through the surf zone and subsequently release as free waves is poorly modeled. 



Gallagher (1971) expanded on the Longuet-Higgins and Stewart (1962) two- 

 dimensional mechanism of forcing by wave groups (the BLW model) to three 

 dimensions by considering forcing from nonlinear interactions of incident swell 

 pairs approaching the shore at oblique angles. These triad interactions provide a 

 mechanism for generating infragravity waves with nonzero alongshore wave 

 numbers, allowing for resonant excitation of edge waves. Since this model 

 neglects processes inside the surf zone it is regarded as an offshore generation 

 mechanism. Bowen and Guza (1978) verified in the laboratory the importance 

 of resonance on edge wave growth, concluding that resonant response was strong 

 even through incident wave breaking. They suggested that due to resonance, 

 edge waves would dominate the infragravity motions in the nearshore. 



The generation mechanism proposed by Symonds, Huntley, and Bowen 

 (1982) considers long waves produced from a time- varying breakpoint position 

 induced by wave groupiness on a plane sloping beach. Their model predicts free 

 long waves propagating seaward from the breaker zone, and shoreward, 

 reflecting from the beach and traveling seaward. This model was based on 

 spectra of bichromatic incident waves with periodic wave groups and excluded 

 incident BLW forcing. Kostense (1984) verified the model in a wave flume and 

 found good qualitative agreement. Since only two dimensions were considered 

 and shoreline resonance was not addressed, this models the generation of leaky 

 waves. 



List (1992) proposed a finite difference model for the generation of two- 

 dimensional surf beat (leaky waves) using random wind waves as input. 

 Contributions from both bound and breakpoint forced long waves were 

 simulated in the time domain. His analysis of data from the DUCK85 

 experiment determined that the primary forcing was associated with bound long 

 waves, which were amplified and strongly modified in the surf zone before 

 reflecting from the shoreline as a leaky wave. He believed that the lower 

 contribution of breakpoint forced waves, roughly half that of the bound wave, 

 was due to radiation stress gradients in the breaker zone associated with 

 breakpoint-forced long waves being transient. These gradients would have 

 alternating signs within the surf zone that change at the group period, resulting in 

 nonstationary forcing. In comparison, seaward of the surf zone, gradients from 

 bound long waves would have a constant sign, allowing continual forcing of long 

 waves into the breaker zone. 



Schaffer (1990) extended this mechanism of breakpoint forced waves to three 

 dimensions thus allowing for the generation of edge waves. In addition, Schaffer 

 and Svendsen (1988) included incident BLW forcing and the propagation of 

 wave groupiness through the surf zone. 



Lippmann, Holman and Bowen (1997) also formulated a theoretical surf zone 

 mechanism for driving edge waves based on variations in the breakpoint location 

 for a plane sloping beach (following Symonds, Huntley, and Bowen 1 982). 



Chapter 2 Infragravity Wave Dynamics 1 5 



