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

Time-averaged concentration profiles over bedforms, measured using acoustic backscatter (ABS) instrumentation on a macrotidal beach at Whitsand Bay, C ornwall, UK, were dominated by vortex-type suspension profiles. Wave height s were typically in the range 0.25-1.0 m, with wave periods of between 5 an d 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 conc entration (C-ref) models [Smith, J.D., McLean, S.R., 1977. Boundary layer a djustments to topography and suspended sediment. Mem. Sec. R. Sci. Liege 11 , 123-151; Nielsen, P., 1986. Suspended sediment concentrations under waves . Coastal Eng. 10, 23-31], The resuspension coefficient (gamma(0)) showed a strong dependence on the maximum skin friction Shields parameter (theta'). The Smith and McLean [Smith, J.D., McLean, S.R., 1977. Boundary layer adju stments to topography and suspended sediment. Mem, Sec. R. Sci. Liege 11, 1 23-151.] model implies that C-ref(z(0)) increases with excess skin friction at the bed(theta'(excess)), however, field data showed no such correlation and the inclusion of a variable gamma(0) gave poor model predictions that tended to have C-ref(z(0)) decreasing with increasing theta'excess. Referen ce concentrations at the bed (C-0) were determined from the time-averaged c oncentration profiles using regression analysis. The Nielsen [Nielsen, P., 1986. Suspended sediment concentrations under waves. Coastal Eng. 10, 23-31 .] model predicts that Co increases rapidly with the modified skin friction Shields parameter (8,), however, extrapolated values of C-0 showed no depe ndence on theta(r). In addition, model predictions were orders of magnitude higher than extrapolated values of C-0. The influence of the bedforms on t he magnitude of near-bed concentration measurements has been examined. We c onclude that the observed variations in C-ref over different bedforms (vort ex 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 sed iment entrainment. Proc. 3rd Int. Conf. on Coastal Engineering, Venice, 4-9 Oct. 1992, pp. 3202-3215.] and the Vincent and Osborne [Vincent, C.E,, Osb orne, P.D., 1995. Predicting suspended sand concentration profiles on a mac rotidal beach. Cent. Shelf Res. 15, 1497-1514.] models have been compared w ith measured data. Although the predictions of both models were typically w ithin 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. Cent. 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 theta', and model predictions do not rely on be dform data. (C) 1999 Elsevier Science B.V. All rights reserved.