TM No. 377 



of about 6 cm sec~- and an azimuth of about 290°T» Three variables are 

 plotted in figure IV-6 as a function of times "f s the height of the tide 

 (upper curve); § , the speed of the current (middle curve); and 8, the 

 direction of the current (lower curve)* 



The time variation of the free surface level ^ is clearly governed by 

 the semidiurnal tidal component M^ whose period of 12,4 hours coincides with 

 the mean period of the recorded oscillation (f Igure IV=6). The mean range for 

 the period, which should be quite representative for year-round values, is 

 of the order of 80-90 cm. However* there is evidence of a strong diurnal 

 inequality (i.e., an alternating increase and decrease in amplitude), and of 

 a long period constituent, which appears as a slow 'decrease in amplitude 

 toward the middle of the record and as an increase toward the end« This 

 latter component is probably the so-called fortnightly inequality (see Defant, 

 1958). The iff portrays a sinusoidal pattern, which appears slightly steeper 

 on the ebb than on the flood, side. The diurnal inequality is relatively 

 intense , giving as much as a 20 percent difference in amplitude between 

 two consecutive periods . Also, the diurnal inequality appears tc weaken 

 as the lunar fortnightly modulation approaches a maximum* According to 

 Defant (1958), one may classify this tide system between semidiurnal form 

 and mixed (predominantly semidiurnal) form. A much larger sample would be 

 required to establish the relative magnitudes of the semidiurnal constituents 

 (i.e.., Up) S^, N and K~) with respect to the diurnal components (i.e., K-^, 

 0,, P, ) and to tne lunar fortnightly constituent MLp » 



The current speed £ is quite variable; however, it displays oscillations 

 that seem to correlate with ebbing and flooding intervals of y (figure 17-6). 

 Velocity peaks tend to occur near the time of high water. The fluctuations of 

 the tidal period, however, show a. strong diurnal inequality. Oddly enough, 

 the strongest current is associated with the minimum oscillation of y • This 

 is particularly evident from 30 April to 3 May. Another interesting feature is 

 the sharp drop in current speed after the maximum is obtained; e.g.,, at 1 May 

 at 0930, 2 May at HOC, and 3 May at 1300. 



The § record contains relatively high frequency bumps, which appear to 

 have periods ranging from 2 hours down to 20-40 minutes or less, and which 

 fluctuate as much as 10=15 cm sec"l within a 20-minute sampling interval. 

 The data as plotted are 20=minute averages of speed and direction. Thus, the 

 many small peaks displayed are suggestive of the still higher unresolved 

 frequencies; i.e.. above the Nyquist frequency - f// of 0.4 cycle per kilosecond 

 (0.025 cycle min-1). 



The current direction © depicts clearly a semidiurnal anti-cyclonic 

 (clockwise) rotation of the tide vector The abrupt drop in the & trace 

 occurs when the flow shifts through 360°T, usually about one hour- after high 

 water. The current direction remains north for not more than an hour, then 

 swings rapidly to the south (at one or two hours after high water) and 

 proceeds to rotate clockwise almost linearly with time. Its direction is west 

 (270 o ) at about mean low water and,, rotating steadily, it reaches 36O again 

 at high water plus one hour. 



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