Long-term and post-storm dynamic patterns of the subtidal rhythmic morphology along the East Frisian island coast, Germany

Abstract

This report evaluates the dynamic pattern of the subtidal, longshore-rhythmic morphology along the East Frisian barrier island coast. Analysis of high-resolution sounding charts from the shoreface of Spiekeroog Island, which span a period of 37 years, leads to the following: (a) the alongshore spacing as well as the eastward increasing shore-normal orientation of the channels of the morphology were markedly constant over time; (b) the main pattern of migration of the channels was rotational about well-defined nodal points, rather than translational; (c) four rotational patterns are distinguished, two of which are symmetric, i.e. the seaward and shoreward segments (relative to a nodal point) of the channel are characterized by a similar sense (clockwise or anti-clockwise) in rotation, whereas the two asymmetric patterns display opposite rotation at both segments; (d) the sense and degree of channel rotation showed no time-dependent progression; (e) the frequency of the clockwise angular displacement > 20° was higher than that of the anti-clockwise counterpart, and (f) the Mode 1 rotational pattern, in which both the seaward and shoreward segments of the channel depict a clockwise rotation, was found to be typical of, but not exclusively associated with, storm conditions in which the storm surge height exceeded 3 m. These observations are inconsistent with the dynamic behaviour of a flow-transverse (sand wave or dune) morphology. The channels of the morphology are considered to represent channels of storm rip-currents. The regularity in the alongshore spacing of the channels suggests an edge-wave control. However the quality and type of hydrodynamic data required to verify the above assertion are presently unavailable. In the rock record, the channels would be best recognized in laterally-extensive, paleocoast-parallel vertical sections as regularly-spaced channel-fill sequences, in which epsilon crossbeds are lacking; seaward-dipping crossbeds may be widespread or rare depending on environmental conditions prior to preservation.