On the circulation in the East Frisian Wadden Sea: numerical modeling and data analysis

Abstract

In this paper we use a combination of nu- merical modeling and data analysis to gain a better understanding of the major characteristics of the circu- lation in the East Frisian Wadden Sea. In particular, we concentrate on the asymmetry of the tidal wave and its modulation in the coastal area, which results in a com- plex pattern of responses to the sea-level forcing from the North Sea. The numerical simulations are based on the 3-D primitive equation General Estuarine Transport Model (GETM) with a horizontal resolution of 200 m and terrain-following vertical coordinates. The model is forced at its open boundaries with sea-level data from an operational model for the German Bight (German Hydrographic Office). The validation data for our model simulations include time series of tidal gauge data and surface currents measured at a pile in the back-barrier basin of the Island Langeoog, as well as several ADCP transects in the Accumer Ee tidal inlet. Circulation and turbulence characteristics are inves- tigated for typical situations driven by spring and neap tides, and the analysis is focused on dominating tem- poral and spatial patterns. By investigating the response of five back-barrier basins with rather different mor- phologies to external forcing, an attempt is made to elucidate the dominating physical balances controlling the circulation in the individual sub-basins. It is dem- onstrated that the friction at the seabed tends to slow down the tidal signal in the shallow water. This leads to the establishment of flood dominance in the shallowsea north of the barrier islands. South of the islands, where the water volume of the channels at low tide is smaller than the tidal prism, the asymmetry of the tidal signal is shifted towards ebb dominance, a feature which is par- ticularly pronounced at spring tide. At the northern open boundary, the tidal wave propagating from west to east generates a sea-level dif- ference of � 1 m along the boundary, and thereby trig- gers vigorous alongshore currents. The frictional control in the model is located in the inlets, as well as along the northern boundary. The correlation between velocity and turbulent kinetic energy tends to the establishment of a net southward transport, giving theoretical support to the observed accumulation of sediments on the in- tertidal flats. Weak turbulence along the northern shores of the barrier islands and the small magnitude of the residual currents there promote accumulation of sus- pended matter in these areas, although wave action will generally counteract this effect.