propagating from the same directional quadrant are sufficiently correlated, such 

 that linear regression can account for more than 90 percent of the up-coast and 

 downcoast variance (Table 4). The correlation of edge wave variance is much 

 lower with seas (either direction) or with incident swell from the opposite 

 quadrant (Table 4). 



The high correlation between edge waves and swell from the same directional 

 quadrant is consistent with a triad interaction description of edge wave forcing 

 (Gallagher 1971). In these triad interactions, two incident waves generate a 

 difference interaction that satisfies an edge wave mode dispersion relationship, 

 thus producing the resonant excitation of that edge wave mode. Theoretical 

 examination of these nonlinear interactions shows that directional swell wave 

 pairs can generate interactions traveling from the same quadrant as the edge 

 wave propagation (Bowen and Guza 1978; Oltman-Shay et al. 1992). It was 

 found unlikely that sea-sea interactions would create a difference interaction 

 capable of exciting an edge wave mode with a preferential direction. 







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Figure 22. Incident directional wave spectrum on 4 Nov 1990. Peak wave 

 direction is from 32 deg south of shore normal 



44 



Chapter 4 Field Observations 



