Rolf Riethmüller

Impact of Density Gradients on Net Sediment Transport into the Wadden Sea

Abstract This study tests the hypothesis that horizontal density gradients have the potential to significantly contribute to the accumulation of suspended particulate matter (SPM) in the Wadden Sea. It is shown by means of long-term observations at various positions in the Wadden Sea of the German Bight that the water in the inner regions of the Wadden Sea is typically about 0.5–1.0 kg m−3 less dense than the North Sea water. During winter this occurs mostly because of freshwater runoff and net precipitation; during summer it occurs mostly because of differential heating. It is demonstrated with idealized one-dimensional water column model simulations that the interaction of such small horizontal density gradients with tidal currents generates net onshore SPM fluxes. Major mechanisms for this are tidal straining, estuarine circulation, and tidal mixing asymmetries. Three-dimensional model simulations in a semienclosed Wadden Sea embayment with periodic tidal forcing show that SPM with sufficiently high se...

COSYNA, an integrated coastal observation system for Northern and Arctic Seas

The development of an integrated coastal observing system for the German Bight and Arctic Coastal Seas (COSYNA) is one of the focal issues of German marine research in the next decade. A major challenge of COSYNA is to tightly merge data from a dense observational network and modeling via data assimilation. The integrated system will focus on daily-to-weekly processes providing objective measures of uncertainty in the state estimates and forecasts. The creation of a centrally managed, operational, large-scale monitoring and modeling system is expected to strengthen und bundle expertise within Helmholtz Association centres, members of the German marine research consortium and the German Federal Maritime and Hydrographic Agency. The layout of COSYNA is based on the knowledge and open scientific questions of the German North Sea research carried out in the last two decades. COSYNA will intensively use platforms-of-opportunities like North Sea ferries, freights ship and offshore research platforms and wind mills. This participation to externally financed infrastructure of power and broadband communication and maintenance logistics will enable a cost effective construction of the system and maintain its operational mode over a long-term period. Adding to the running governmental monitoring, COSYNA seeks to pinpoint the German role in the international development of marine monitoring and long-term observational strategies in cooperation and agreement with the German Federal Maritime and Hydrographic Agency.

Observational evidence for the general presence of estuarine circulation in the Wadden Sea

ABSTRACT Flöser, G., Nauw, J., Burchard, H., Riethmüller, R., 2013. Observational evidence for the inward transport of suspended matter by estuarine circulation in the Wadden Sea. Observational evidence is presented that corroborates the hypothesis that estuarine circulation represents a generic process in the Wadden Sea, independent of (the amount of) river runoff. Long-term current velocity data from three locations in the Wadden Sea, taken on moored acoustical current meters, onboard anchored ships and across-channel ship-of-opportunity platforms show in all cases features concurring with the predictions of the theory: a tidally residual outflow of Wadden Sea waters in the upper part and a residual inflow of seawater in the lower part of the water column. This may be regarded as a further confirmation for a generic process to import suspended sediments from the German Bight / North Sea into the Wadden Sea against the prevailing concentration gradient.

Detection of salient features in surface current maps from dopplerized X-band radar

Fig. 2 shows the maps of water depth and current magnitude. For consistency the finer bathymetry map was mapped onto the 15m ·15m grid of the RDCP analysis. The large bed dunes are clearly visible and position and orientation of the crests identified in most cases by the structure tensor method. Fig. 3. Digital terrain model with the current edges detected on an RDCP current map overlayed, demonstating the influence of bottom on the surface current (left). ADCP vertical current profile, showing hydrodynamic modulation (right). The current fields show a modulation in magnitude that obviously resembles the sea bed topology in the middle end eastern part (x > 600 m), although the algorithm did identify more salient structures here. This may be due to the fact that the two coherency thresholds for selecting the edges do not consistently mirror the fact that the edges are more pronounced in the digital terrain model than in the current map in the algorithm now implemented. The current modulation is most likely due to the flow continuity through the varying cross sections of the water column. This hdyrodynamic interaction of surface current with bottom topography was already indirectly observed in a modulation of the normalized radar cross section [6]. With the RDCP, however, this effect can be observed directly [7]. It shows up also in the vertical current component (s. Fig. 3). In the western part, the orientation of the current “edges” is nearly perpendicular to the bed dunes. A thorough topological analysis of bathymetry and current features is presently under investigation.