EurOCEAN 2000 - Vlaams Instituut voor de Zee

liquefaction (e.g. induced by wave action). The model allows the implementation of more
realistic constitutive rheological equations (i.e. stress – strain relationships).
With regard to deposition/ erosion the use of a new empirical stress – density relationship has
been proposed which accounts for the fact that no strength is developed below the gelling point
of the mud. The erosion rate parameter is proposed to be a function of the bed surface density.
For deposition the total settling flux is considered, i.e. no critical stress for deposition. In order
to distinguish between the settling sediment, which attaches to the bed and the deposited
sediment which remains mobile and can readily be entrained, the erosion law has been
generalised.
Applied modelling
The goal of the “Applied Modelling” was to provide results using numerical models, including
the knowledge resulting from the theoretical aspects of the project. For this purpose, various
test cases have been defined, in order to test and validate the new formulations, and to compare
the different numerical models. The final goal is to apply the models to the cases of real
estuaries, to show their capability to reproduce actual cohesive sediment phenomena.
The first test case is a one-dimensional vertical case, designed to compare the models regarding
vertical processes, and particularly the modelling of turbulence damping by suspended
sediment. Several sets of conditions for hydrodynamics and sediment have been tested. The
computed results show that stratification and saturation effects are very sensitive to the choice
of damping functions. It appears, looking at the viscosity and diffusivity profiles, that the
influence of the shear velocity at the bottom is an important parameter to make correct
sediment transport predictions. However, theoretical work has shown that the shear velocity is
not correctly estimated by traditional methods when sediment is involved. Therefore a new
formulation has been proposed.
Experimental and theoretical studies of flocculation processes have produced new information,
which has been used to develop parameterisations of the sediment settling velocity. Several
approaches exist, which have been implemented in the one-dimensional model. Comparisons
and sensitivity tests have been carried out. On the other hand, the effects of entrainment of bed
materials, resulting from the instability of the lutocline, have also been examined. A
parameterisation has been established, which has been tested in the numerical models.
Similarly, the results of detailed studies of bed properties have been used to consider
parameterised representations of the bed. There are two main aspects to this: consolidation of
bed deposits and erosion. The model developed for consolidation is a variant of the Gibson
equation based on a fractal representation of the floc structure (Winterwerp, 1999). It has been
tested in the one-dimensional model. Resistance of the bed to erosion is a crucial parameter in
the modelling of cohesive sediment transport, but less well understood. Based on experimental
results and theory new formulations have been proposed and incorporated in the model
parameterisation.
Simulations have been carried out with a second test case, a schematic estuary, considered as a
two-dimensional vertical model. The results will prove the ability of the parameterisation
developed to represent correctly the cohesive sediment processes when including advection
and realistic estuarine processes, such as unsteady tidal hydrodynamic forcing, river
discharges, and stratification due to salinity.
Finally, simulations have been carried out in the cases of three different real estuaries: the
Weser, the Tamar, and the Loire estuaries. To validate the model results, these are compared
with extensive experimental data from the three estuaries, partly collected during the project.
The comparison between the models results and these experimental data show that it is now
possible to predict cohesive sediment processes correctly in real estuaries.
All data have been stored in a data base, which is accessible to the public.
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