5 Summary 



Measurements of nearshore waves and currents have shown that a significant 

 amount of the total energy can be contained in the infragravity band (nominally 

 0.003 to 0.05 Hz). On highly dissipative beaches the infragravity wave variance 

 often dominates over energy in the incident wave band (0.05 to 0.3 Hz), 

 particularly inside the surf zone and in the runup. Infragravity waves are surface 

 gravity waves that are classified as either free waves, which are nonlocally 

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

 waves generated by wave groups and are bound to the group. These forced 

 waves are also referred to as bound waves since they are bound to the group and 

 travel at the group velocity. There are two types of free waves, edge waves that 

 are refractively trapped to the shoreline and leaky waves that escape out to deep 

 water upon reflection from the shoreline. Edge and leaky waves are free surface 

 gravity waves, free to propagate away from the generating source. 



Infragravity waves are generally believed to be produced by the nonlinear 

 interactions of incident wind-generated surface gravity waves in the 0.05 to 0.03- 

 Hz band. Longuet-Higgins and Stewart (1962) showed theoretically that group- 

 forced waves could be generated by variations in radiation stress (momentum 

 flux tensor). Gallagher (1971) extended the work of Longuet-Higgins and 

 Stewart (1962) from two to three dimensions and described wave triad 

 interactions approaching the shore at oblique angles. Gallagher's model is the 

 only spectral edge wave generation model, and it is consistent with observations 

 of positive correlation between edge wave and incident wave propagation 

 directions. Field investigations of infragravity energy have also verified a 

 positive correlation between incident wave energy and nearshore infragravity 

 wave energy (e.g., Guza and Thornton 1982; Elgar et al. 1992; Oltman-Shay et 

 al. 1992). Another proposed mechanism considers a forcing of low-frequency 

 waves by a group-modulated variation in the breakpoint location (Symonds, 

 Huntley, and Bowen 1982; Schaffer and Svendsen 1988; Schaffer 1990). Based 

 on field observations (e.g., List 1992), it is reasonable to assume that both 

 mechanisms have a role in the production of low-frequency motions in the 

 nearshore region. Clearly, more research is needed to improve our 

 understanding of infragravity wave generation processes. 



4g Chapter 5 Summary 



