Sediment transport processes and morphodynamics on a reflective beach under storm and non-storm conditions

Abstract

New field experiments of sediment transport on the shoreface of a reflective beach (R=0.4–0.6) during pre-storm, storm and post-storm conditions are reported. Data were collected outside the breakpoint and include water surface elevations, cross-shore and long-shore flow velocity, suspended sediment concentrations, bedform dimensions and the morphological response. Instruments included two pulse-coherent ADPs, an ADV, multiple OBS and a pencil-beam sonar. The depth-averaged mean velocity was always offshore and increased by a factor five during the storm, and this increase is attributed to undertow being enhanced by strong backwash plumes from cusp bays. Estimates of cross-shore suspended sediment transport are landward during the pre- and post-storm phases, consistent with the morphological response. During the storm, the shoreface quickly appeared to reach equilibrium, despite energy levels being maintained, with sediment transport rates reducing rapidly after the initial phase of the storm. Streaming in the wave boundary layer was observed and strongest during the pre-storm phase, consistent with the observed shoreward transport. During the storm the near-bed streaming was too small to reverse the increased offshore current, consistent with the observed net offshore transport. The generally small orbital velocity skewness observed on this steep reflective beach suggests that wave reflection may reduce the overall skewness of the velocity, in turn contributing to the stability of reflective beach types.