2 Infragravity Wave Dynamics 



Infragravity Wave Classifications 



As their name suggests, infragravity waves are at frequencies below wind- 

 generated gravity waves, or wind waves. Both wind waves and infragravity 

 waves are gravity waves; the restoring mechanism for their motion is gravity. 

 However, unlike wind waves, infragravity waves are not generated directly by 

 the wind because in deep water they travel faster than the wind. It is generally 

 accepted that infragravity waves are generated through the nonlinear interaction 

 of wind waves, either offshore of the surf zone (Longuet-Higgins and Stewart 

 1964; Gallagher 1971; Sand 1982; Herbers, Elgar, and Guza 1994; Herbers et al 

 1995), and/or in the surf zone through a varying breaker location (from wave 

 groupiness) and concomitant setup (Schaffer and Svendsen 1988; Schaffer 1990; 

 Schaffer and Jonsson 1992; Lippmann, Holman, and Bowen 1997). 



The specific frequency chosen to separate the infragravity and incident wave 

 bands (nominally 0.05 Hz) varies in the literature, generally depending on 

 location. For example, on the east coast of the United States the 0.04- to 0.06-Hz 

 band is often considered to be included in the infragravity band, whereas this 

 transition is often dominated by incident swell waves on the west coast. Howd, 

 Oltman-Shay, and Holman (1991) found in the analysis of SUPERDUCK 

 experiment data (Crowson et al. 1988) that variance from swell below 0.05 Hz 

 was insignificant at this east coast site. Varying the cutoff between 0.04 and 

 0.06 Hz resulted in typical changes of ±5 percent in the total infragravity 

 variance. Okihiro, Guza, and Seymour ( 1 992) determined that a 0.04-Hz cutoff 

 was more appropriate for their observations in the Pacific Ocean, where long 

 period swell is more common. Selection of an appropriate cutoff is generally a 

 trade-off between underestimating infragravity variance to avoid the 

 contamination from low-frequency swell and overestimating infragravity 

 variance by including swell energy. 



Infragravity waves are classified as free waves, which can be nonlocally 

 forced and freely propagating, and forced waves, which are second-order forced 

 waves generated by wave groups and are bound to them (Longuet-Higgins and 

 Stewart, 1964; Figure 2). There are two types of free waves: edge waves that are 



Chapter 2 Infragravity Wave Dynamics 



