Circulation in the vicinity of a submerged stream groyne under mesotidal impact (East Frisian Islands, Southern North Sea)

Abstract

<p>In this study, the major characteristics of the circulation in the tidal inlet between the East Frisian Islands Spiekeroog and Wangerooge (Harle, Southern North Sea) are depicted by investigating current parameters using in situ Acoustic Doppler Current Profiler (ADCP) transect data. The focus is to reveal flow patterns in the area as a response to the submerged training wall Groyne H (GH) and the mesotidal impact prevailing at the East Frisian Islands. The circulation in the Harle is determined by semidiurnal tides, forcing the water to the southwest (ebb) resp. northeast (flood), thereby surrounding and overflowing the GH. The structure enhances the discharge and velocity in the main channel by blocking two thirds of the inlet horizontally and vertically.  It is assumed that depending on the water level above the groyne crest and the morphology of the inlet, vertical and/or horizontal recirculation cells develop on either the upstream or downstream side. An attempt is made to detect such cells and to give an explanation of the underlying physical processes. Therefore, ADCP transect data were obtained during two tidal cycles performing cross-sectional transects parallel to GH and a cross pattern in the groyne field south of GH. Data analysis particularly targets the computation of flow fields to resolve the circulation on a spatial scale and decompose velocity profiles into their 2D-components. Moreover, flow data is divided into different depth layers comparing surface versus average velocity over the transect to ascertain areas of velocity shearing. The consequences on each downstream side are a) the deceleration of flow in the groyne field by about 30-50 % b) a dynamic eddy at the groyne tip forming a low-velocity zone and c) a vertical recirculation cell rotating towards the groyne sole. For the latter, additional measurements need to be conducted to resolve such a cell in more detail and to analyze a broader area for the presence of horizontal circulation cells. </p> <p> </p>