110 



U. S. COAST AND GEODETIC SURVEY 



the diurnal inequality in the current at any given place is, in general, only about 

 half as great as that in the tide. This brings about differences in the corre- 

 sponding features of tide and current as between morning and afternoon. How- 

 ever, in such cases it is frequently possible to refer the current at a given place 

 to the tide at some other place with comparable diurnal inequality. 



EFFECT OF NONTIDAL CURRENT 



The tidal current is suljject to the disturbing influence of nontidal currents 

 which affect the regularity of its occurrence as regards time, velocity, and direc- 

 tion. In the case of the rectilinear current the effect of a steady nontidal current 

 is, in general, to make both the periods and the velocities of flood and ebb 

 unequal and to change the times of slack water but to leave unclianged the 

 times of flood and ebb strengths. This is evident from a consideration of Figure 

 F, which represents a simple rectilinear tidal current, the time axis of which is 

 the line AB, flood velocities being plotted above the line and ebb velocities 

 below. 



When unaffected by nontidal currents, the periods of flood and ebb are, in 

 general, equal as represented in the diagram, and slack water occurs regularly 

 three hours and six minutes after the times of flood and ebb strengths. But if 

 we assume a steady nontidal current introduced which has, in the direction of 

 the tidal current, a velocity component represented by the line CD, it is evident 



Fig. F.— Effect of nontidal current on tidal current 



that the strength of ebb will be increased by an amount equal to CD, while the 

 flood strength will be decreased by the same amoimt. The current conditions 

 may now be completely represented by drawing, as a new axis, the line EF 

 parallel to ^jB and distant from it the length of CD. 



Obviously, if the velocity of the nontidal current exceeds that of the tidal 

 current at the time of strength, the tidal current will be completely masked 

 and the resultant current will set at all times in the direction of the nontidal 

 current. Thus, if in Figure F the line OP represents the velocity component 

 of the nontidal current in the direction of the tidal current, the new axis for 

 measuring the velocity of the combined current at any time will be the line GO 

 and the current will be flowing at all times in the ebb direction. There will be 

 no slack waters; but at periods 6 hours 12 minutes apart there will occur mini- 

 mum and maximum velocities represented, respectively, by the lines RS and TU. 



In so far as the effect of the nontidal current on the direction of the tidal cur- 

 rent is concerned, it is only necessary to remark that the resultant current will 

 set in a direction which at any time is the resultant of the tidal and nontidal 

 currents at that time. This resultant direction and also the resultani velocity 

 may be determined either graphically by the parallelogram of velocities or by 

 the usual trigonometric computations. 



VELOCITY OF TIDAL CURRENTS AND PROGRESSION OF THE TIDE 



In the tidal movement of the water it is necessary to distinguish clearly 

 between the velocity of the current and the progression or 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. 



