TM No. 377 

 Column F shows the depth at which a gravity wave with the attributes 

 listed in B, C, D and E has particles oscillating at a velocity of at 

 least 10 cm sec o For the last three values in column F a water depth 

 of 20 meters forced an intermediate wave calculation (using equation 11=7); 

 all other values are estimated using the deep water wave equations (e g a ; 

 II-5)= Similarly, column G gives the water depths at which an orbital 

 diameter of 10 cm exists* Deep water equations (e g<>, II-9) are used 

 except for the bottom three terms , which are estimated using equation 

 11-11 for intermediate waves 



Winds observed in Narragansett Bay and Rhode Island Sound frequently 

 occur in the range of speeds given in table II-l (i.e„, 2-15 m sec~l)„ 

 Hence s at the higher wind speeds, the surface waves can be expected to 

 produce the minimum particle velocities and displacements at depths to 

 18 meters Thus, it appears feasible to measure perturbations of surface 

 waves (which fall within the somewhat conservative limits established for 

 velocity and scale) at a variety of depths and under realistic wave condi- 

 tions occurring in the area of measurement 



Since measurements are to be made of the dynamic perturbations associated 

 with the motions of wind waves , the measuring instrument should respond to 

 frequencies somewhat higher than the wave frequencies „ Table II-2 is a sea 

 state chart taken from Marks (l$6k) The wave characteristics have been 

 converted from English to metric units . The table lists wave periods 

 associated with various wind speeds, fetches, and durations For wind 

 speeds between 2 and 15 m sec "1 the average periods of the waves run from 

 lo4 to about 8«5 seconds 8 It is obvious that the sensor selected must 

 resolve oscillations of less than one second 



Kinsman (i960), in making free surface elevation measurements of rela- 

 tively small (and thus, relatively high frequency) wind driven waves, sampled 

 at o 2 second intervals from the analog output of his wave staff . This 

 study and other experiiaents discussed in Ocean Wave Spectra (1961) indicate 

 that sampling intervals of 0o5 to 0,2 second are used to obtain data for 

 statistical analysis of even the highest frequency ocean waves „ Thus, the 

 response time of the sensor must be equal to or less than C„2 second,. 



Choice of an Instrument 



The brief historical review in chapter I indicates that past experience 

 offers no firm basis to assist in choosing a method of wave measurement » 

 To embark on some instrumentation development, although tempting, would 

 certainly result in extreme complexity (even if it were based upon conven- 

 tional systems) and considerable expense* Moreover, the guarantee of success 

 would be very tenuous e 



The purpose here is to explore the conceptual approach to studying wave 

 particle motions in the hope of providing the impetus for farther concerted 

 effor& Tne success of tnis preliminary study will determine the worth of 

 an all-out assault on the problems associated with the measurement of wave 

 motions and the development of properly sophisticated instrumentation Thus, 



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