Oscillatory infragravity wave contribution to surf zone sediment transport: the role of advection

Field measurements reported in the literature demonstrate that suspended sediment transport due to infragravity wave motions can sometimes be very large and dominate the net sediment transport at a given measurement location within the surf zone. At other times, however, this transport component is insignificant. Moreover, infragravity transport directions are inconsistent across the shoreface. This study tests the hypothesis that net sediment transport caused by infragravity waves may be due to advection (at infragravity frequencies) of sediment that has been suspended by breaking wind/swell waves. It is shown that infragravity sediment transports are onshore directed at the landward side of relative (incident) wave height maxima, and offshore directed at the seaward side of such maxima. If a longshore infragravity wave structure exists, such as in the case of standing edge waves, the advection process can create a perturbation of the bar crest and/or generate a crescentic bar. These results provide support for the template model for crescentic bar formation, first proposed by Bowen and Inman (Bowen, A.J. and Inman, D.L., 1971. Edge waves and crescentic bars. J.Geophys.Res., 76, 8662-8670) although the template mechanism is distinctly different from the template originally suggested and morphodynamic feedbacks between the evolving bar perturbations and the forcing mechanisms are important.