Field observations of intra-wave sediment suspension and transport in the intertidal and shallow subtidal zones

Abstract

Sandy beaches are highly dynamic due to the transport of sediment by waves and currents. While beach erosion is mainly driven by cross-shore currents, beach recovery is driven by the onshore-directed transport of sediment by incoming short waves. This latter process is complex and often inaccurately predicted by morphodynamic models. This is partly because of the omission of the effects of bed form-induced and surface-generated turbulence on sediment pick-up in these models. In this paper, the phase relationship between the oscillatory flow, turbulence and suspended sediment concentration was investigated based on field measurements obtained beneath shoaling waves, spilling breakers and surf bores. Wave-averaged and intra-wave variability in turbulence and suspended sediment concentration were analysed on a wave-by-wave basis, where individual waves were extracted and then grouped according to relative wave height. Beneath shoaling waves, small but steep wave ripples were shown to affect the phasing of suspended sediment concentration and to cause an offshore-directed short-wave suspended sediment flux. Beneath spilling breakers and surf bores, sediment was mainly stirred under the wave crest and the short-wave suspended sediment flux was onshore-directed. Based on the analysed data, several mechanisms were suggested to cause sediment stirring: friction between the wave orbital motion and the seabed, penetration of surface-generated turbulence towards the seabed, increased bed shear stress at the wave front due to acceleration skewness, and during conditions with strong undertow, wave-current interactions.