Abstract

Patterns of surficial sediment distribution across shoreface-connected sand ridges located along the barrier-island coast of the southern North Sea generally show a strong affinity between textural parameters and ridge morphology, comparable to that observed in other parts of the world. Thus, the sediment of the upper shoreface and the inner ridge crest consists of fine sand (water depths < 12 m). The troughs between the ridges, by contrast, comprise medium to coarse sands with occasional gravel, whereas the ridge flanks are characterized by fine to medium sands with a coarsening trend towards the troughs (water depths > 12 m). In order to assess the mobility of the sediments and the consistency of the observed textural patterns, two high-resolution multi-beam bathymetric surveys were carried out along selected corridors across a ridge and trough system off Spiekeroog island, located along the southern North Sea coast of Germany, one in 2003, the other in 2007. The bathymetric data of the two surveys show only small-scale topographic changes. These were produced mainly by local changes in the bedform patterns on the inner ridge. The bathymetric surveys therefore document a remarkable morphological stability of the ridges over time periods of several years. Spatial and temporal variations in the distribution of standard textural parameters were assessed on the basis of 95 sediment samples recovered on the same grid in 1989 and 2005. These data show that, for example, the distribution of coarse sand (0–1 phi) became enhanced along the axis of the outer trough, while expanding onto the lower ridge flanks. Current measurements in fair weather demonstrate that the coarse sediments cannot be moved by the local tidal currents. The observed changes in the distribution of coarse sand, and in particular its expansion onto the adjacent ridge flanks, must therefore be associated with wave action, probably in combination with tidal currents. This is supported by an assessment of the entrainment potential of coarse sand (D = 1 mm) by typical waves during storms (T = 8 s; H = 2.5 m) and fair weather (T = 4 s; H = 0.7 m). The results demonstrate that the effective wave base for coarse sand during fair-weather conditions is limited to a water depth of 4.8 m, whereas during storms it exceeds a depth of 26.4 m. The mobility of the coarse sands in the outer trough at water depths > 16 m, and with it the morphodynamics of the ridges as a whole, is thus clearly controlled by the superimposition of storm waves on the tidal current regime.