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WAVE MEASUREMENTS* 

 F. E. Snodgrass 



INTRODUCTION 



Although considerable progress has been made in the development of in- 

 struments to measure the height and period of wind waves and swell as they ap- 

 proach the shore, few instruments have been developed to measure other charac- 

 teristics of these ocean waves or the characteristics of other ocean waves. Ex- 

 ceptions to this have been the development of techniques to measure very short 

 period waves (surface ripples) and very long period waves (tsunami, surf beat, 

 etc.) by the Scripps Institution of Oceanography . Harbor surge recorders de- 

 veloped by the California Institute of Technology also provide an exception. In 

 addition, basic studies have been made toward the development of instruments to 

 measure waves in deep water and to measure wave direction; but no instruments 

 suitable for routine studies were developed. 



The purpose of this paper is to review the instruments that have been de- 

 veloped for the measurement of ordinary gravity waves (periods of 1 to 30 sec- 

 onds) and to suggest possible solutions to other measurement problems that ex- 

 ist today. Only the measurement of ocean waves that generally are of concern 

 to the engineer are discussed. Many other measurements of the sea surface 

 are important, but are considered beyond the scope of this paper. 



WAVE DIRECTION MEASUREMENT 



The measurement of wave direction is as important as the measurement 

 of wave height and period, yet comparatively little effort has been made to de- 

 velop an instrument to obtain this information. Consequently, no instrument 

 has been designed which will measure and record the direction of waves in the 

 open sea. Visual observation of wave direction also has been unsatisfactory. 

 Short period waves generated by local wind often hide the more important ground 

 swell arriving from distant storms. The observer, therefore, will report the 

 direction of the wind chop instead of the swell which is of primary importance. 



At the present time wave direction can be deternnined best through a 

 study of weather maps. The swell will have the direction of the path connecting 

 the fetch area to the shore site while the wind chop will have the direction of the 

 local wind. Several difficulties arise when wave direction is determined from 

 weather maps. First of all, the weather maps may not have sufficient informa- 

 tion to locate the fetch areas; this is especially true of storms in the southern 

 hemisphere. In fact, the study of ocean waves is one source of information 



* - Technical Report, University of California, Series 3, Issue 342. 



