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| Titel |
Patch behaviour and predictability properties of modelled finite-amplitude sand ridges on the inner shelf |
| VerfasserIn |
N. C. Vis-Star, H. E. Swart, D. Calvete |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 15, no. 6 ; Nr. 15, no. 6 (2008-12-08), S.943-955 |
| Datensatznummer |
250012810
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| Publikation (Nr.) |
 copernicus.org/npg-15-943-2008.pdf |
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| Zusammenfassung |
| The long-term evolution of shoreface-connected sand ridges is investigated
with a nonlinear spectral model which governs the dynamics of waves,
currents, sediment transport and the bed level on the inner shelf. Wave
variables are calculated with a shoaling-refraction model instead of using a
parameterisation. The spectral model describes the time evolution of
amplitudes of known eigenmodes of the linearised system. Bottom pattern
formation occurs if the transverse bottom slope of the inner shelf, β,
exceeds a critical value βc. For fixed model parameters the
sensitivity of the properties of modelled sand ridges to changes in the
number (N−1) of resolved subharmonics (of the initially fastest growing
mode) is investigated. For any N the model shows the growth and subsequent
saturation of the height of the sand ridges. The saturation time scale is
several thousands of years, which suggests that observed sand ridges have not
reached their saturated stage yet. The migration speed of the ridges and the
average longshore spacing between successive crests in the saturated state
differ from those in the initial state. Analysis of the potential energy
balance of the ridges reveals that bed slope-induced sediment transport is
crucial for the saturation process. In the transient stage the
shoreface-connected ridges occur in patches. The overall characteristics of
the bedforms (saturation time, final maximum height, average longshore
spacing, migration speed) hardly vary with N. However, individual time
series of modal amplitudes and bottom patterns strongly depend on N,
thereby implying that the detailed evolution of sand ridges can only be
predicted over a limited time interval. Additional experiments show that the
critical bed slope βc increases with larger offshore angles of wave
incidence, larger offshore wave heights and longer wave periods, and that the
corresponding maximum height of the ridges decreases whilst the saturation
time increases. |
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