16. Gallagher (1971) suggested the possibility of second-order non- 

 linear forcing of long waves through the difference frequency and longshore 

 wave number interaction of wind waves. This mechanism permits forcing from 

 the full spectrum of directionally distributed wind waves. Thus, it allows 

 for the generation of directionally distributed long waves and as such is a 

 mechanism that can generate edge waves. Since edge wave energy is contained 

 in the nearshore with no leakage, even a weak coupling with wind waves could 

 build up large edge waves. 



Field Evidence 



17. Field studies have clearly identified both bounded long waves and 

 low-mode edge waves in the very nearshore. Leaky and/or high-mode edge waves 

 have also been observed, but not resolved. During the 1970' s, cross-shore 

 arrays of sensors were used to compare the observed cross-shore structure of 

 elevation or velocity with theory (Figure 3). Suhayda (1974) used a run-up 

 meter, wave staff, and two pressure sensors in a cross-shore line to first 

 demonstrate that the cross-shore structure of infragravity energy was in good 

 agreement with the theoretical cross-shore standing structure of leaky 

 waves. Guza (1974) pointed out that the cross-shore structure of leaky waves 

 and high-mode edge waves are almost indistinguishable for the first few zero 

 crossings. To separate edge waves from leaky waves using a cross-shore array, 

 sensors would have to extend considerably beyond the last antinode of the the- 

 oretically highest edge wave mode that can be trapped (Figure 3). The added 

 complications of partial reflection at the shoreline and the presence of sev- 

 eral modes made cross-shore arrays generally unsatisfactory for distinguishing 

 edge wave modes and leaky waves (Snodgrass, Munk, and Miller 1962). Neverthe- 

 less, Suhayda and others that followed (Huntley 1976; Sasaki, Horikawa, and 

 Hotta 1976; Sasaki and Horikawa 1978; Huntley, Guza, and Thornton 1981; Holman 

 1981) clearly demonstrated that in the nearshore the infragravity band was 

 dominated by waves that have standing structure in the cross shore. 



Longshore Array Data 



18. Edge waves were most convincingly observed in the nearshore using a 

 longshore array of bidirectional current meters in the surf zone (Huntley, 



12 



