Modeling the impact of wind and waves on suspended particulate matter fluxes in the East Frisian Wadden Sea (southern North Sea)

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.

A review on substances and processes relevant for optical remote sensing of extremely turbid marine areas, with a focus on the Wadden Sea

“…The positive relation between SPM concentrations and higher wind speeds was found by several researchers (e.g. Badewien et al 2009;Stanev et al 2009;Bartholomä et al 2009;Lettmann et al 2009). However, even when weather conditions were similar, winter SPM concentrations were higher (Grossart et al 2004).…”

“…The variation in tidal level depends on the location: the highest tidal ranges in the Wadden Sea are found in the corner of the German Bight (>3 m) and the least diVerences are found near the islands Texel and Fanø (»1.5 m) (Postma 1982;Dijkema et al 1980). Storm events alter the water level and thus also the water depth in the tidal area (Stanev et al 2009;Lettmann et al 2009). While most South-westerly storms will elevate the water level, strong easterly wind will lower the water level signiWcantly.…”

“…Models of SPM variation in the Wadden Sea including the eVect of location, tide and wind (Van Ledden 2003;Stanev et al 2006Stanev et al , 2007Stanev et al , 2009Lettmann et al 2009) can be used to predict SPM patterns, which can be compared with satellite data. Gayer et al (2006) showed good agreement between their model, in situ data and satellite data in the German Bight, derived with the modelling technique of Pleskachevsky et al (2005).…”

Coupling of wave and circulation models in coastal–ocean predicting systems: a case study for the German Bight

“…In addition to the storm surge, unusually high waves have been measured in the southern North Sea developing on northern North Sea and propagated southward under the influence of strong north winds with a long fetch. The storm Britta has been given particular attention in our analyses for the types of changes that may occur during single event (Bartholomä et al, 2009;Lettmann et al, 2009;Stanev et al, 2011;Grashorn et al, 2015).…”

“…The ocean waves affect not only the sea level but also the currents and mixing, the latter being of utmost importance for the sediment dynamics (Lettmann et al, 2009). Prandle et al (2000) demonstrated the need of accounting for surface waves with a significant wave height larger than one metre in the sediment modelling.…”