TIDES AND CURRENTS IN BOSTON HARBOR 



109 



RELATION OF TIME OF CURRENT TO TIME OF TIDE 



In simple wave motion the times of slack and strength of current bear a con- 

 stant and simple relation to the times of high and low waters. In a progressive 

 wave the time of slack water comes, theoretically, exactly midway between high 

 and low water and the time of strength at high and low water; in a stationary 

 wave slack comes at the times of high and low water, while the strength of current 

 comes midway between high and low water. 



The progressive-wave movement and the stationary-wave movement are the 

 two principal types of tidal movements. A progressive wave is one whose crest 

 advances, so that in any body of water that sustains this type of tidal movement 

 the times of high and low water progress from one end to the other. A stationary 

 wave is one that oscillates about an axis, high water occurring over the whole 

 area on one side of this axis at the same instant that low water occurs over the 

 whole area on the other side of the axis. 



The tidal movements of coastal waters are rarely of simple wave form; never- 

 theless, it is very convenient in the study of currents to refer the times of current 

 to the times of tide. And where the diurnal inequality in the tide is small, 

 as is the case on the Atlantic coast, the relation between the time of current 

 and the time of tide is very nearly constant. This is brought out in Figure E, 

 which represents the tidal and current curves in New York Harbor for October 

 9, 1919, the current curve being the dashed-line curve, representing the velocities 





Fig. E.— Tide and current curves, New York Harbor, October 9, 1919 



of the current at a station in Upper Bay, and the tide curve being the full-line 

 curve, representing the rise and fall of the tide at Fort Hamilton, on the eastern 

 shore of the Narrows. 



The diagrams of Figure E were drawn by plotting the heights of the tide 

 and the velocities of the current to the same time scale and to such velocity 

 and height scales as will make the maximum ordinates of the two curves approxi- 

 mately equal. The time axis or axis of X represents the line of zero velocity for 

 the currents and of mean sea level for the tide, the velocity of the current being 

 plotted in accordance with the scale of knots on the left, while the height of the 

 tide reckoned from mean sea level was plotted in accordance with the scale in 

 feet on the right. 



From Figure E it is seen that the corresponding features of the tide and current 

 in New York Harbor bear a very nearly constant time relation to each other, 

 and this constancy in time relation of tides and currents is characteristic of tidal 

 waters in which the diurnal inequality is small. This permits the times of slack 

 and of strength of current to be referred to the times of high and low water. 

 Thus, from Figure E we find strength of ebb occurred about 0.6 hour after the 

 time of low water, both morning and afternoon; slack before flood occurred 2.2 

 hours before high water; strength of flood 0.4 hour after high water; slack before 

 ebb 3.0 hours before low water. In this connection, however, it is to be noted 

 that the time relations between the various phases of tide and current are subject 

 to the disturbing effects of wind and weather. 



Apart from the disturbing effect of nontidal agencies, the time relations 

 between tide and current are subject to variation in regions where the tide 

 exhibits considerable diurnal inequality, as, for example, on the Pacific coast of 

 the United States. This variation is due to the fact, previously mentioned, thai 



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