Flocsize Distribution Over The Water Depth

From the analysis of the Lagrangian flocculation model in Section 12.2, we obtained the time scales for flocculation (e.g. Equation (12.11)). The flocculation time can vary between a few minutes to many days, and even more, depending on the hydrodynamic conditions and the suspended sediment concentration. As a result, flocs of cohesive sediment, observed in marine environments, are often not in equilibrium with the local hydrodynamic conditions.

Let us first compute the time evolution of floc size, as predicted with the Lagrangian flocculation model (12.7). The results are shown in Figure 12.6, together with some data obtained in a settling column (for details, see Winterwerp8). We observe that for two sets of experimental data, equilibrium conditions are obtained within a few hundred seconds. For one experiment though, the flocculation time exceeds 1 h. From a qualitative point of view, Figure 12.6 shows the same trend as Figure 12.4. However, we cannot analyze the results in Figure 12.4 further, as no details of the experimental conditions are given.

These observations have important implications for the flocculation behavior in marine conditions. Flocs settle continuously due to gravity and are remixed/resuspended by turbulence. Hence, they experience varying hydrodynamic conditions (shear rates) during their journey through the water column. This implies that the ratio between flocculation time and residence time in a specific turbulent environment becomes an important parameter.

This can be further analyzed for schematic conditions by considering a situation where the turbulence field is homogeneous over the water depth, as can be realized in a settling column, for example. The mean residence time Tr for all particles in the

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