COASTAL CURRENTS ALONG THE ATLANTIC COAST 

 OF THE UNITED STATES 



TIDAL CURRENTS, GENERAL CHARACTERISTICS 



DEFINITIONS 



Tidal currents are the horizontal movements of the water that 

 accompany the rising and falling of the tide. The horizontal move- 

 ment of the tidal current and the vertical movement of the tide are 

 intimately related parts of the same phenomenon brought about by 

 the tide-producing forces of sun and moon. Tidal currents, like the 

 tides, are therefore periodic. 



It is the periodicity of the tidal current that chiefly distinguishes 

 it from other kinds of currents in the sea, which are known by the 

 general name of nontidal currents. These latter currents are brought 

 about by causes that are independent of the tides, such as winds, 

 fresh-water run -off, and differences in density and temperature. 

 Currents of this class do not exhibit the periodicity of tidal currents. 



Tidal and nontidal currents occur together in the open sea and in 

 inshore tidal waters, the actual current experienced at any point 

 being the resultant of the two classes of currents. In some places 

 tidal currents predominate and in others nontidal currents predomi- 

 nate. Tidal currents generally attain considerable velocity in narrow 

 entrances to bays, in constricted parts of rivers, and in passages from 

 one body of water to another. Along the coast and farther offshore 

 tidal currents are generally of moderate velocity; and in the open 

 sea, calculation based on the theory of wave motion, gives a tidal 

 current of less than one-tenth of a knot. 



REVERSING TIDAL CURRENTS 



In the entrance to a bay or in a river and, in general, where a 

 restricted width occurs, the tidal current is of the reversing or recti- 

 linear type; that is, the flood current runs in one direction for a period 

 of about 6 hours and the ebb current for a like period in the opposite 

 direction. The flood current is the one that sets inland or upstream 

 and the ebb current the one that sets seaward or downstream. The 

 change from flood to ebb gives rise to a period of slack water during 

 which the velocity of the current is zero. An example of this type of 

 current is shown in figure 1 , which represents the velocity and direction 

 of the current as observed on August 8-9, 1922, in The Narrows, the 

 entrance to New York Harbor. 



The curve of figure 1 was drawn by plotting the velocity of the 

 current as observed at the beginning of each hour and drawing a 

 smooth curve that conformed as nearly as possible with the plotted 

 velocities. The northerly setting or flood velocities were plotted above 



