TM No. 377 



The wave meter systems (OMDUM and LIMDUM) , when suspended from the BBELS 

 platform at various depths beneath the free surface, permitted some interesting 

 observations of the vertical gradient of wave motion. Four sets of observations 

 from BBELS are particularly valuable for an examination of this phenomenon. 



BBELS-5 Observations - The first series of wave measurements reported herein 

 (BBELS- 5) produced relatively crude results. The suspension system was untried, 

 and less attention was paid to precise positioning of the wave meter system than 

 in later experiments. However, in some respects the results of BBELS-5 were quite 

 meaningful. Two factors contributed favorably to this series of measurements: 

 (l) wind and sea conditions were relatively constant during the sampling period; 

 and (2) the sequence of observations was made over a short time interval (less than 

 two hours). Both factors should lend credence to the desired assumption of station- 

 arity, thus permitting a better comparison of individual observations. The 

 results provided a vivid picture of wave motion attenuation with depth. Before 

 considering this aspect, a brief review of the general experiment may be helpful. 



The OMDUM II system was suspended without the benefit of a back guy (dis- 

 cussion of suspension systems is given in chapter IV ) . As usual, the u meter was 

 aimed in the upwind (up-wave) direction. For convenience, the observations of 

 BBELS-5 are divided into two series, A and B (see appendix B). The former were 

 taken between 1135 and 1257 hours, and the latter between 1305 and 1333 hours. 

 (Results of the measurements are given in table IV-3)- 



Referring to the table IV-3, the mean values of the horizontal component u 

 indicate a fairly steady flow moving in an up-wind direction at about 30-^-0 cm 

 sec"-'-. The mean vertical velocity component w ranges from -3-9 to +1.2 cm sec"!. 

 In general, there is a non-zero value obtained for the mean vertical velocities. 

 This effect cannot be attributed to the vertical tidal velocity since, for the 

 100 cm tide range at BBELS, the maximum vertical velocity component would be less 

 than 0.1 mm sec _ l. The chief cause is probably that the vertical w meter was 

 tilted slightly off -vertical during the measurements. When the w values are 

 large, there is reason to doubt the records. The majority of the w values 

 obtained were of the order of ± 1.5 cm sec"!, which is not considered excessive. 



The variance of the u values is roughly one-tenth that of the w values. 

 From this discrepancy in the variances and, as will be seen, from the auto- 

 spectra curves of the u and w components, there is reason to doubt the horizontal 

 stability of the suspension system used in BBELS-5 and 7 observations. Since 

 the u component shows such strong damping, the discussion based on these data is 

 concentrated on the statistical properties of w. 



Figure V-20 shows the plot of the variance Ow 7 - as a function of depth for 

 both A and B series of BBELS-5 (triangles). The attenuation is clearly of an 

 exponential nature, with the variance Ou, - *- decreasing by about one-half every 

 2 meters. This exponential decrease is illustrated by the solid and broken lines 

 drawn through the triangles. The linear relation of the semi -logarithmic plot is 

 indicative of a genuine exponential relationship. 



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