TIDAL CURRENTS 



In figure 2 a purely tidal current is represented by the curve 

 referred to the line AB as the line of zero velocity. The strengths 

 of the flood and ebb are equal, as are also the durations of flood and 

 ebb. In this case slack water occurs regularly 3 hours and 6 minutes 

 (one-quarter of the current cycle of 12 hours and 25 minutes) after the 

 times of flood and ebb strengths. If now a nontidal current is intro- 

 duced which sets in the ebb direction with a velocity represented by 

 the line CD, the strength of ebb will obviously be increased by an 

 amount equal to CD and the flood strength will be decreased by the 

 same amount. The current conditions may now be represented by 

 drawing, as the new line of zero velocities, the line EF parallel to AB, 

 and distant from it the length of CD. 



Figure 2 now shows that the nontidal current not only increases 

 the ebb strength while decreasing the flood strength, but also changes 

 the times of slack water. Slack before flood now comes later, while 

 slack before ebb comes earlier. Hence the duration of ebb is increased 

 while the duration of flood is decreased. 



FiGUEE 2. — Effect of nontidal current on reversing tidal current. 



If the velocity of the nontidal current exceeds that of the tidal 

 current at time of strength, the tidal current in the opposite direction 

 will be completely masked and the resultant current will set at all 

 times in the direction of the nontidal current. Thus, if in flgure 2 

 the line OP represents the velocity of the nontidal current, the new 

 axis for measuring the velocity of the combined current at any time 

 will be the line OH and the current will be flowing at all times in the 

 ebb direction. There will be no slack waters; but at periods 6 hours 

 12 minutes apart there will occur mmimum and maximum velocities 

 represented, respectively, by lines RS and TU. 



Insofar as the effect of the nontidal current on the direction of the 

 tidal current is concerned, it is only necessary to remark that the 

 resultant current will set in a direction which at any time is the 

 resultant of the tidal and nontidal currents at that time. This 

 resultant direction and also the resultant velocity may be determined 

 either graphically by the parallelogram of velocities or by the usual 

 trigonometric computations. 



VARIATIONS IN STRENGTH OF CURRENT 



Tidal currents exhibit periodic changes in the strength of the cur- 

 rent that corresponds closely with the periodic changes in range 



