Wave and tidal current sorting of shelf sediments southwest of England (summary only)

Abstract

The relative roles of waves and tidal currents in sediment transport processes on the continental shelf off Lands End, southwest of England, are discussed in the light of ( a ) sediment grain size and boundary layer measurements in tidal currents, ( b ) regional variation in sediment parameters in relation to peak tidal and wave-induced currents, and ( c ) photographic and television observations of bedforms. ( a ) The sediments in this temperate study area are mainly zoogenic sands. The size parameters have been determined by settling velocity in a specially developed sedimentation tower. The average median diameter of sediments from 144 stations is dm^ — 1.40^ (medium grade sand), with a standard deviation of 0.43^. The linear bottom current which will just move this range of particle size must attain a drag velocity ( U *) of 2.37 cm s - 1 . This value is exceeded, only slightly, by the maximum drag velocity of 2.6 cm s - 1 measured in a bottom tidal current in the area. Thus, tidal currents alone are just competent to move the sediments. This movement is intermittent and limited to certain states of the tide.