Observations and Measurements of Ocean Waves 57 



energy is indicated for 8 8, 9 and 9-5 sec. The lower diagram shows in the same 

 form, a wave spectrum of a pressure record. This is a typical swell spectrum 

 with a narrow period range of 11-15 sec. 



Seiwell (1949) and Wadsworth (1949) have doubted the results of wave 

 analysators, which they considered as physically unfounded. They feel that 

 a period analysis by autocorrelation will lead to more plausible and physically 

 more simple results. Frequently, there was only one single harmonic wave 

 upon which local oscillations are superposed by chance. The former, the 

 "cyclic" component, corresponds to the situation forced by the wind, the 

 latter, the "oscillatory" components, are regarded as depending upon ac- 

 cidental local disturbances. These doubts are only in part justified. In some 

 cases with a narrow frequency range Seiwell and Wadsworth may be right, 

 but in general the wave pattern is much more complicated, and the assumption 

 that this pattern is formed by the superposition of a great number of waves 

 with different periods, amplitudes and heights will be more correct. 



7. Statistical Relations Between the Different Apparent Wave Characteristics 

 and their Interpretation 



It is possible to compute the average period T, that is, the average of time 

 intervals between succeeding wave crests at a fixed position, from the 

 theoretical energy spectrum of wind generated waves. This value is cal- 

 culated by 



(/ oo \ 1/2 



JdUada \ 

 i- • (HI. 13) 



fda*U a do 

 ' 



We obtain for the average period 



f/v = 7t\/3/g or ? = 0-555fl (III. 14) 



where v is given in m/sec _1 . If v is given in knots, then T = 0-258?'. 

 With equation (III. 8) we obtain the ratio 



T ma x/T = \/2 = 1-414 . (III. 15) 



Therefore, the apparent wave with the "average period" T is not identical 

 with the energy-richest wave of the spectrum, but its period is considerably 

 smaller than the period T mas . 



From the energy spectrum of sea motion one can easily estimate in which 

 period range the most important wave components must be found. The 

 energy richest part of the spectrum, which contains 92 % of the total wave 

 energy, can be said to give the actual pattern of the sea motion. This main 

 part can be limited such that 3 % of the total energy at the short-wave end 



