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Abstract
The present thesis considers undulations on sandy shorelines. The aim of the
study is to determine the physical mechanisms which govern the morphologic
evolution of shoreline undulations, and thereby to be able to predict their
shape, dimensions and evolution in time. In order to do so a numerical model
has been developed which describes the longshore sediment transport along
arbitrarily shaped shorelines. The numerical model is based on a spectral
wave model, a depth integrated flow model, a wavephase resolving sediment
transport description and a oneline shoreline model.
First the theoretical length of the shoreline undulations is determined in
the linear regime using a shoreline stability analysis based on the numerical
model. The analysis shows that the length of the undulations in the linear
regime depends on the incoming wave conditions and on the coastal profile.
For larger waves and flatter profiles the length of the undulations increases.
Secondly the evolution of the shoreline undulations from the linear regime
to the fully nonlinear regime is described using the numerical shoreline evolution
model. In the fully nonlinear regime down drift spits and migrating
shoreline undulations are described by the model. The shoreline evolution
is considered for both constant and varying wave forcing and both periodic
model domains with a single undulation as well as periodic model domains
with multiple undulation are considered. Three different shoreline shapes
are found depending on the wave conditions and the coastal profile: undulations
with no spits, undulations with flying spits and undulations with
reconnecting spits. It is further shown that the evolution of the shoreline
undulations is governed mainly by the angle between the shoreline and the
incoming waves and the curvature of the shoreline.
Thirdly the shoreline evolution model is tuned to two naturally occurring
shorelines. On one of the shorelines, the west coast of Namibia, the shoreline
model is able to describe the observed shoreline features in both a qualitative
and quantitative way. The model overpredicts the scale of the feature and
under predicts the migration speeds of the features. On the second shoreline,
the shoreline model predicts undulations lengths which are longer than the
observed undulations.
Lastly the thesis considers field measurements of undulations of the bottom
bathymetry along an otherwise straight coast at the DanishWest Coast. Two bathymetric datasets and two time series of wave measurements are
used in order to determine the following properties: The offshore extent
of shoreline undulations, the amount of sediment transported alongshore in
the shoreline undulations, the relationship between the shoreline undulations
and longshore bars and the relationship between the morphology and
the hydrodynamics. In one of the data sets the shoreline undulations are
well correlated with undulations on the depth contours between 5 m and
+2 m relative to mean sea level. An analysis of the wave climate shows
that this shoreline is right at the limit between a stable and an unstable
shoreline.
Original language  English 

Publisher  DTU Mechanical Engineering 

Number of pages  284 
Publication status  Published  2011 
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Projects
 1 Finished

Numerisk modellering af kystmorfologi
Kærgaard, K. H., Fredsøe, J., Deigaard, R., Hjelmager Jensen, J., Falqués, A. & Larson, M.
Technical University of Denmark
01/02/2008 → 21/12/2011
Project: PhD