Predicting suspended sand concentration profiles on a macro-tidal beach

Suspended sand concentration profiles were measured using acoustic backscatter 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 of 6 or 14 min were dominantly of the vortex type, reflecting the importance of steep bedforms in maintaining the suspended sand profile. Bedform dimensions spanned the range between those occurring under regular laboratory waves and those observed under field conditions [Nielsen, 1981 (Journal of Geophysical Research, 86, 6467–6472)]. The concentration profiles can be defined by a mixing height zh and a near-bed concentration C0, in this case the concentration at 2 cm above the sea bed. There was considerable variability in zh resulting from the irregular spacing of the bedforms [Osborne and Vincent, 1994 (Marine Geology, 115, 207–226)], but zh showed a significant dependence on the bed friction defined through the Shields number θ′max based on the skin friction. The variation in the near bed concentration was observed to be only weakly dependent on the excess skin friction at the bed, supporting earlier observations that the resuspension parameter γ0 [Smith and McLean, 1977 (in: Bottom turbulence, Elsevier, New York, pp. 123–152)] decreased rapidly as the excess skin friction increased. We conclude that a mixing height given by zh = 2.2 exp (3.05θ′max) and a reference concentration at 2 cm defined using γ0 = 5.25 × 10−6 (θ′max)−1.64 was appropriate to predict the mean vertical suspension profile on this macro-tidal beach for a wide range of low wave conditions.