The quality and quantity of available wave climate data often do not justify 

 elaborate statistical analysis. Even where adequate data are available, a 

 simple characterization of wave climate meets many engineering needs. Thus 

 mean values of height and, to a lesser degree, period are useful. Hovever, 

 data on wave direction are generally of insufficient quality for even mean 

 value use. 



Mean annual wave heights and periods determined from data collected at a 

 number of wave gages and by visual observers at exposed sites along the coasts 

 of the United States are presented in Table 4-4. The visual height observa- 

 tions, made from the beach, represent an average value of the higher waves 

 just before their first break. They can be considered as estimates of sig- 

 nificant height H . The wave gage data were measured by gages fixed in 

 depths of 3 to 8.5 meters (10 to 28 feet). Manual analysis of waves recorded 

 on chart paper is discussed in Chapter 3 and by Draper (1967), Tucker (1961), 

 Harris (1970), and Thompson (1977). Spectral analysis of wave records is 

 discussed in Chapter 3 and by Kinsman (1965), National Academy of Sciences 

 (1963); Neumann and Pierson (1966); Harris (1974); Wilson, Chakrabarti, and 

 Snider (1974); and Thompson (1980a). While gage measurements are more 

 accurate than visual observations, visual observations define wave conditions 

 at breaking which account for onshore-offshore variation in surf zone position 

 as a function of water level and wave height. 



Wave data treated in this section are limited to nearshore observations 

 and measurements. Consequently, waves were fully refracted and had been fully 

 affected by bottom friction, percolation, and nonlinear changes in waveform 

 caused by shoaling. Thus, these data differ from data that would be obtained 

 by simple shoaling calculations based on the deepwater wave statistics. In 

 addition, data are normally lacking for the rarer, high-wave events. However, 

 the nearshore data are of use in littoral transport calculations. 



Mean wave height and period from a number of visual observations made by 

 the Coast Guard at shore stations are plotted by month in Figures 4-17 and 

 4-18, using the average values of stations within each of five coastal 

 segments. Strong seasonal variations are evident in Figure 4-17. 



The minimum monthly mean littoral zone wave height averaged for the 

 California, Oregon, and Washington coasts exceeds the maximum mean littoral 

 zone wave height averaged for the other coasts. This difference greatly 

 affects the potential for sediment transport in the respective littoral zones 

 and should be considered by engineers when applying experience gained in a 

 locality with one nearshore vave climate to a problem at a locality with 

 another wave climate. 



The climatological importance of prominent secondary wave trains occurring 

 simultaneously with the dominant wave train has been considered by Thompson 

 (1980b). Probabilities associated with multiple wave trains, obtained by 

 counting prominent spectral peaks over approximately 1 year of data from each 

 site, are presented in Figure 4-19. About 70 percent of the Atlantic coast 

 records and 60 percent of the southern California and gulf coast records 

 indicate the existence of more than one prominent wave train. 



b. Mean versus Extreme Conditions . Chapter 3, Section II contains a 

 discussion of the distribution of individual wave heights for a wave condition 



4-36 



