formation due to coagulation will occur if the initial particles

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are primary or dispersed and conducive to cohesion (Mehta, 2014). Depending on the mineralogy of the sediment involved, this stage may last for seconds or minutes. The onset of rapid fall in the water-sediment interface at the end of the stage (Figure 2.11) shows a smooth transition to the next stage (i.e. settling stage; Figure 2.11) [Imai, 1981; etc.].

Settling stage

This stage is characterised by two phases, namely: uniform settling/constant flux settling phase and hindered settling phase (see Figure 2.11). In the former, the rate of fall of water-sediment interface is relatively uniform due to negligible hindrance against the fall; in the latter phase, however, the fall of the interface (i.e. lutocline) is hindered, this phase is characterised by the decrease in the interface falling rate with time and the formation of second interface below the lutocline (see Figure 2.11), which defines the building up of bed above the bottom (Imai, 1981; Mehta, 2014). Detailed description of this stage has been given earlier in this chapter (i.e. section 2.3.4)

Consolidation stage

The last identified stage is the consolidation stage, and the transition from the settling stage to this stage happens at a time (t) when the lutocline meets the rising bed height (see Figure 2.12). Beyond the meeting point between the lutocline and the rising bed height, the surface of the freshly formed bed falls very slowly until there is no significant reduction in the bed deposit height, and this is due mainly to consolidation (Imai, 1981; Mehta, 2014). At the initial stage of consolidation, as mud flocs settle, subsequently settled flocs will squeeze the flocs that settle before them and in the process pore-water is expelled out of the flocs and out of the space between the flocs. This process has been described as self-weight consolidation process by Terzaghi (1943). Self-weight consolidation process basically describes the transition from a fluid-supported suspension to a solid-supported suspension (e.g. soil), which is characterised by a change of state in which pore-fluid pressures and vertical total stress are equal, to a state where pore-fluid pressures are less than the total vertical stress (i.e. there is existence of effective stress) [Sills (1998)]. Figure 2.12 shows (a) a typical density profile; and (b) the calculation of its effective stress (i.e. difference between total pressure and pore pressure) for experiment REDMO5 from Sills (1998). Figure 2.12(a) clearly shows, the fluid supported part, the hindered settling phase, solid-supported part, and the consolidation phase. In Figure 2.12(b), the hindered settling phase is characterised by equal pore-fluid pressures

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and total pressure; in the consolidation phase, however, the pore pressures are less than the total pressure (i.e. showing existence of effective stresses). The process described above is widely used in soil mechanics to characterise soil structures; for example, the onset of consolidation is identified by the development of effective stresses, therefore from Figure 2.12(b), it is clear that consolidation starts at Pressure = ca 0.17 kPa. The pressure value corresponds to a density of ca 1200 Kg m-3 _{in Figure 2.12(a), this referred}

to as the structural density (i.e. the concentration at which a space-filling network occurs where particles within the mixture support each other at their loosest packing).

Figure 2.12 Sketch of a typical density profile (a) and the calculation of its effective stress [i.e. difference between total pressure and pore pressure] (b), for experiment REDMO5 fro m Sills (1998). [Adapted from Lintern, 2003]

Figure 2.13 shows the density profiles for the initial 79 hours of a settling column experiment to investigate sedimentation process of estuarine mud (from Combwich, Somerset in England), Been and Sills (1981), described the density (i.e. 1070 Kg m-3 _or

1.07 g cm-3_{) of the initial mud suspension as almost uniform (i.e. at flocculation stage),}

but with time, as settling continues, an interface separating the pool of clear water and the sediment slurry forms which falls linearly with time and maintains an almost constant concentration (i.e. settling stage, similar to Figure 2.11). Meanwhile, at the bottom, a layer of relatively high density is formed, due to (i) coarse particles settling quickly before becoming involved in the flocculation process (which is the case most of the time with sand-mud mixtures) and (ii) partly because of rapid consolidation at the base while the sediment layer is still relatively thin (Been and Sills, 1981).

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Figure 2.13 Density profiles in a settling suspension with initial density of 1.07 g cm-3

(From Been and Sills, 1981)