2 U. S. COAST AND GEODETIC SURVEY 



the line of zero velocity and the southerly setting or ebb velocities 

 were plotted below this line. The velocities are given in Icnots, which 

 is the unit generally used in measuring tidal currents and represents 

 a velocity of 1 nautical mile per hour. Since a nautical mile has a 

 length of 6,080 feet, laiots may be converted into statute miles per 

 hour by multiplying by 1.15, or into feet per second by multiplying 

 by 1.69. 



The curve of the reversing current resembles the tide curve. The 

 maximum velocity of the flood current, called the strength of flood, 

 corresponds to the high water of the tide curve, while the maximum 

 velocity of the ebb, called the strength of ebb, corresponds to the low 

 water. The current day, like the tidal day, has a length averaging 24 

 hours and 50 minutes. 



The current curve shown in figure 1 represents the current near the 

 surface in the axis of the channel of The Narrows. From observation 

 and also from theory it is laiown that the tidal current extends from 

 the surface to the bottom. In general it may be said that the velocity 



Figure 1.— Current curve, The Narrows, New York Harbor, August 8-9, 1922. 



of the tidal current decreases from the surface to the bottom, the veloc- 

 ity near the bottom being about two thirds that at the surface. But 

 the effects of wind and fresh -water flow may bring about considerable 

 variation in the vertical velocity distribution. 



The current in a channel is also characterized by a variation in 

 the horizontal distribution of velocity. In a rectangular channel of 

 miiform cross-section, the velocity is greatest in the center of thii 

 channel, and decreases uniformly to both sides. Combining both the 

 vertical and horizontal variations, it may be said that the average 

 velocity of the current in a section of a regular channel is about three- 

 quarters that of the central surface velocity. 



Wliere the current is undisturbed by wind or fresh-water flow, the 

 flood and ebb velocities, and the durations of flood and ebb are approx- 

 imately equal. In this case, too, the characteristics of the current 

 from the surface to the bottom are much the same. That is, the 

 strengths of the flood and ebb currents, and also the slacks, occur at 

 about the same time from top to bottom. If, however, nontidal cur- 

 rents are present, the characteristics of the tidal flow are modified 

 considerably. The effect of nontidal currents on tidal currents may 

 be derived from general considerations. 



