Generation of Free Infragravity Waves by Time-Varying Breakpoint with Real Wave and Bathymetry Conditions

Breakpoint forcing (Symonds et al., 1982) is one of the major mechanisms for generation of free infragravity waves in the surf-zone. Baldock (2012) introduced a surf beat similarity parameter based on the normalized beach slope for infragravity waves and the square root of the short wave steepness. Using the surf beat similarity parameter, Baldock (2012) suggests that conditions such as a steep normalized bed slope and short wave steepness were favourable to breakpoint forcing.

Here, the sensitivity of the time-varying breakpoint model to various parameters: beach slope, group frequency, short wave amplitudes, is tested for a plane beach and the results assessed in terms of the surf beat similarity parameter.

Field observations on a barred beach are used to test the model sensitivity to different real spectra on complex beach profiles. The predicted infragravity response is shown to be consistent with the observations suggesting breakpoint forcing is a major mechanism for the generation of free infragravity waves in the surf zone.

References:

Baldock, T. (2012), Dissipation of incident forced long waves in the surf zone-Implications for the concept of "bound" wave release at short wave breaking, Coastal Engineering, 60, 276-285, doi:10.1016/j.coastaleng.2011.11.002.

Symonds, G., D. Huntley, and A. Bowen (1982), Two-dimensional surf beat – Long-wave generation by a time-varying breakpoint, Journal of Geophysical Research-Oceans and Atmospheres, 87(NC1), 492-498, doi:10.1029/JC087iC01p00492.