nonbreaking, breaking, and broken waves during different stages of a tidal 

 cycle. The wave action a structure is subjected to may also vary along its 

 length at a given time. This is true for structures oriented perpendicular to 

 the shoreline such as groins and jetties. The critical section of these 

 structures may be shoreward of the seaward end of the structure, depending on 

 structure crest elevation, tidal range, and bottom profile. 



Detailed discussion of the effects of astronomical tides and wind- 

 generated surges in establishing water levels is presented in Chapter 3, WAVE 

 AND WATER LEVEL PREDICTIONS. In Chapter 7, it is assumed that the methods of 

 Chapter 3 have been applied to determine design water levels. 



The wave height usually derived from statistical analysis of synoptic 

 weather charts or other historical data to represent wave conditions in an 

 extreme event is the significant height Hg . Assuming a Rayleigh wave height 

 distribution, H may be further defined in approximate relation to other 

 height parameters of the statistical wave height distribution in deep water: 



H. ,^ or H = average of highest 1/3 of all waves (an alternate defini- 

 tion of H sometimes applied is 4 times the standard 

 deviation of the sea surface elevations, often denoted as 



m 

 o 



H,Q « 1.27 H = average of highest 10 percent of all waves (7-1) 



Hr « 1.37 H = average of highest 5 percent of all waves (7-2) 



-J O 



Hi « 1.67 H = average of highest 1 percent of all waves (7-3) 



Advances in the theoretical and empirical study of surface waves in recent 

 years have added great emphasis to the analysis of wave energy spectra in 

 estimating wave conditions for design purposes. Representation of wave 

 conditions in an extreme event by wave energy as a function of frequency 

 provides much more information for use in engineering designs. The physical 

 processes which govern the transformation of wave energy are highly sensitive 

 to wave period, and spectral considerations take adequate account of this 

 fact. An important parameter in discussing wave energy spectra is the energy- 

 based wave height parameter H , which corresponds to the significant wave 



height, H , under most conditions. An equally important parameter is the 

 peak spectral period, T , which is the inverse of the dominant frequency of 

 a wave energy spectrum. The peak spectral period, Tp , is comparable to the 

 significant wave period, T , in many situations. The total energy, E , and 

 the energy in each frequency band, E(u)) , are also of importance (see Ch. 3, 

 Sec. 11,3, Energy Spectra of Waves). 



3 . D etermin a tion of Wave Conditio ns. 



All wave data applicable to the project site should be evaluated. Visual 

 observation of storm waves, while difficult to confirm, may provide an indica- 

 tion of wave height, period, direction, storm duration, and frequency of 

 occurrence. Instrumentation has been developed for recording wave height, 



7-2 



