PART III: GENERATION MECHANISMS 



12. Basic to the understanding of infragravity energy is its associ- 

 ation with the incident wind waves. Munk (1949) and Tucker (1950) were not 

 only the first to observe these low-frequency motions in the nearshore, but 

 were also the first to propose such a link. They noticed significant corre- 

 lations between the fluctuation of wind wave heights (groupiness) and the 

 infragravity motions. (The term "surf beat" is often used in place of infra- 

 gravity. Its origins are with Munk (1949), who coined the term to describe 

 this association with wind wave groups or beats.) Field evidence has contin- 

 ued to support this relation. For instance, infragravity energy has repeat- 

 edly been shown to increase with the incident wind wave energy (Guza and 

 Thornton 1982; Sallenger and Holman 1984; Holman and Sallenger 1985). 



13. Field observation suggests the presence of both freely propagating 

 and bounded surface gravity waves in the infragravity band. Bounded long 

 waves are directly coupled to the local incident wind waves, whereas free 

 (edge and leaky) waves are decoupled from its wind wave forcing. Both are 

 probably generated through mechanisms that transfer energy from the groupy 

 structure of the wind waves. However, the free waves most likely have enough 

 nonlocal and/or broad-banded wind wave contribution to their total variance to 

 decouple them from the local wind waves. 



14. Longuet-Higgins and Stewart (1962) first suggested a generation 

 mechanism for bounded long waves. They showed through radiation stress argu- 

 ments that there is depression of the mean sea level under groups of high 

 waves and a corresponding rise under low waves. This second-order bounded 

 wave travels with the groups at the group velocity of the wind waves. They 

 further speculated that it became a free wave, traveling back out to deep 

 water upon reflection at the breakpoint. However, this was not theoretically 

 justified. 



15. A mechanism for generating free, long waves was proposed by 

 Symonds, Huntley, and Bowen (1982). In a two-dimensional model of the surf 

 zone, they demonstrated that the time-varying position of the breakpoint due 

 to groupy structure could cause a concomitant variation in setup and generate 

 freely propagating long waves. They predicted both seaward and shoreward 

 propagation of the wave with the shoreward wave reflecting at the shoreline, 

 thus setting up a standing wave in the surf zone. 



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