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Suspended sediment transport in the German Wadden Sea—seasonal variations and extreme events
Alexander Bartholomä, Adam Kubicki, Thomas H. Badewien and
Burghard W. Flemming
Ocean Dynamics, Special Issue - WATT
Abstract
The German Wadden Sea (southern North Sea) sediments are composed of both
cohesive and non-cohesive deposits. The spatial distribution patterns are
mainly driven by wind-induced waves and tidal currents. Transport intensity
and duration depend on the hydrodynamic conditions, which vary over time.
In this paper, the transport of suspended sediment was investigated on
seasonal, tidal and hourly time scales in the back-barrier system of
Spiekeroog Island. Long- and short-term data of fair weather periods and
two storm events were investigated based on stationary and mobile measurements
of currents and waves by Acoustic Doppler Current Profiler (ADCP), in situ
particle size and suspended sediment concentration (SSC) measurements by
laser in situ scattering and transmissometry (LISST) as well as wind records.
The ADCP backscatter intensities were calibrated by means of LISST volume
concentration data in order to quantify longer term SSCs and fluxes in the
back-barrier system. Values up to 120 mg l-1 were recorded, but concentrations
more commonly were below 60 mg l-1. The long-term results confirm former
observations of a balanced budget during low-energy (fair weather) conditions
in the study area. In general, SSCs were higher during spring tides than
during neap tides. The data also clearly show the remobilisation of sediment
by tidal current entrainment. The records include two severe storm events,
"Britta" (1st November 2006) and "Kyrill" (18th January 2007). The data
reveal very complex temporal flow and transport patterns. During both storm
events, the export of material was mainly controlled by the interaction of
wind, waves and tidal phase. The typical ebb-dominance occurring during
fair-weather conditions was temporarily neutralised and even reversed to a
flood-dominated situation. During "Kyrill", the wind and high-waves setup
in conjunction with the tidal phase was even able to compress the duration
of two successive ebb cycles by over 70%. Although SSCs increased during both
storms and higher turbulence lifted particle clouds upwards, an export of
suspended matter towards the North Sea was only observed under the conditions
taking place during "Britta". Such fluxes, however, are currently still
difficult to quantify because the backscatter intensity during high energy
events includes a substantial amount of noise produced by the high turbulence,
especially near the water surface.
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