Le Hir Guan et al.30

Fluid Mud Concentration (g/l)

Appr. 10


Above cons. bed 0.35 m from bed 0-2 m from bed 1 m thick layers 0.35 m from bed included in the lDV-computations. If relaxation effects are omitted, the computed fluid mud concentrations would be much smaller (e.g. the values of Figure 12.16 at nf = 2) because the floc size would become unrealistically large.

It is further noted that the computed values are also in the range of fluid mud concentrations encountered in estuaries and coastal areas, as reported in the literature, and presented in Table 12.1. These values show a considerable scatter, however. The large values on the Amazon shelf may be attributed to the large flow-induced shear stresses, reducing the floc size significantly, yielding large fluid mud concentrations.

We have shown that the suspended sediment behavior in/near the turbidity maximum in the Ems estuary is characterized by a rapid settling around slack water and a highly stratified concentration profile during ebb. Our analysis of the data with a 1DV-model reveals that this behavior is governed by two processes:

1. Sediment-induced buoyancy effects (not treated here).

2. Flocculation:

(a) The large growth in floc size, hence settling velocity, causes rapid settling around slack water.

(b) The stable floc size, hence settling velocity profile during ebb enhances the effects of stratification.

The flocculation time, in relation to the time scale of the driving forces, is important in limiting the growth of the flocs, hence the settling velocity around low-turbulent slack water conditions. Moreover, these relaxation effects result in fluid mud concentration (gelling concentration) of a few tens of grams per liter.

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