FIGURE 12.2 Conceptual flocculation diagram of Dyer.6

Based on this diagram, Van Leussen5 further suggested that large flocs, formed in the upper part of the water column are broken down near the bed, as sketched in Figure 12.3. This diagram would also explain the sedimentation formula found by Krone,1 which predicts that cohesive sediment particles can only settle if the bed shear stress is smaller than a threshold value. This threshold value would be determined by the strength of the flocs.

In this chapter, we analyze the data and concepts in Figures 12.1 through 12.3 further. To a large extent it is based on the work presented in Winterwerp.8,9 However, here we focus on the role of the flocculation time, that is, the time necessary for flocs to achieve their equilibrium size at given hydro-sedimentological conditions, and the consequences on the time evolution of floc size. Our main presumption herein is that flocculation takes time, as shown in Figure 12.4. This figure presents the time evolution of flocs, as measured by Hogg et al.10 on a suspension of kaolinite clay, peptized with a polymeric flocculant. The clay concentration was about 80 g/l, and

Collisions fine sediment particles

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