TIDAL CURRENTS 9 



rate of advance of the tide. In the former case reference is made to 

 the actual speed of a moving particle, while in the latter case the 

 reference is to the rate of advance of the tide phase or the velocity 

 of propagation of wave motion, which generally is many times greater 

 than the velocity of the current. 



It is to be noted that there is no necessary relationship between 

 the velocity of the tidal current at any place and the rate of advance 

 of the tide at that place. In other words, if the rate of advance of 

 the tide is known we cannot from that alone infer the velocity of 

 the current, nor vice versa. The rate of advance of the tide in any 

 given body of water depends on the type of tidal movement. In a 

 progressive wave the tide moves approximately in accordance with 

 the formula r=-y/gh, in which r is the rate of advance of the tide, g 

 the acceleration of gravity, and h the depth of the waterway. In 

 stationary-wave movement, since high or low water occurs at very 

 nearly the same time over a considerable area, the rate of advance is 

 theoretically very great ; but actually there is always some progression 

 present, and this reduces the theoretical velocity considerably. 



The velocity of the current, or the actual speed with which the 

 particles of water are moving past any fixed point, depends on the 

 volume of water that must pass the given point and the cross section 

 of the channel at that point. The velocity of the current is thus 

 independent of the rate of advance of the tide. 



ROTARY TIDAL CURRENTS 



Within the channel of a bay or river, the current is compelled to 

 follow the direction of the channel, upstream on the flood and down- 

 stream on the ebb. Out in the open sea, however, this restriction 

 no longer exists, the current having complete freedom so far as direc- 

 tion is concerned. Offshore, therefore, tidal currents are generally 

 not of the 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 5, which represents 

 the velocity and direction of the current at the beginning of each 

 hour of the forenoon of July 30, 1922, at Nantucket Shoals Lightship, 

 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 a little more than 12 hours it is 

 seen that the current has shifted in direction completely round the 

 compass. 



It will be noted that the tips of the arrows, representing the veloc- 

 ities and directions of the current at the beginning of each hour, 

 define a somewhat irregular ellipse. If a number of observations are 

 averaged, eliminating accidental 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 

 for the rotary current, like the tidal day, is 24^ 50°^ in length. 



