156 



in practice analysis of the ordinate distribution can be used to determine the 

 characteristic wave height and the complete wave height distribution as deter- 

 mined by measuring individual waves. 



Further studies are now in progress to determine the accuracy of this 

 method if fewer levels are measured. The purpose of this study is to investi- 

 gate the possibility of designing an instrument to analyze and record character- 

 istic wave heights directly. The instrument would measure and record the per- 

 centage of time the water surface, or the differential pressure, exceeded a given 

 level. For example, if the water surface elevation exceeded a level of 2 feet 

 (above still water) 10 percent of the time the characteristic wave height theoret- 

 ically would be 62 feet. 



Reports are now being written at the University of California which de- 

 scribe the ordinate distribution analyzer and the results that have been obtained. 



Analysis of Wave Recorders for Wave Period - Wave period has been defined in 

 terms of the time history of the surface elevation at a point location as follows 

 (Folsom, 1948): 



Wave period is the tinne interval between the appearance at a fixed point 

 of successive wave crests. 



Significant wave period is the average wave period for the well defined 

 series of highest waves observed. 



Effectively, the significant wave period is defined as the average wave 

 period of the waves measured to determine the characteristic wave height. The 

 significant wave period as determined from a wave by wave analysis determines 

 the wave period distribution, although not as simply as the characteristic wave 

 height determined the wave height distribution (Putz, 1951). 



Fourier analysis of wave records for wave period indicate that the wave 

 period spectrum depends upon the meteorological condition, the age of the waves 

 and the distance the waves have traveled to reach the site. A study of the wave 

 period spectrums obtained from pressure type wave recorders by the Admiralty 

 Research Laboratories, Teddington, England (1947) and later by W.H. Munk 

 (1947) indicated that this type of information was useful in tracking storms at 

 sea and in correlating meteorological and wave data. The analyzers used to 

 obtain the frequency spectrum have been described repeatedly in the literature, 

 so only a brief description of the principle of operation will be given here. 



The wave record is fastened to the circumference of a wheel which is 

 rotated at high speed revolving the record past a scanning system. The electri- 

 cal output of the scanner is passed through a high Q filter to a recording system. 

 As the speed of the wheel is gradually decreased the recorder produces a curve 

 consisting of a series of peaks representing the Fourier amplitude spectrum of 

 the curve on the wave record. 



The analysis of wave records by means of the auto-correlation function, 

 and the design of analyzers based on the auto-correlation function, have been 

 investigated by several groups (Seiwell, 1949) (Rudnick, 1949). Although the 

 application of this technique to ocean wave studies is still in the process of de- 

 velopment, this method of analysis has proven to be helpful in special problems, 

 such as those involved in the study of transformation of waves and the study of 

 the motion of ships. 



The correlogram obtained from an auto-correlation analysis contains in- 

 formation regarding the frequency components presented in the wave record as 



