EurOCEAN 2000 - Vlaams Instituut voor de Zee

eal estuaries ( Tamar, Loire and Weser). Data to set-up and calibrate the model applications
are stored in the database. From the experience with the large-scale applications feed back has
been produced towards the process module development and their parameterisations.
RESULTS
Sediment - turbulence interaction
For the numerical modelling of sediment - turbulence interaction, the most commonly used
engineering turbulence models have been used, i.e. the Prandtl mixing length (PML) and the kε
model.
Suspended sediment particles cause damping of turbulent energy in the flow. Traditionally, this
effect is parameterised by the use of semi-empirical damping functions, which are applied to
correct the turbulent eddy viscosity (in the PML model) and the sediment mixing coefficient
(or eddy diffusivity) for neutral conditions. The k-ε model includes the buoyancy effect
explicitly but still needs the damping functions in the bed boundary conditions and the
buoyancy term. Data on turbulence damping in stratified flows from the literature have been
reanalysed, together with numerical data generated with the k-ε model. Based on these results
and on theoretical considerations, new damping functions have been proposed and tested.
High density gradients at the bed result in an apparent reduction of the bottom roughness with
consequently higher transport and erosion rates than expected when the model would not
account for these buoyancy effects in the bed boundary conditions. A new bottom boundary
treatment method has been proposed, which yields the correct bed shear stress.
When a flowing suspension decelerates during a tidal cycle, hindered settling may lead to the
formation of a two-layer stratified flow with a distinct density interface, the lutocline.
Turbulence can be completely suppressed at this interface, as a result of which no sediment can
be suspended in the upper layer, increasing the amount of sediment in the lower layer further.
Hence a snow-ball effect occurs resulting in a collapse of the turbulence field and the vertical
concentration profile. Under certain conditions this interface becomes unstable, resulting in
internal waves which generate new turbulence and mixing across the lutocline. It has been
proposed to model this turbulence generation in a parameterised form as an additional eddy
viscosity. An experimental and theoretical study was carried out to establish the effects of nonlocally
produced turbulence on the mixing
Ratio of the mixing to the momentum damping function (or normalised inverse
Schmidt number) as a function of the gradient Richardson number (symbols =
various data from the literature, fine line = Munk & Anderson (1948), dashed
line = Ellison (1956), dotted line = Kranenburg (1999), thick line = new
proposal)
Fs/Fm
1
0.8
0.6
0.4
0.2
0
0.001 0.01 0.1 1 10 100
Ri
395