FIGURE 12.16 Variation of gelling concentration Cgei with Wf and G for equilibrium conditions. (Equation (12.20).)

Using the various model parameters assessed in Section 12.2, cgel (G, nf) can be established for equilibrium conditions. The results are presented in Figure 12.16, showing the high sensitivity of cgel to nf at larger values of nf. For nf = 3, we obtain the trivial result that cgel = ps. We observe that for nf = 2.6 to 2.7, we find cgel ^ 100 g/l, which is a characteristic concentration for fluid mud occurrences. However, these nf values are unrealistically large.

It appears impossible to obtain cgel values of the order of 100 g/l by modifying the other parameters of the flocculation model within reasonable limits. Hence, by applying the flocculation model for equilibrium conditions, fluid mud concentrations of about 100 g/l can only be obtained with fractal dimensions substantially larger than the average nf values encountered for the mud flocs in the water column. This is an important conclusion, which is explained by the response time of the flocs to a variation in hydrodynamic conditions. Apparently, the fluid mud forming flocs are much smaller than the equilibrium values to be expected within the fluid mud layer because the flocs (settling from the upper part of the water column) get insufficient time to grow. Again, the residence time is the limiting factor here.

Next, 1DV-simulations for Station 3 of Figure 12.10 in the turbidity maximum are discussed. As the horizontal gradients in concentration are fairly large in this area (Van Leussen5), such simulations can only be indicative to predict qualitative trends. Again, the measured variation in water-depth and depth-mean velocity are prescribed, and the depth-mean suspended sediment concentration is set at the measured value of C0 = 5 g/l.

The computed vertical concentration distribution at three times is compared with the measurements in Figure 12.17, showing that the near-bed concentrations (fluid mud) are reasonably predicted. It is noted that the observed near-bed concentrations depicted in Figure 12.17 can only be reproduced when the full flocculation model is

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