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 collecte d from the 300 km(2) 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 sedi ments 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 un der the occasional influence of waves. The mobile sands are commonly bedfor med and rest on a highly backscattering lag. The flanks below - 140 m mainl y exhibit transverse tidal dunes and sand ribbons, whereas the top of the b ank mostly displays discontinuous sand patches and wave ripples. The sand p atches are interpreted as the remnants of tidal bedforms reworked by waves. Calculations of the threshold of motion of four modal grain sizes under va rious conditions at the seabed show that tidal bedforms are active during s pring tides sometimes associated with waves, whereas only the largest annua l waves may explain the observed wave ripples, As in active tidal banks, th ere is a reversal of tidal bedload transport from one bank side to the othe r. The sedimentation rate is very low on the Kaiser bank. In addition, ther e is a loss of sediment at both bank extremities and the tidal bedload cros s-bank transfer is very small, probably controlled by shelf residual curren ts and long-term drift of sediment resuspended by waves. However, the prese nt-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. (C) 1999 Elsevier Science B.V. All rights reserved.