the bathymetry of the continental shelf than the shorter waves. Though the 

 above argument is largely hypothetical, it seems quite reasonable to expect a 

 refraction- induced concentration of low- frequency energy near the shore -normal 

 direction and a lesser effect on high-frequency waves. This effect is also 

 suggested, but less dramatically, by Figures 5 and 6. 



132. Refraction would have the effect of spreading the total energy 

 (integrated with respect to frequency) over a greater arc in direction. At 

 any given frequency in the contour plot of Figure 7b, the total directional 

 spread, as indicated by the 10-percent contour line, is about the same, 

 roughly 60 deg, being just slightly greater at the lower frequencies. The 

 integrated direction spectrum, which adds contributions from all frequencies 

 for each direction, shows the total energy spread over an arc of about 100 

 deg, based on the arc for which the curve exceeds 10 percent of the spectral 

 maximum. It will be necessary to keep this effect in mind when considering 

 parametric representations of directional spread. 



133. Another feature in this example is that most of the high-frequency 

 components appear to have two peaks (or modes) in direction. This is seen in 

 both plots of Figure 7. Since the peaks are well separated in direction and 

 the distributions drop to rather low values between the peaks, this suggests 

 that there may be superimposed wave fields from two separate processes or 

 sources. Another possibility is that wave energy has been reduced in certain 

 directions and frequencies from a more complete, single mode distribution. 

 Whatever the cause, the bimodal distributions are evident. These features are 

 examined in more detail below. 



134. It should also be noted that, with all the directional structure 

 just discussed, the integrated frequency spectrum of Figure 7a is not par- 

 ticularly distinguished. It appears like many other observed frequency 

 spectra but gives no obvious clue as to the directional structure of the 

 spectrum. 



53 



