PART II: IMFRAGRAVITY WAVE DYNAMICS 

 Long Wave Categories 



4. Originally, the term "infragravity" identified the band of frequency 

 that contains fluid motions falling between those of the wind-generated sur- 

 face gravity waves and astronomical tides. However, it has become more spe- 

 cifically identified with nearshore motions of periods from 30 sec to several 

 minutes (Figure 1). Munk (1949) and Tucker (1950) were the first to observe 

 infragravity motions. Using wave pressure recorders, they found small but 

 measurable motions 300 m offshore with periods of 2 to 3 min. (Later studies 

 have shown that these motions are larger closer to shore.) 



5. Since these early observations, researchers have come to understand 

 that infragravity energy is composed of organized motions in the form of long 

 waves. These waves fall into three categories. They are (a) forced locally 

 by the wind wave groups and bounded to them, traveling at the group velocity 

 of the wind waves (bounded long waves); (b) forced nonlocally by the wind 

 waves in shallow water, freely propagating and refract ively trapped toward the 

 shoreline (edge waves); and (c) forced nonlocally in deep or shallow water, 

 freely propagating and escaping out to deep water upon reflection at the 

 shoreline (leaky waves). Edge and leaky waves are free surface gravity waves, 

 free to propagate away from the generation source. Bounded long waves, as 

 their name implies, must remain with the forcing. 



Free Surface Gravity Waves 



Edge waves 



6. The edge wave was first postulated as a component of infragravity 

 energy by Isaacs, Williams, and Eckart (1951) immediately following the 

 observations by Munk (1949) and Tucker (1950). Issacs, Williams, and Eckart 

 conceived of the notion of the topographically trapped edge wave. This 

 shallow-water surface gravity wave would travel alongshore in a natural wave 

 guide, trapped on one side by reflection at the shoreline and on the other 

 side by refraction over a sloping bathymetry. 



