Implications of Small-Scale Superimposed Bedforms on the Morphodynamics and Sediment Transport Patterns on the Lower Shoreface in the Southern North Sea

Shoreface connected ridges (length up to a few kilometers, height up to a few meters) on the shoreface of the Island of Spiekeroog in the southern North Sea have been shown to be rather stable over decades and possibly hundreds of years (Flemming and Davis 1994). Recent high-resolution multibeam bathymetric surveys have revealed patterns of small-scale bedforms (length and height up to 10 m and 0.3 m, respectively) superimposed on the ridge-trough morphology across the entire lower shoreface. In general, the flood current velocities, measured at 1 m above the bottom are as much as 47% higher than ebb counterparts (Antia et al., 1995). Exceptions are the landward flanks of and troughs between the shoreface connected ridges where higher ebb velocities have been recorded. The superimposed megaripple-scale bedforms imply a net shoreward sediment transport on the seaward flanks of the shoreface connected ridges at a low angle to the ridge crest. Sediment reaching the crest and being pushed over to the landward side is then transported seaward by occasionally dominating ebb currents there. Storm events, having generally an erosive effect on the shoreface, tend to increase the amplitude of the shoreface connected ridges, i.e. amplify the morphodynamic feedback mechanisms. Recovery of the seabed after ordinary winter storms occurs in time scales less than a month. Hence, the ridge maintenance seems to be controlled by the fair-weather tidal currents that are competent to transport fine to medium sands during a considerable part of the tidal cycle. The suggested sediment transport circulation mechanism is consistent with the long term stability of the shoreface connected ridges as well as high sediment mobility on the seabed. References Antia, E.E., Flemming, B.W. and Wefer, G. 1995. Calm-weather spring and neap tidal current characteristics on a shoreface-connected ridge complex in the German Bight, southern North Sea. Geo-Marine Letters 15, 30-36. Flemming, B.W. and Davis, R.A. Jr. 1994. Holocene Evolution, Morphodynamics and Sedimentology of the Spiekeroog Barrier Island System (Southern North Sea). Senckenbergiana maritima 24, 117-155.