Sediment Transport in the Swash Due to Obliquely Incident Wind-Waves Modulated by Infragravity Waves

Swash is described by obliquely incident wind-waves superposed on, and interacting with, long standing waves. The wind waves of the inner surf zone are modulated in amplitude, wavenumber and direction by the relatively slowly varying depth changes caused by the long waves. The energy in the wind-wave band is enhanced by side-band growth at the sum and difference frequencies of the short and long waves (order 10 percent at the shoreline). A non-vanishing steady longshore current is found at the shoreline (swash) as a result of the interaction between incident and infragravity waves. Sediment transport in the swash can then be considered as a stirring of the sediments by the combined energetics of the short and long waves and a net transport due to the mean longshore current induced by their interaction. Including the infragravity waves in the alongshore sediment transport formulation gives qualitative comparisons with field measurements in the swash.