The influence of bed forms on reference concentration and suspension under waves and currents

摘要

Hydrodynamic, suspension and bed-form measurements were made 2km off the Dutch coast near Noordwijk aan Zee in similar to 14 m water depth for a period of 32 days in 2003. Tidal currents were just able to suspend sand at the bed at peak spring tide but most suspension and transport occurred as a result of the combination of waves and currents. Burst-average (17 min) sand concentration profiles (C-profiles) from an acoustic backscatter instrument were used to define the (varying) location of the sea-bed, following the method used by Green et al. [Green, M.O., Dolphin, T.J., Swales, A., Vincent, C.E., 1999. Transport of mixed-size sediments in a tidal channel. Coastal Sediments '99, edited by N.C. Kraus, and W.G. McDougal, ASCE, Long Island, New York, pp. 644-6581. Reference concentrations at the sea-bed (CO) and at 1 cm (C-1) were examined in relation to both the hydrodynamic conditions and the type of bed forms present. The C-0 predictive equations of Green and Black [Green, M.O., Black, K.P., Suspended sediment reference concentration under waves: field measurements and critical analysis of two predictive models, Coastal Engineering, 38, 115-141,1999](short-wave ripples) and Nielsen [Nielsen, P., Suspended sediment concentrations under waves, Coastal Engineering, 10, 23-31, 19861(all bed forms; includes ripple steepness), both of which require knowledge of the bed-form type, were not as successful in explaining the variance in our Co data as a regression of Co against the skin-friction Shields parameter theta'(cw) that ignored bed-form type (73% of variance explained). The values of the reference concentration C, were compared with the Lee et al. [Lee, G.-H., Dade, W.B., Friedrichs, C.T., Vincent, C.E., Examination of Reference Concentration Under Waves and Currents on the Inner Shelf, Journal of Geophysical Research, 109, 1-10, 2004] equation which predicts C, from the product of the Shields parameter and the inverse Rouse parameter; 51% of the variance in C, was explained. The shapes of the C-profiles were very variable as reported by Dolphin et al. [Dolphin, T.J., Grasmeijer, B.T.Vincent, C.E., Sand suspension due to waves and tidal flow over short and long-wave ripples and flat beds on the Dutch Coast, in Sand Transport and Morphology of Offshore Mining Pits, edited by L.C. van Rijn, R.L. Soulsby, P. Hoekstra, and A.G. Davies, Aqua Publications, Amersfoort, The Netherlands, 2005.]., particularly over larger and steeper bed forms when the position of the measurements in relation to the bed-form geometry became important. However, a Rouse-shaped profile (in which the sediment diffusivity increases linearly with height) could be fitted (R-2 > 95%) to the lowest 0.2 m of 89% of the suspended sand profiles. The apparent separation of C-profiles, when grouped by bed-form type, is due to the variation in the reference concentration and not to the dependence on the friction velocity, which shows large scatter and no systematic variation with bed-form type. It is concluded that, for most practical applications, reference concentration at I cm can be predicted by Lee et al. [Lee, G.-H., Dade, W.B., Friedrichs, C.T., Vincent, C.E., 2004. Examination of Reference Concentration Under Waves and Currents on the Inner Shelf., Journal of Geophysical Research, 109, 1 -10.] and that the suspended sediment profiles are Rouse-shaped with a shape parameter p of 0.80 +/- 0.25; knowledge of the bed-form type does not, at the Noordwijk site, help to refine either the reference concentration or the shape of the suspended concentration profiles. (C) 2009 Elsevier Ltd. All rights reserved.