PREDICTING SUSPENDED SAND CONCENTRATION PROFILES ON A MACRO-TIDAL BEACH

Suspended sand concentration profiles were measured using acoustic bac kscatter on a macro-tidal beach at Whitsand Bay, Cornwall, U.K. over a period of 9 days when wave heights were generally between 0.75 m and 0.25 m and wave periods from 5 to 7 s. Average profiles over periods o f 6 or 14 min were dominantly of the vortex type, reflecting the impor tance of steep bedforms in maintaining the suspended sand profile. Bed form dimensions spanned the range between those occurring under regula r laboratory waves and those observed under field conditions [Nielsen, 1981 (Journal of Geophysical Research, 86, 6467-6472)]. The concentra tion profiles can be defined by a mixing height z(h) and a near-bed co ncentration C-0, in this case the concentration at 2 cm above the sea bed. There was considerable variability in z(h) resulting from the irr egular spacing of the bedforms [Osborne and Vincent, 1994 (Marine Geol ogy, 115, 207-226)], but z(h) showed a significant dependence on the b ed friction defined through the Shields number theta(max)' based on th e skin friction. The variation in the near bed concentration was obser ved to be only weakly dependent on the excess skin friction at the bed , supporting earlier observations that the resuspension parameter gamm a(0) [Smith and McLean, 1977 (in: Bottom turbulence, Elsevier, New Yor k, pp. 123-152)] decreased rapidly as the excess skin friction increas ed. We conclude that a mixing height given by z(h) = 2.2 exp (3.05 the ta(max)') and a reference concentration at 2 cm defined using gamma(0) = 5.25 x 10(-6) (theta(max)')(-1.64) was appropriate to predict the m ean vertical suspension profile on this macro-tidal beach for a wide r ange of low wave conditions.