2; MANUAL OF CURRENT OBSERVATIONS 
Reversing Tidal Currents 
6. Reversing tidal currents alternate in direction of flow. When the movement 
is towards the shore or up a stream, the current is said to be flooding and when in the 
opposite direction, it is said to be ebbiny. In a strait connecting two tidal bodies of 
water the application of the terms must be somewhat arbitrary and a special definition 
may be necessary to avoid ambiguity. 
7. In the normal semidiurnal type of reversing current, the flow is approximately 
6 hours in each direction with slack water occurring as the current changes direction. 
Beginning with slack water, the velocity of the current increases for about 3 hours to 
a maximum known as strength of current, then decreases for about 3 hours to the follow- 
ing slack. The current then reverses and -begins running in the opposite direction 
with the velocity increasing to another maximum and then decreasing to zero, thus com- 
pleting the semidiurnal tidal cycle in an average time of 12.42 hours. In the diurnal 
type of current a cycle covering 24.84 hours will include one complete flood period and 
one complete ebb period. 
8. A reversing current may be graphically represented by a curve in which veloci- 
ties are plotted as ordinates with times as the abscissas. By considering flood velocities 
as positive and ebb velocities as negative, the resulting graph will roughly approximate 
a cosine curve. The maximum and minimum points of the curve will usually represent 
the strengths of flood and ebb respectively, and the pomts where the curve crosses the 
line of zero velocity will indicate the times of slack water. Figures 1-8 are graphic 
representations of some different types of reversing currents. 
9. In a true cosine curve referred to its own axis, it may be shown that the mean 
value of all positive ordinates within any cycle is equal to 2/m (or 0.637) times the maxi- 
mum ordinate or amplitude of the curve. Numerically the mean of all negative 
ordinates within the cycle will be the same. Insofar as the current velocity curve 
Flood 
-- Strength of flood 
jy Slack water 
& 
[s 
‘Slack water 
*--Strength of ebb 
Ebb 
FIGURE 1.—Current in New York Harbor Entrance at time of equatorial tides. (A normal semidiurnal reversing current.) 
