The large effect wave breaking can have on significant height meas- 

 ured at most of the CERC gages is shown in Figure 3. The figure shows 

 a time history of significant wave height for three Atlantic coast gages 

 during a large winter storm. The standard Nags Head, North Carolina, 

 gage is represented along with two staff gages on the new CERC Field 

 Research Facility (FRF) pier 30 miles north of Nags Head. The FRF gage 

 farthest seaward shows peak significant heights about 1 meter higher 

 than the FRF gage 390 meters farther shoreward. During the peak of the 

 storm, significant heights at the Nags Head gage drop well below heights 

 at the seaward FRF gage. The figure suggests that breaking has a major 

 influence on significant height at both the nearshore FRF gage and the 

 Nags Head gage. 



t 1 



LOW 



TIDE LEVEL 



HiGH LOW 



\^ FRF Pier End (Depth 8ir.) 



/ 





'^^:j:AV^' 



Nogs Head (Depth 5m) 



FRF Mid Pier (Depth 2m) 



16 20 



19 Jan 76 



00 



04 08 12 



Time (Eostern Stondord Time) 



16 20 



20 Jon 78 



Figure 3. Time history of significant wave height for three North 

 Carolina gages during a large winter storm. 



For some of the gage sites included in this report, and perhaps for 

 all except the acceleroraeter buoy sites, a sandbar was often seaward of 

 the gage. The bar is a somewhat transient feature which can have a major 

 influence on the waves by causing high waves to break before they arrive 

 at the gages and by acting to reflect energy seaward or trap it between 

 the bar and shore. Evidence presented by Wang and Yang (1976) indicates 

 the bar acts mainly as an energy dissipator at low tide. At high tide 

 for nonbreaking waves, the bar tends to reflect wave energy seaward 

 during onshore winds and to trap wave energy between the bar and beach 

 during offshore winds. 



Since ocean waves in a sea state are never strictly uniform in either 

 height or frequency, a sea state is conveniently characterized by the 

 distribution of wave energy as a function of frequency (the spectrum). 

 During active wave growth, most energy from the atmosphere is added at 

 frequencies slightly higher than the dominant frequency. The energy is 

 then redistributed by nonlinear transfers within the wave field to fre- 

 quencies slightly lower than the dominant frequency and to very high 

 frequencies where it is lost to breaking processes. These phenomena 

 result in a gradual shift of the spectral peak to lower frequencies. 



16 



