EurOCEAN 2000 - Vlaams Instituut voor de Zee

microflocs separated at a size of 160 microns was observed to increase with both increasing
concentration and shear stress. This suggests that the influence of concentration on aggregation
is greater than that of shear on floc break-up. The biochemical results suggest that high
carbohydrate levels acts as an adhesive assisting the production of the larger faster settling
macroflocs formed during low concentrations at neap tides. It also appears that the faster
settling macroflocs can selectively scavenge the very small microflocs at a rate faster than that
for the medium sized flocs. Lower organic and chlorophyll-a content sediment is eroded from
the bed at spring tides, reducing the contents observed at neap tides.
A 3D flocculation model in an Eulerian frame, describing turbulence induced aggregation and
floc break-up, has been developed. The flocs are described as self-similar fractal entities. The
model is calibrated against experimental data reported in the literature and deployed to describe
the processes in the turbidity maximum on the Ems estuary (The Netherlands). Observed
variations in vertical concentration distributions could be predicted only when both
flocculation and sediment-induced buoyancy effects are taken into account.
CBS dynamics
In 2D models, the modelling of entrainment is important. From flume experiments is has been
found that
- due to generation of turbulence in the lower, dense CBS layer, material from the upper,
less dense and less turbulent layer is entrained into the lower layer, which thickened
accordingly.
- the entrainment velocity appears to be constant in time, which is consistent with theory.
- a freshly deposited CBS behaves as a viscous fluid
- A relationship of the form E a 1/Ri* was found, in which E is the dimensionless
entrainment rate and Ri* the overall Richardson number.
From grid tank experiments the formation of CBS layers reaching an equilibrium thickness was
observed for different concentration conditions. The time averaged sediment concentration
appears to be uniform in the CBS layer for all concentrations. The turbulent kinetic energy
decreases with increasing distance from the grid. No decay of turbulent kinetic energy was
found for sediment concentrations up to 200 g/l. The flux Richardson number below the
lutocline varies by more than two orders of magnitude when the variations of the settling
velocity versus the concentration was taken into account.
Bed dynamics
Extensive settling column experiments have been used to develop and verify numerical models
of the consolidation process. The relationship between the properties of the settling flocs, the
deposition rates and the properties of the deposited bed (in particular the consolidating density
profile and strength development) have been investigated. Using in situ measurements the
critical shear stress for erosion has been related to other properties.
A 1DV sedimentation-consolidation model, solving the classical Gibson equation in an
Eulerian frame, has been deployed to simulate the hindered settling and consolidation process
in an annular flume. Material functions and strength evolution are described using fractal
theory. The model has been extended to include the effects of sand-mud mixtures in the case of
small sand fractions.
A 2DV bed dynamics model based on the generalised Biot theory, extended to deal with
extremely large deformations and corresponding density changes, has been developed to study
the strength development in cohesive sediment beds during consolidation, fluidisation and
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