EurOCEAN 2000 - Vlaams Instituut voor de Zee

ISVA, turbulence under spilling breakers and broken waves has been investigated by wave
flume experiments.
Topic 1.2c. Influence of breaking waves on sediment transport. The work undertaken by
DHI/ISVA under Topic 1.2a (see above), constitutes a key element for this research. The
sediment transport has been included in an advanced Navier-Stokes solver in two-dimensions.
DHI/ISVA will continue the development of the free-surface method for breaking and broken
waves. Also, comparison of the sediment transport rates to experimental results, and
sensitivity analysis of for instance the fall velocity, will be made.
Topic 1.2d. Wave-induced currents. The objective is to develop a 3-D wave-driven current
model. A start has been made in testing the 3D implementation against 2D (laboratory) cases.
Furthermore the present implementation has been combined with a research version which has
on-line sediment transport module included. The wave effects are implemented in a code that
will also be used for morphological modelling of complex bathymetries. Testing of 3D
implementation against 3D cases will be sought. The first comparisons have shown that
especially the streaming effects are not yet included correctly. Research will focus on this
aspect for the rest of the project period.
Topic 2.1a. Morphological modelling of the surf zone without structures. Two research
groups have been working on this topic, namely DHI and DH. The work DHI undertakes
basically deals with vertical structure of motion and associated sediment transport plus
morphological modelling of coastal profiles. A mathematical model has been developed that
describes the 3D distribution of the mean shear
stress, the time varying eddy viscosity, velocity profiles, suspended sediment concentrations
and sediment transport. The sediment transport model has been extended to include irregular
wave trains. A phase resolving model has been developed to simulate the morphological
evolution of a cross-shore beach profile. The depth averaged-hydrodynamics are calculated
from a wave model based on the Boussinesq equations. The Quasi 3D sediment transport
model has been incorporated into a phase averaged morphological model for cross-shore
profile evolution. The morphological model has been used to investigate the morphological
development of beach profiles under the attack of obliquely incident waves. Regarding the DH
study under this topic, the objective is to model the erosion/accretion of dry beaches in a
morphodynamic area model.The principle idea is to extrapolate sediment transport from the
last wet point to the highest dry point of a grid row or column, where transport is assumed to be
a linear function of the height; in this way, profiles can uniformly shift in horizontal direction.
A 1-D version of this approach has been successfully tested. Subsequently, a 2-D
implementation has been carried out which has been tested successfully on the Keta Lagoon
case.
Project 3. Horizontal structure of wave- and breaker-induced motion and
area modelling.
Goal of the project: to investigate the hydrodynamics, sediment transport and morphodynamics
in the surf and swash zone, with focus on the 2DH (two dimensions in the horizontal) view
point. Most important results until now are as follows
• for a realistic longshore current, in presence of eddy viscosity, resonant triads, which can
involve unstable and/or stable modes, can exist and a resonant triad, comprising 3 linearly
stable modes derived from a linear stability analysis, can exhibit explosive instabilities.
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