dominant) in shallow water or during large storm events. The magnitude of the 

 free infragravity waves varied dramatically with wave conditions, as expected, 

 and also with the configuration of the shelf. For instance, energy levels on 

 wide, sandy continental shelves, such as off the FRF, were found to have free 

 wave energy levels 2-4 times greater than those on a narrow shelf under similar 

 incident wave conditions, suggesting a sensitivity to the refractive 

 characteristics of the shelf and the distance over which nonlinear interactions 

 occur during shoaling. 



Herbers, Elgar, and Guza (1995) again use their data from the 13-m site at 

 the FRF to investigate the forcing and propagation of infragravity waves. Much 

 of their analysis is based on comparison of energy fluxes of waves progressing 

 either on or off-shore, and either direction along the beach. In general terms 

 they found that during storms the outgoing infragravity wave energy flux is 

 several times larger than the shoreward energy flux, arguing for a surf-zone 

 origin of the waves, and significant energy dissipation on the continental shelf. 

 The opposite holds during low swell conditions, with shoreward energy flux 

 commonly exceeding the offshore flux, suggesting non-locally generated 

 motions may be important during those time periods. Examination of the 

 partitioning of the energy fluxes is very helpful in focusing the future needs for 

 infragravity wave research on generation and dissipative mechanisms. 



Okihiro, Guza, and Seymour (1993) and Okihiro and Guza (1996) report on 

 observations and model studies of the forcing and amplification of harbor 

 seiches at infragravity frequencies. The three harbors they studied were all 

 small with surface areas of approximately 1 km 2 , and water depths ranging 

 from 5 to 12 m. All were located in the Pacific, two in Hawaii and one in 

 California. The sources of seiche energy were determined by correlation 

 analysis between offshore swell (recorded just seaward of harbor entrances) 

 and pressure sensors from within the harbors. They found, for these harbors, 

 that below a frequency of -0.0015 Hz the seiche and swell energies were only 

 weakly correlated. This frequency band contained the grave mode of oscillation 

 and appeared to be excited by meteorological events and tsunamis. In contrast, 

 seiche energy at frequencies above this cutoff (i.e., the infragravity frequencies) 

 are highly correlated with swell energy. They also noted secondary 

 dependencies on swell frequency (increasing correlation with longer period 

 waves) and more energetic seiching at high tide. Swell-driven seiches 

 dominated over the lower frequency seiche band when waves were high, as 

 could be expected. 



Amplification of the sieche motions was found to depend on the seiche 

 frequency and was largest for the grave mode. However the harbors they 

 studied all showed a decrease in grave-mode amplification with increasing 

 overall seiche energy. Significant seiche heights were reported to be 

 approximately 10-15 percent of the offshore significant wave height when it 

 exceeds 2 m. 



Chapter 2 Infragravity Wave Dynamics 1 7 



