the computational domain, that is, neither sedimentation nor erosion is modeled. The results of the computations are shown in Figure 12.12 in the form of isolutals and as vertical concentration profiles at t = 1420 and 1780 min in Figure 12.13. Note that the model is first run for 1000 min to mediate for spin-up effects; these 1000 min have to be subtracted from the horizontal axes.

The computational results reproduce the vertical structure of the measured concentration distribution, including the characteristic rapid settling around HWS. It is noted that it was not possible to reproduce the character of the measured data with 1DV-computations with a constant settling velocity (no flocculation) or with a constant fluid density (no sediment-fluid interaction), for example, Winterwerp.9

During flood, at t = 1420 min, the sediment is almost homogeneously distributed over the water column, whereas at t = 1780 min, around MEV, the concentration profile is highly stratified.

As the turbulence intensities vary over the tidal cycle, floc size, hence settling velocity, is expected to vary over the tidal cycle. This is indeed the case, as depicted in Figure 12.1, showing the time variation in settling velocity, measured by Van Leussen.5 Such a variation in floc size, hence settling velocity, was also computed with the 1DV-model, as shown in Figure 12.14, though the time phase of ws does not match the measured pattern exactly. The absolute value of the computed settling velocity, however, agrees very well with the measurements. The large value of ws around slack water explains the rapid settling of the sediment around slack water (t = 1500 min), as observed during the measuring campaign and predicted with the 1DV-model. The values of ws around slack water are limited (i.e., much smaller than the equilibrium values) by the large flocculation time Tf around slack water, as shown in Figure 12.15: Tf increases to many hours around slack water, as a result of which equilibrium floc sizes cannot be achieved. The very large flocculation time

Concentration, c (g/l)

Concentration, c (g/l)

FIGURE 12.13 Computed and measured vertical concentration profiles at (a) t = 1420 minutes and (b) t = 1780 minutes. (Subtract 1000 min from time axis to compare with Figure 12.11.)

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