TM No, 377 



Historical Background 



Measurements of surface gravity waves have traditionally been made by 

 recording either the time- variable height of the free surface, using various 

 wave staff devices t or the dynamic pressure obtained from shallow bottom- 

 mounted transducers o The latter method, in particular s is not well suited 

 to studies of wind-driven waves of periods less than 4-5 seconds (greater 

 than 0,25 cps) because the dynamic pressure induced by higher frequency 

 surface waves attenuates with depth in a fashion similar to that of the 

 velocity and acceleration of the orbital motions „ Hence, pressure measure- 

 ments reveal little of what is happening at the surface in the higher fre- 

 quency bands , 



Thus s a discussion of the background of work related to directly 

 measuring the motions beneath the surface need not dwell on either of these 

 methods of measurement s with one exception*, 



A series of wind wave measurements was conducted by Kinsman (1960) 

 in which the actual observations were of the time variable fluctuations 

 of the free surface. His report provides a concise statement of the 

 problems involved with instrumentation, data processing, and analysis, This 

 work involved a study of 24 point records of surface waves made in Chesapeake 

 Bay at short fetches (2-5 km) and relatively low wind speeds (3,8 = 8„2 m 

 sec-l). In his results and analysis Kinsman deals with such problems as de- 

 viation of observations from Gaussian distribution, evidence of nonlinear 

 interaction in the spectra, and examination of the so-called "equilibrium 

 range" of the spectra. His work offered a means of comoarison of the SDec- 

 tral characteristics of free surface fluctuations with the spectra of the oscilla- 

 tory motions presented in this-report i Kinsman's work will be discussed further ii 

 Chapter V. 



There is no account in the published literature (known to the author) of 

 a deliberate attempt to make insitu measurements of the particle motions in 

 wind-driven waves in a Eulerian manner . Therefore , no source is available 

 to obtain estimates of wave-induced turbulent fluctuations, variances, co» 

 variances s and the associated spectra. Measurements have been conducted 

 in tidal estuaries in an effort to observe turbulent quantities present in 

 tidal flows. Measurements have also been made in laboratory tanks to study 

 orbital motions of artificially produced waves , 



This historical review is concerned not only with the past measurements 

 of wave motion per se, but also with the results of experiments made to 

 measure turbulence on the surface layers of the ocean — specifically 9 measure- 

 ments of motions in which special care was taken to obtain a statistical es- 

 timate of the vector quantities and correlations, A general review of the 

 measurements of oceanographic turbulence is given by Bowden (1962-A), Unfor- 

 tunately , Bowden emphasizes in his introduction that "turbulent motions" are 

 generally not considered in the same catagory as wave motions, which he states 

 are of "more regular" fashion. It is this conception that should be avoided. 



