LONG-TERM AND POST-STORM DYNAMIC PATTERNS OF THE SUBTIDAL RHYTHMIC MORPHOLOGY ALONG THE EAST FRISIAN-ISLAND COAST, GERMANY

This report evaluates the dynamic pattern of the subtidal, longshore-r hythmic morphology along the East Frisian barrier island coast. Analys is 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 rotat ional 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 o pposite rotation at both segments; (d) the sense and degree of channel rotation showed no time-dependent progression; (e) the frequency of t he clockwise angular displacement > 20 degrees 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 depi ct a clockwise rotation, was found to be typical of, but not exclusive ly associated with, storm conditions in which the storm surge height e xceeded 3 m. These observations are inconsistent with the dynamic beha viour of a flow-transverse (sand wave or dune) morphology. The channel s of the morphology are considered to represent channels of storm rip- currents. The regularity in the alongshore spacing of the channels sug gests an edge-wave control. However, the quality and type of hydrodyna mic data required to verify the above assertion are presently unavaila ble. In the rock record, the channels would be best recognized in late rally-extensive, paleocoast-parallel vertical sections as regularly-sp aced channel-fill sequences, in which epsilon crossbeds are lacking; s eaward-dipping crossbeds may be widespread or rare depending on enviro nmental conditions prior to preservation.