Discussion And Conclusions

We have developed a simple model for turbulence-induced flocculation of cohesive sediment. This model predicts floc sizes, settling velocities, and fluid mud concentrations in agreement with observations in natural systems. We predict large flocculation times, that is, of the order of many hours and more, at low sediment concentration and low turbulence intensity (turbulent shear stresses). These large flocculation times explain the evolution of floc size as a function of shear stress, as hypothesized by Dyer6 (e.g. Figure 12.2): at low shear stress, the cohesive sediment particles have insufficient time to form large flocs.

This implies that for field conditions at low concentration, as met in many rivers and coastal areas, the floc size is governed by near-bed hydrodynamic processes only. Only at higher concentrations, as often encountered in estuaries, variations in floc size over the water depth, as sketched in Figure 12.3, are expected.

Under such high concentrations, large temporal variations in floc size are also expected (e.g. Figure 12.1 and Figure 12.14). In particular, during slack water, floc size and settling velocity are expected to grow significantly. This growth explains the rapid settling around slack water as often observed in turbid environments (e.g. Figure 12.11). However, their growth is limited by a limited residence time of these flocs in the turbulent water column. In other words, around slack water, no equilibrium conditions are to be expected, contrary to the conditions around MFV (and MEV), when flocculation times are small and equilibrium can be attained.

Sediment concentrations in fluid mud layers are also affected by history effects. The fluid mud forming flocs settling from the upper part of the water column have insufficient time to grow to the equilibrium values that might be expected within the fluid mud layer. Moreover, the vertical density gradients induced by the settling sediment decrease turbulence intensity, augmenting the effect of floccula-tion time. The large sediment-induced stratification observed in the Ems River is probably responsible for the very large flocculation times predicted around slack water.

These conclusions imply that at low concentrations, floc sizes throughout the water column are in equilibrium with the near-bed processes. Under high concentrations, equilibrium is expected only around MFV and MEV. Around slack water, the flocs can have almost any size, depending on the local hydrodynamic conditions and history effects. This means that measurements of floc size, for example, settling velocity can only be interpreted properly if the hydro-sedimentological conditions and their history are measured as well.

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