TM Wo. 377 



CHAPTER V 

 RESULTS AND DISCUSSION 



The large amount of wave meter data gathered at BBELS makes it difficult to 

 examine it all in detail in the remaining pages. Moreover, the intention here is 

 to present brief discussions on the most meaningful aspects of the data in the 

 hope of deriving some basic information and, perhaps more importantly, of demon- 

 strating the potential of this new approach to the study of surface waves and their 

 energy relationships. 



Velocity Data 



General Characteristics - Before examining the velocity data, it is worthwhile 

 to note that the electrical output of the wave meters can be used to present a 

 graphic or analog picture of wave motions. The wave meter record, a sample of which 

 is shown in figure II-7, represents a train of voltage pulses generated by the 

 impeller magnets moving by the induction coil of the meter. As explained in 

 chapter II, the amplitude of the pulse should be proportional to the current speed. 

 This amplitude modulation property is graphically demonstrated in figure V-l 

 (A and b). Shown are raw wave records of the LTMDUM I wave meter recorded at chart 

 speeds of 5 mm sec - -'- (Record A) and 1 mm sec" (Record B). The records were made on 

 20 July 1965 at BBELS, with the LIMDUM I system configured to measure simultaneously 

 the horizontal velocities Um and w m (see equation 111-55). On this day, there was 

 an exceptionally large swell radiating from the south. This swell had an estimated 

 height of about 50 cm and wave lengths of over 50 meters. The winds were light from 

 the NE at 2-3 m sec"-*-. The u meters were spaced with a vertical separation of 2„5 

 meters and were aimed at about 170° T - directly into the swell. 



The top and bottom traces of each record represent the upper u meter (at 

 0.0 m) and the lower u meter (at 2.5 m), respectively. The polarity of the 

 voltage pulses on the bottom trace of the A record was reversed; i.e., the 

 oscillations of both u channels should be in phase. The polarity is correct for 

 the bottom trace of the B record. 



The motions caused by the passage of the long swell are excellently portrayed, 

 appearing as the 5-7 second undulations produced by the amplitude modulation of the 

 voltage output. The wave motions at the two levels are of similar phase, and the 

 damping effect on the amplitudes of the horizontal component of the wave motions 

 with depth is evident. 



The amplitude bias downward in the B record is caused by the superposition 

 of the observed mean tidal current flowing northward in the positive sense 

 (appearing downward on the chart). At the time of the A record, the tidal current 

 was weak, and bias does not appear. 



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