Sedimentary Deposits Related to Inlet-Shoreface Storm Flow Interaction in the German Bight

Detailed grain-size studies coupled with data on fair-weather flow direction offshore of Spiekeroog Island (German Bight) were used to evaluate the nature of interaction between storm inlet-ebb currents and their shoreface counterparts. Two primary sedimentary deposits are related to the above interaction. The first is a shore-oblique, WNW-ESE trending, band of high mean settling-velocity (coarse-grained) sediments blanketing the shoreface-connected ridges, and is flanked both landward and seaward by fine-grained sands. Sediments along the axes of the ridge morphology progressively fine north-westward, a direction which is opposite to that of the major storm and mean current in the region. The second deposit is an accumulation of very fine sand at an offshore location which is diametrically opposite to the main inlet-channel axis, and is embedded within slightly coarser sands. It is suggested that: (1) the coarse-grained deposits owe their origin to storm-flow-induced scouring of the inlet-channel floor, and are transported offshore by the inlet jet, largely close to the sea-bed; (2) the shore-oblique orientation of the coarse sediment band reflects the vector resultant of the near-bottom northward-directed inlet jet and the westward-directed shoreface ebb storm-surge; and (3) the inlet-diametric offshore accumulation of fines represents fall-outs of the waning inlet ebb-jet. The increasing veering from shore-normal with offshore distance of the fair-weather shoreface ebb-current (U100) direction is consistent with the above interpretation.