Sediment transport associated with morphological beach changes forced by irregular asymmetric-skewed waves

Abstract

The waveshape effects on sediment transport are investigated for cross-shore beach profile changes. This study is based on experiments performed in the Laboratoire des Ecoulements Géophysiques et Industriels wave flume for irregular waves. The interest of such experiments resides in presenting complex combinations of wave skewness and asymmetry in bed load, ripple, and sheet flow regimes. Net sediment transport rates on typical beach morphodynamics are analyzed in regard to wave skewness and asymmetry, undertow, and ripple occurrence. Onshore bar migration is mainly associated with onshore-directed sediment transport, whereas terrace profile and offshore bar formation correspond to offshore sediment transport. As for natural beaches, energetic (moderate) wave climates mostly induce offshore (onshore) sediment fluxes. For a given significant wave height, an increase (decrease) in the wave climate peak period is associated with an increase (decrease) in wave skewness and leads mostly to offshore (onshore) sediment fluxes. The experiments are fully characterized by unsteady behavior. Consequently, several conditions exhibit phase-lag effects where the sediment is mobilized by the wave crest and transported by the following trough, which produces a net offshore transport even for a weak undertow. The presence of ripples clearly contributes to enhance this behavior. An original concept, due to its application to skewed asymmetric irregular waves, presents the important interaction between wave nonlinearities driving the sediment fluxes. The net sediment transport rate under strongly skewed waves is either offshore directed due to phase-lag effects or onshore directed when the wave asymmetry is large enough. Both these mechanisms probably largely contribute to bar formation and migration