REDUCTION OF CURRENT RECORDS 55 
with the line of zero velocity will in general be taken as the time of slack water; but if 
the velocity remains near zero for a considerable period of time, the time of slack may 
be taken at the middle of the period during which the velocity is less than 0.1 knot. 
144. The velocity of the current at the time of each strength is also taken from the 
plotted curve and entered in the form to the nearest tenth of a knot. This value will 
usually be the actual observed maximum velocity unless a smoothed curve has been 
drawn in which case it will be the maximum velocity indicated by the smoothed curve. 
145. The direction of the current at the time of each strength is to be taken directly 
from the original record (Form 270) or from the plotting sheet and will usually be ex- 
pressed in true azimuth. If the series of observations does not exceed 3 days, it is 
recommended that the direction for each strength be taken as the average of the 3 
observed directions nearest the time of the strength. For longer series, the single ob- 
served direction nearest the time of the strength may be used provided it is consistent 
with the generally prevailing direction for the particular phase of current. 
146. Sometimes a nontidal current may be strong enough to overcome the tidal 
flow in one direction as explained in paragraphs 10-11. The current then flows con- 
tinuously in the same direction throughout the tidal cycle with a velocity varying be- 
tween maximum and minimum without coming to a slack. When tabulating a record 
of this kind, the minimum velocity may be tabulated as a negative velocity of opposite 
phase and the time of its occurrence as the time of the strength of that phase. The 
direction of such minimum velocity, however, should be enclosed in parentheses as it 
will not be comparable with the direction of the current of positive velocity of the same 
phase. There being no intervening slack water, the corresponding spaces in Form 451 
are to be left vacant or filled with dashed lines. 
147. Double phases.—In some places the current may be characterized by a double 
flood or a double ebb as illustrated in Figure 4. When tabulating currents of this type 
it is recommended that Form 451 be modified so as to separate the phases applying to 
the flood period and the ebb period as illustrated in Figure 29. In the example, both 
flood and ebb have a double maximum, but in other cases it may be found that only one 
of the strengths is doubled. At the time of the minimum strength the direction of the 
current may reverse itself, in which case the velocity is to be tabulated with a negative 
sign. Only a single column is provided for the direction of the current during either 
the flood or ebb period. The direction to be entered is the one applicable to the greatest 
velocity occurring during the period. 
148. Weak reversing currents.—When a reversing current is weak and irregular 
the times of the several current phases are not well defined and considerable uncer- 
tainty will exist in any attempt to select the times of the individual strengths and slacks. 
In such cases it is desirable to tabulate the observed velocities and directions at hourly 
or half-hourly intervals as referred to the times of the high and low waters at a reference 
station. The method of tabulation is similar to that used for rotary currents and is 
described in paragraphs 164-165. The average velocity for each hourly or half-hourly 
group is plotted and a mean velocity curve drawn through the plotted points. The 
times of the several current phases as referred directly to the high and low waters at 
the reference station are taken from this curve of mean velocities and entered in Form 
451 in the columns of current intervals. The current velocities at the times of flood 
and ebb strength may also be taken from the same curve. The average direction of 
the current for the time of each strength may be obtained by interpolating between the 
hourly or half-hourly group averages. 
