Persistence of Wave Heights 



Table D5 shows the number of times in 1992 when the specified wave height was equaled or 

 exceeded at least once during each day for the duration (consecutive days). Data for 1980 through 

 1992 are averaged and given in Table D6. An example is shown below: 



Height 

 m 













Consecutive 



Dav(s) or Longer 











1 



2 



_3 



4 



5 



6 7 8 9 



10 11 12 13 14 



15 



16 17 



18 



19+ 



0.5 



18 



15 





14 



13 



12 11 10 



9 8 





7 







1.0 



50 



34 



24 



21 



18 



14 12 8 7 



3 2 











1.5 



41 



19 



8 



6 



2 



1 













2.0 



22 



9 



5 



1 

















2.5 



10 



5 



2 



















3.0 



6 



1 





















3.5 





1 





















4.0 



1 























This example indicates that wave heights equaled or exceeded 1.0 m 50 times for at least 1 day; 34 

 times for at least 2 days; 24 times for at least 3 days, etc. Therefore, on 16 occasions the height 

 equaled or exceeded 1.0 m for 1 day exactly (50 - 34 = 16); on 10 occasions for 2 days; on 3 

 occasions for 3 days, etc. Note that the height exceeded 1 m 50 times for 1 day or longer, while 

 heights exceeded 0.5 m only 18 times for this same duration. This change in durations occurred 

 because the longer durations of lower waves may be interspersed with shorter, but more frequent, 

 intervals of higher waves. For example, one of the times that the wave heights exceeded 0.5 m for 

 16 days may have represented three times the height exceeded 1 m for shorter durations. 



Spectra 



Monthly spectra for the offshore staff gage (Gage 625) are presented in Figure D8. The plots 

 show "relative" energy density as a function of wave frequency. These figures summarize the large 

 number of spectra for each month. The figures emphasize the higher energy density associated with 

 storms, as well as the general shifts in energy density to different frequencies. As used here, 

 "relative" indicates the spectra have been smoothed by the three-dimensional surface drawing routine. 

 Consequently, extremely high- and low-energy density values are modified to produce a smooth 

 surface. The figures are not intended for quantitative measurements; however, they do provide the 

 energy density as a function of frequency relative to the other spectra for the month. 



Monthly and annual wave statistics for Gage 625 for 1992 and for 1980 through 1992 are 

 presented in Table D7. 



Figure D9 plots monthly time histories of wave height and period. 



D2 



Appendix D Wave Data for Gage 625 



