Hydrodynamics of Infragravity Waves in Surf and Swash Zones

In a train of irregular wind waves, the high and low waves usually appear in groups. The existence of the wave groupings will induce secondary long waves with a period corresponding to that of the groupings. The typical period of the long waves is around 100s. The long waves, sometimes called infragravity waves. Formerly, wind waves were considered to be the main external forces of beach erosion. Although the larger waves break further offshore making the surf zone wider, the wave heights in the inner surf zone remain the same as those during non-storm conditions because the wave heights after breaking are limited by the water depth. In contrast, infragravity waves do not break in the surf zone and reach their maximum height at the shoreline. Thus, the infragravity waves are now considered to play a significant role on the beach erosion. The present study investigates the coupling field of grouped wind waves and infragravity waves propagating over a plane slope beach and a bar-type beach. Based on the calculated wave fields, the contributions of the wind waves and infragravity waves to the sediment transport efficiency are discussed.