Persistence of Wave Heights 



6. Table D5 shows the number of times in 1991 when the specified wave 

 height was equaled or exceeded at least once during each day for the duration 

 (consecutive days) . Data for 1980 through 1991 are averaged and given in 

 Table D6 . An example is shown below: 



Height Consecutive Day(s) or Longer 



m _1_2_3_4_5_6_7_5_2 1Q n 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 3 times the height exceeded 1 m for shorter 

 durations . 



Spectra 



7. Monthly spectra for the offshore Waverider buoy (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, 



D2 



