Suspended sediment transport in the German Wadden Sea—seasonal variations and extreme events

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.