An 8-month data set of infragravity variance measured at 8-m depth and at 

 the shoreline (runup) was compared with incident wave variance. Analysis of 

 the 8-m-depth data showed that high mode edge waves account for about 50 

 percent of the total infragravity variance, and as high as 80 percent at times. 

 Significant edge wave heights greater than 20 cm were observed at the 8-m 

 depth. Infragravity wave variance was shown to have a higher correlation with 

 swell variance (C = 0.95) than with sea variance (C = 0.61). Correlations 

 between swell and edge waves were found to be dependent on their propagation 

 direction. Swell and edge waves propagating from the same directional quadrant 

 had a much higher correlation than when propagating from different directional 

 quadrants. The correlation between swell and edge waves, propagating from the 

 same quadrant, is approximately linear, with linear regression accounting for 

 more than 90 percent of the upcoast and downcoast variance. Measurements of 

 swash infragravity variance was shown to be significantly correlated with 

 incident swell, however, the correlation was lower (C = 0.70) than that measured 

 at the 8-m depth. Ratios of infragravity variance in the swash relative to the 8-m 

 depth had a mean value of 35, and ranged from 10 to 100. This amplification of 

 infragravity energy at the shoreline is expected when the infragravity field is 

 dominated by trapped edge waves. 



Many studies have shown the infragravity spectrum is broadbanded (white) 

 during storms (e.g., Holman 1981, Oltman-Shay and Guza 1987), implying it is 

 unlikely that selective edge wave modes are interacting to reshape the nearshore 

 morphology. Other studies (e.g., Wright et al. 1986, Aagaard 1988,1990) 

 observed preferential selection of infragravity modes that matched resonant 

 conditions on a beach with bar-trough topography. A basic concept model 

 presently exists for bar formation. It is suggested that in the surf zone, sediment 

 suspension is dominated by incident waves, and that infragravity waves 

 influence surf zone morphology by altering the net drift currents. Future 

 modeling of nearshore bar morphodynamics will improve as we learn more about 

 the interplay of wind waves, infragravity waves, mean currents, and sediments. 



Chapter 5 Summary 47 



