TIDES AND CURRENTS IN BOSTON HARBOR 107 



RECTILINEAR TIDAL CURRENTS 



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

 occurs, the tidal current is of the rectilinear or reversing type; that is, the flood 

 current runs in one direction for a period of about six 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 down- 

 stream. 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 C, which represents the velocity and direction of the current 

 as observed in the Hudson River off Fort Washington on July 22, 1922. 



In Figure C the upper curve represents the velocity of the current in knots, 

 flood being plotted above the axis of X and ebb below the axis. The velocity 

 curve represents approximately the form of the cosine curve. The maximum 

 velocity of the flood current is called the strength of flood and the maximum ebb 

 velocity the strength of ebb. The knot is the unit generally used for measuring 

 the velocity of tidal currents and represents a velocity of 1 nautical mile per 

 hour. Knots may be converted into statute miles per hour by multiplying by 

 1.15, or into feet per second by multiplying by 1.69. 



The lower curve of Figure C3 is the direction curve of the current, the direction 

 being given in degrees, north being 0°, east 90°, south 180°, and west 270°. 

 The directions are magnetic and represent the direction of the current as derived 

 from hourly observations. During the period of flood the direction curve shows 

 that the current was running practically in the same direction all the time, 

 making an abrupt shift of about 180° to the opposite direction during the period 

 of slack water. For the ebb period the direction curve likewise shows the cur- 

 rent to have been running in approximately the same direction with an abrupt 

 change of about 180° during slack. 



ROTARY TIDAL CURRENTS 



Offshore the tidal currents are generally not of the rectilinear or reversing type. 

 Instead of flowing in the same general direction during the entire period of the 

 flood and in the opposite direction during the ebb, the tidal currents offshore 

 change direction continually. Such currents are therefore called rotary currents. 

 An example of this type of current is shown in Figure D, which represents the 

 velocity and direction of the current at the beginning of each hour of the after- 

 noon on September 24, 1919, at Nantucket Shoals Light Vessel, stationed off 

 the coast of Massachusetts. 



The current is seen to have changed its direction at each hourly observation, 

 the rotation being in the direction of movement of the hands of a clock, or from 

 north to south by way of east, then to north again by way of west. In a period 

 of about 12 hours it is seen that the current has veered completely round the 

 compass. 



It will be noted that the ends of the radii vectores, representing the velocities 

 and directions of the current at the beginning of each hour, define a somewhat 

 irregular ellipse. If a number of observations are averaged, eliminating acci- 

 dental errors and temporary meteorological disturbances, the regularity of the 

 curve is considerably increased. The average period of the cycle is, from a 

 considerable number of observations, found to be 12^ 25™. In other words, the 

 current day, like the tidal day, is 24'' 50™ in length. 



A characteristic feature of the rotary current is the absence of slack water. 

 Although the current generally varies from hour to hour, this variation from 

 greatest current to least current and back again to greatest current does not 

 give rise to a period of slack water. When the velocity of the rotary tidal current 

 is least, it is known as the minimum current, and when it is greatest it is known 

 as the maximum current. The minimum and maximum velocities of the rotary 

 current are thus related to each other in the same way as slack and strength o'f 

 the rectilinear current, a minimum velocity following a maximum velocity by 

 an interval of about three hours and being followed in turn by another maximum 

 after a further interval of three hours. 



VARIATIONS IN STRENGTH OF CURRENT 



Tidal currents exhibit changes in the strength of the current that correspond 

 closely with the changes in range exhibited by tides. The strongest currents 

 come with the spring tides of full and new moon and the weakest currents with 

 the neap tides of the moon's first and third quarters. Likewise, perigean tides 

 are accompanied by strong currents and apogean tides by weak currents; and when 

 the moon has considerable variation, the currents, like the tides, are characterized 

 by diurnal inequality. 



