EurOCEAN 2000 - Vlaams Instituut voor de Zee

Modelling results for rippled bed regime
Innovative research modelling work on rippled beds has been started in Year 1. Research
models of differing complexity have been under development in Task Group 2. Research
models have been developed by four institutes (DTU, SINTEF, UGe and UCNW). In addition,
IMAR’s model can be used for rippled as well as plane beds, and DHI’s STP model can also be
used for ripples (though it was developed primarily for plane beds). In Year 1, DTU has
completed modelling work on roughness and friction above vortex ripples using a k-ω
turbulence model, and has made systematic comparisons of the (mean) drag coefficient for
wave+current flows and for currents alone. This model has been used also to make
morphological calculations to predict the profiles of fully developed vortex ripples. At
SINTEF a model has been developed of (mainly) steady flow above undulating beds, based on
the Reynolds equations and standard closures for the turbulence and sediments. This model
also performs morphological calculations. A feature of importance suggested by the solution is
the possible role of suspended sediments in suppressing flow separation above the lee slopes of
bedforms. At UGe in Year 1, computer code to perform a 3D direct numerical simulation of
the oscillatory flow over fixed ripples has been developed and validated. This model will be
used to study sediment dynamics using a Lagrangian approach. Also at UGe a study of the
ripple formation process has been completed using a linear stability analysis, with emphasis on
the role of near-bed drift currents, and the resulting migration of ripples. These latter results
have been compared successfully with preliminary experimental data from the University of
Catania. Finally, at UCNW, a new model for wave+current flows above rippled beds has been
developed. Unlike the models above, this is a simplified 1DV model in which vortex shedding
is represented in the near-bed layer by an analytical eddy viscosity formulation. The near-bed
solution is coupled with a standard turbulence model in the outer layer. This model will be
further validated in Year 2.
As in the case of the flat-bed research models, it is still under discussion how best to use the
rippled-bed models in practical applications, and how the results should be systematised for
this purpose.
Engineering model results
As a good starting point, an intercomparison exercise with field data proposed by HR has been
carried out in order to assess the reliability of existing engineering models and methods. The
results showed an unexpectedly large discrepancy between the different model predictions and
the data applied. The intercomparison exercise was joined by DUT, DH, IMAR, UCNW and
LNH.
Existing models and methods need to be improved in order to achieve the overall objective of
the project which is to make sediment transport rate predictions within a factor 2. This has been
started for example by DH, which has extended its engineering model (TRANSPOR) based on
existing field data sets and large scale experiments, in order to include the effect of the waverelated
suspended transport. The work by IMAR can also be mentioned, which has extended
the Dibajnia and Watanabe formula, in order to improve sediment transport rate predictions for
finer sand.
A framework for a new engineering formula has been proposed jointly by HR and UCNW
based on the parameterization of their own model results. It was decided at the first stage to run
improved models for a given set of input parameters and to produce graphs which could be
used later on to fit the different coefficients of the proposed framework.
An exploitation plan for the results of the project has been completed by EDF. A web page and
leaflets has been prepared by DUT. A questionnaire has been sent out by DHI to target/user
groups: national and regional coastal and sea authorities, harbour/port authorities, consulting
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