Model comparisons were made with forcing outside the surf zone from nonlinear 

 interactions of incident wave pairs (after Gallagher 1971). Conceptually, their 

 development can be likened to a directional wavemaker, with the time and space 

 variation in the breakpoint (determined by modulation scales in the incident 

 wave field) forcing the generation of edge waves through similarly varying 

 gradients in the components of the radiation stress. They concluded, based on 

 their model, that this surf zone mechanism would be 2 to 10 times more 

 important than the offshore forcing. 



Recent observations of infragravity waves outside the surf zone have 

 considerably advanced understanding of the relationship between incident wave 

 conditions, the generation of infragravity waves, and the relative contributions of 

 leaky versus high mode edge waves (Elgar et al. 1992; Herbers, Elgar, and Guza 

 1994; Herbers et al. 1995; Herbers, Elgar, and Guza 1995). This series of papers 

 uses data primarily from an array of 24 bottom-mounted pressure transducers 

 deployed in 13-m water depths approximately 2 km offshore of the Field 

 Research Facility. 



The primary conclusions of Elgar et al. (1992) concerned the relationship 

 between incident wave climate and the bound and free infragravity waves. They 

 found that the total infragravity wave energy was well correlated with the local 

 swell (incident) wave energy, suggesting local forcing was important. Using 

 bispectral analysis they were able to demonstrate that anomalous amplification 

 of infragravity wave energy between 13- and 8-m depths was associated with the 

 growth of the bound wave component of the infragravity band. This growth was 

 most important during storm events and most evident in the high frequency end 

 of the infragravity band. Bound waves provided between 70 and 100 percent of 

 the storm infragravity wave energy in 8-m depths and between 30 and 50 percent 

 at the deeper 13-m site. During low wave conditions less that 10 percent of the 

 infragravity wave energy was in the form of bound waves. Ruessink (1998) had 

 similar results from measurements made on a gently sloping multi-bar beach. 



Herbers, Elgar, and Guza (1994) provide a thorough test of the second-order 

 nonlinear theory governing the forcing of bound infragravity waves. Their results 

 confirm that theory can accurately predict the total bound wave contributions, 

 which ranged from 0.1 to 30 percent of the total infragravity energy. As in Elgar 

 et al. (1992), the highest contributions were noted during storms when both the 

 total infragravity energy and incident waves were also large. Less success was 

 had with predicting the bound wave energy levels in specific frequency bands, 

 with errors of up to a factor of 50 observed. They attribute this to statistical 

 uncertainties in the estimates of the incident wave directional spectrum and in 

 the bispectral-based estimates of bound wave energy. 



In a companion paper (Herbers et al. 1995), the sources and variability of free 

 infragravity waves on continental shelves are studied using data from depth 

 ranges of 8 m to 204 m on morphologically variable continental shelves along 

 both the Atlantic and Pacific coastlines of the United States. Consistent with 

 previous studies, they found bound infragravity waves to be important (but not 



1 6 Chapter 2 Infragravity Wave Dynamics 



