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Modeling the impact of wind and waves on suspended particulate matter fluxes in the East Frisian Wadden Sea (southern North Sea)
Karsten A. Lettmann, Jörg-Olaf Wolff and Thomas H. Badewien
Ocean Dynamics, Special Issue - WATT
Abstract
Suspended particulate matter (SPM) fluxes and dynamics are investigated in
the East Frisian Wadden Sea using a coupled modeling system based on a
hydrodynamical model [the General Estuarine Transport Model (GETM)], a
third-generation wave model [Simulating Waves Nearshore (SWAN)], and a SPM
module attached to GETM. Sedimentological observations document that, over
longer time periods, finer sediment fractions disappear from the Wadden Sea
Region. In order to understand this phenomenon, a series of numerical scenarios were formulated to discriminate possible influences such as tidal currents,
wind-enhanced currents, and wind-generated surface waves. Starting with a
simple tidal forcing, the considered scenarios are designed to increase the
realism step by step to include moderate and strong winds and waves and,
finally, to encompass the full effects of one of the strongest storm surges
affecting the region in the last hundred years (Storm Britta in November 2006).
The results presented here indicate that moderate weather conditions with wind
speeds up to 7.5 m/s and small waves lead to a net import of SPM into the East
Frisian Wadden Sea. Waves play only a negligible role during these conditions.
However, for stronger wind conditions with speeds above 13 m/s, wind-generated
surface waves have a significant impact on SPM dynamics. Under storm
conditions, the numerical results demonstrate that sediments are eroded in
front of the barrier islands by enhanced wave action and are transported into
the back-barrier basins by the currents. Furthermore, sediment erosion due
to waves is significantly enhanced on the tidal flats. Finally, fine sediments
are flushed out of the tidal basins due to the combined effect of strong
erosion by wind-generated waves and a longer residence time in the water
column because of their smaller settling velocities compared to coarser
sediments.
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