Comparison of time-averaged acoustic backscatter concentration profile measurements with existing predictive models

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Time-averaged concentration profiles over bedforms, measured using acoustic backscatter (ABS) instrumentation on a macrotidal beach at Whitsand Bay, Cornwall, UK, were dominated by vortex-type suspension profiles. Wave heights were typically in the range 0.25–1.0 m, with wave periods of between 5 and 8 s. Mean currents measured at 0.2 m above the bed were generally weak (<0.1 m s −1). Near-bed concentrations were compared with two reference concentration ( C ref) models [Smith, J.D., McLean, S.R., 1977. Boundary layer adjustments to topography and suspended sediment. Mem. Soc. R. Sci. Liege 11, 123–151; Nielsen, P., 1986. Suspended sediment concentrations under waves. Coastal Eng. 10, 23–31]. The resuspension coefficient ( γ 0) showed a strong dependence on the maximum skin friction Shields parameter ( θ′). The Smith and McLean [Smith, J.D., McLean, S.R., 1977. Boundary layer adjustments to topography and suspended sediment. Mem. Soc. R. Sci. Liege 11, 123–151.] model implies that C ref( z 0) increases with excess skin friction at the bed ( θ excess′), however, field data showed no such correlation and the inclusion of a variable γ 0 gave poor model predictions that tended to have C ref( z 0) decreasing with increasing θ excess′. Reference concentrations at the bed ( C 0) were determined from the time-averaged concentration profiles using regression analysis. The Nielsen [Nielsen, P., 1986. Suspended sediment concentrations under waves. Coastal Eng. 10, 23–31.] model predicts that C 0 increases rapidly with the modified skin friction Shields parameter ( θ r), however, extrapolated values of C 0 showed no dependence on θ r. In addition, model predictions were orders of magnitude higher than extrapolated values of C 0. The influence of the bedforms on the magnitude of near-bed concentration measurements has been examined. We conclude that the observed variations in C ref over different bedforms (vortex ripples, lunate ripples, lunate mega-ripples) will need to be taken into account if model predictions of C ref, as a function of the skin friction Shields parameter, are to improve. The mixing height ( L) predictions of the Nielsen [Nielsen, P., 1992. Combined convection–diffusion modelling of sediment entrainment. Proc. 3rd Int. Conf. on Coastal Engineering, Venice, 4–9 Oct. 1992, pp. 3202–3215.] and the Vincent and Osborne [Vincent, C.E., Osborne, P.D., 1995. Predicting suspended sand concentration profiles on a macrotidal beach. Cont. Shelf Res. 15, 1497–1514.] models have been compared with measured data. Although the predictions of both models were typically within a factor of two of L measurements, the Vincent and Osborne [Vincent, C.E., Osborne, P.D., 1995. Predicting suspended sand concentration profiles on a macrotidal beach. Cont. Shelf Res. 15, 1497–1514.] model is believed to be more appropriate for modelling L over bedforms; as field data showed a strong dependence of L on θ′, and model predictions do not rely on bedform data.