TIDAL CURRENTS 7 



is due to the fact, previously mentioned, that the diurnal inequality 

 in the current at any given place is, in general, only about half as 

 great as that in the tide. This brings about differences in the corre- 

 sponding features of tide and current as between morning and after- 

 noon. However, in such cases it is frequently possible to refer the 

 current at a given place to the tide at some other place with com- 

 parable diurnal inequality. 



DISTANCE TRAVELED DURING A TIDAL CYCLE 



The vertical distance traveled by a floating object during the 

 tidal cycle at any place can be easily determined from the tide curve 

 at that place, for the tide curve represents the successive heights 

 of the surface of the water during the tidal cycle. Hence the vertical 

 distance on the tide curve between a high water and low water gives 

 the vertical distance through which a floating object moved during 

 that tidal cycle. 



The close resemblance between the curve of the reversing current 

 and the tide curve might lead one to conclude that from the current 

 curve the horizontal distance traveled by a floating object can be as 

 readily derived as the vertical distance is from the tide curve. The 

 current curve, however, gives the successive speeds of the horizontal 

 movement, and not the successive positions of a floating object. 

 Hence the current curve does not give directly the horizontal distance 

 traveled by a floating object. 



If the velocity of the current during a tidal cycle were constant, the 

 horizontal distance traveled by the water particles or by any object 

 floating in the water would be given by multiplying the velocity by 

 the period of duration. The velocity of the current, however, is not 

 constant but changes continuaDy throughout a tidal cycle. The 

 distance traveled by the water particles is, therefore, the average 

 velocity during the flood or ebb period in question, multiplied by the 

 duration. 



The average velocity of the current during any given interval may 

 be determined in several different ways. By measuring the velocity 

 on the current curve at frequent intervals, say every 10 or 15 minutes, 

 the average velocity during the interval is easily derived. Or the 

 area of the surface bounded by the current curve and the zero line of 

 velocities may be determined by means of a planimeter and the 

 average velocity derived by dividing this area by the length of the 

 zero line included within the current curve. 



The simplest method, however, consists in making use of the fact 

 that the current curve approximates the cosine curve. And on the 

 cosine curve it is known that the ratio of the mean ordinate to the 

 maximum ordinate is 2-.-ir, or 0.637. Since the strength of the tidal 

 current corresponds to the maximum ordinate, it follows that during 

 any given flood or ebb period the average velocity will be the strength 

 of the current multiplied by 0.637. 



In the semidaily or mixed types of current the duration of a flood 

 or ebb period approximates 6.2 hours. Hence, in the case of such a 

 current which has a velocity at strength of one knot, a floating object 

 will, during a flood or ebb period, be carried a distance of 0.637X6.2 — 

 3.95 nautical miles, or 24,000 feet. In a daily current of the same 

 strength the distance will be twice as great. 



