VII. OCEAN WAVE MEASUREMENTS 



Pasquale DeLeonibus 



A. INTRODUCTION 



Data on the properties of ocean waves are sought for a variety of 

 reasons depending upon the needs of the user. Some of these reasons 

 are: (1) To attack the problems associated with undersea warfare, 

 such as the naval minefield, by studying the responses of various 

 mines to actual sea conditions; (2) to obtain the "wave climatology 

 of a region by extended measurements (at least a year at one or more 

 locations) of the wave systems in that region; (3) to check the correct- 

 ness of the mathematical models which have been proposed to describe 

 and explain the properties of ocean waves; (4) to study the eroding 

 effect of waves on beaches and coasts so that better breakwater sys- 

 tems can be devised; and (5) to obtain basic information on what is 

 probably the outstanding unsolved problem associated with the impul- 

 sive generation of the waves by wind: namely, the initial growth of 

 very small wavelets (capillary waves). 



The ocean surface wave system is composed of a spectrum of wave 

 periods. By this is meant that the sea surface is made up of many 

 individual sinusoidal components which combine to produce the com- 

 plex ocean surface wave pattern. Since interest in waves may range 

 from tiny capillary waves (whose wave lengths are of the order of 

 centimeters) to mountainous waves (whose wave lengths are of the 

 order of hundreds of feet), the required measuring instruments vary 

 widely in their basic principles and designs. To complicate the wave 

 measuring problem, in the open ocean the disturbance which is being 

 measured disturbs the entire wave measuring system, so that it is 

 very difficult to design a stable reference level which is at the same 

 time a floating reference level. In order to provide a fixed reference 

 level, wave measurements have been made by recording the changes 

 in pressure which the wave system produces on the ocean bottom. 

 In addition, surface wave measuring instruments have been mounted 

 on piers, docks, and the so-called Texas Towers. However, since 

 these structures are in relatively shallow water, the observations 

 all include the distorting effect that the bottom has on the surface 

 wave profile. 



To measure the wave profile on the open ocean, floating and air- 

 borne instrumentation techniques have been devised with varying 

 degrees of success. The Hydrographic Office made one of the earliest 

 attempts to obtain such wave records by using a damped electric wave 

 staff. In 1952 a pressure recorder-accelerometer unit was installed 



VII-1 



