Tide and Wave Dynamics on A Sand Bank from the Deep Shelf of the Western Channel Approaches

The sedimentary response of deep shelf sediments to the interplay of tides and waves, based on the example of the Celtic Sea sand banks is described. Grab samples and 1500 km of multibeam and side scan data have been collected from the 300 km2 densely surveyed Kaiser bank area. The bank is 140–170 m deep, 30 m high and 60 km long, oriented perpendicular to the shelf edge and nearly parallel to the major axis of the tidal ellipse. Surficial sediments of the bank consist of medium to gravely biolithoclastic sands. They are swept by tidal currents that reach 0.9 m/s 1 m above the seabed, and under the occasional influence of waves. The mobile sands are commonly bedformed and rest on a highly backscattering lag. The flanks below −140 m mainly exhibit transverse tidal dunes and sand ribbons, whereas the top of the bank mostly displays discontinuous sand patches and wave ripples. The sand patches are interpreted as the remnants of tidal bedforms reworked by waves. Calculations of the threshold of motion of four modal grain sizes under various conditions at the seabed show that tidal bedforms are active during spring tides sometimes associated with waves, whereas only the largest annual waves may explain the observed wave ripples. As in active tidal banks, there is a reversal of tidal bedload transport from one bank side to the other. The sedimentation rate is very low on the Kaiser bank. In addition, there is a loss of sediment at both bank extremities and the tidal bedload cross-bank transfer is very small, probably controlled by shelf residual currents and long-term drift of sediment resuspended by waves. However, the present-day action of waves at the bank top is less intense than it was at lower sea levels during the Holocene, as evidenced by seismic studies.