The role of density gradients on tidal asymmetries in the German Bight

Abstract

The dynamics of the German Bight associated with river plumes and fresh water intrusions from tidal flats have been studied with numerical simulations. The horizontal and vertical patterns of the M2, M4 and M6 tides revealed complex distortions along the bathymetric channels connecting the coast and the open sea. A major focus was on the surface-to-bottom change in tidal asymmetries, which provides a major control on draining the tidal flats around the Elbe and Weser River mouths. Comparisons between baroclinic and barotropic experiments demonstrated that the estuarine gravitational circulation is responsible for pronounced differences in surface and bottom asymmetries. These differences could be considered as a basic control mechanism for sediment dynamics. The most prominent area of tidal distortions, manifested by a delay of the tidal wave, was located between the estuarine turbidity maximum and the estuarine mouth north of Cuxhaven. This area was characterized by the strongest periodic convergence and divergence of the flow and by the largest salinity gradients. The enhancement of the gravitational circulation occurred during the transition between spring and neap tides. The large-scale dynamics and small-scale topographic features could impact the sediment distribution as there was a marked interplay in the channels between stratification and turbulence. Also an explanation has been given for the mechanisms supporting the existence of a mud area (Schlickgebiet) south of Helgoland Island, associated with trapping suspended particular matter.