TM No. 377 



In conclusion, it has been shown that the values of estimated potential 

 energy of waves are similar to the values of wave kinetic energy estimated from 

 direct measurements of u and w within the waves. Obviously, the value of E p 

 in equation (V-2l) is sensitive to the value of A; hence, error in choosing A 

 values could vary Ep by as much as a factor of h. However, the important point 

 is that Ep(H/2) or E p (H) are of the same order of magnitude as E^ w estimated 

 from actual measurements. This not only lends credence to the wave measurements 

 and techniques, but suggests that a real equipartitioning of wave potential and 

 kinetic energy occurs in the ocean. 



Short Term Changes in Wave Energy 



Wind Wave Build-up - Usually, when making a series of wave measurements, the 

 total time involved was short enough so that the wave and wind conditions were 

 relatively constant (i.e., stationary). This was desirable for comparison of 

 observations at different depths. However, during observations made on 

 26 January 1965 (serial 023 and 02^ of BBELS-9), a build-up of sea conditions 

 was visually observed, which was corroborated by the time variation in the 

 auto- spectra of wave motions observed at 0.5 meter depth. 



The measurements were made using the OMDUM III system (with three guy wires) 

 suspended from the west side of the tower. The u meter was aimed at l80°T, with 

 the original intention of observing the unusually strong swell radiating from 

 the south. 



Two sets of 5-minute observations were made. The first (023) started at 

 11^5 hours; the second (02U) at 1224 hours. The oceanographic and meteorological 

 conditions are listed in table IV- 3. At llU7 the wind was recorded at 3.7 m 

 sec - -'- from the ESE. Between measurements, the wind steadily increased until at 

 12^-5 it was recorded at 5.8 m sec~l from the SE. The seas observed at 11^-5 were 

 slight, but became perceptibly larger as the wind speed increased. The well- 

 defined swell observed throughout the observation period had wave lengths of 

 about 30 meters, wave heights of about 50 cm, and periods of 5-6 seconds. 



The tide gauge system aboard the BBELS (discussed in chapter TV) served in 

 this case as an indicator of the strong swell. Figure V-30 shows the tide 

 gauge trace for the period of BBELS-9 observations. The swell appears as high 

 frequency "noise" on the trace. For comparison, a section of a record with 

 little or no swell is shown in the insert at lower left of figure V-30. 



The indicated swell height on the tide chart is about 20 cm, which is some- 

 what less than the visually estimated height of 50 cm. Thus, it appears that 

 swell motions are somewhat damped by the tide gauge system, but are still of low 

 enough frequency so as not to be completely filtered out. With small cost, a 

 minor modification could be made in the time response, enabling the tide gauge 

 to record swell along with tide height over long periods. 



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