For waves propagating outside the wave -gene rat ion area the nonlinear 

 energy transfer to low and very high frequency continues without the 

 accon5)anying replenishment of energy at midfrequencies. Thus, swell 

 waves are characterized by narrow spectra with peaks at low frequency. 



There is evidence that the intensity of nonlinear energy transfer 

 increases rapidly with decreasing water depth. The nonlinear transfer 

 may also narrow the directional spread of energy in shallow-water waves 

 (Herterich and Hasselmann, in preparation, 1980). 



Aerial photos of nearshore ocean waters indicate remarkable organi- 

 zation in the surface waves (Fig. 4) ; the photos also indicate that sea 

 states dominated by a single visible wave train are relatively unusual. 

 Photos typically show two or three apparently independent wave trains 

 with different, well-defined directions and wavelengths. The relative 

 prominence of the trains often varies with proximity to shore due to 

 frequency-dependent shoaling and refraction, and in some cases due to 

 redistribution of energy with frequency by nonlinear interactions 

 between trains. 



For exaii5)le, although a long swell wave train often dominates the 

 breaker zone in photos taken along the southern California coast, the 

 train is invisible two or three wavelengths seaward of the breaker zone. 

 The swell waves are preferentially amplified nearshore. McClenan and 

 Harris (1975) show several examples of this process occurring. 



The swell waves are also refracted more completely than higher fre- 

 quency waves so that at breaking they are often nearly parallel to the 

 shore. The water particle velocity associated with low-frequency swell 

 waves attenuates slowly with depth. High-frequency waves which have not 

 been substantially refracted can travel into the swell-created breaker 

 zone with a large component of longshore motion; yet, their particle 

 velocities attenuate rapidly with depth. Since longshore sediment move- 

 ment requires both a mechanism for initiating sediment movement and a 

 longshore drift to transport it, two concurrent wave trains may move 

 sediment alongshore more effectively than either train acting alone. 



The presence of a current near a gage site can alter the measured 

 characteristics of waves. The longshore current in and near the breaker 

 zone affects some of the gage measurements in this report. Tidal cur- 

 rents and v/ind- generated currents may also affect the measurements. The 

 Gulf Stream may affect deepwater waves approaching the U.S. Atlantic 

 coast by steepening waves enough to cause breaking or, in special cases, 

 by refracting the waves so that they never reach the shore (Kenyon, 1971) 

 The rate of wave growth in the vicinity of the current may also be 

 affected. 



III. WAVE GAGE DATA COLLECTION AND ANALYSIS 



1. Collection. 



The wave gage data used in this report were collected as part of the 

 CERC field wave data collection program from 11 gages af the 9 locations 



17 



