TIDAL CURRENTS 



13 



vious position. The lines drawn to the various hourly points on the 

 ellipse from this new origin will now represent the velocity and 

 direction of the tidal current as affected by the nontidal current. 



The average velocity of the tidal current at the times of flood or 

 ebb strength at Nantucket Shoals Lightship is 0.85 knot. If the 

 nontidal current due to the wind in the case just considered is greater 

 than 0.85 knot, the origin of the velocity lines would lie outside the 

 ellipse. In that case the current would throughout the day be setting 

 either southeasterly or southwesterly, completely masking the rotary 

 character of the tidal current. By plotting the observed hourly veloc- 

 ities and directions of the current, however, the tidal current would 

 appear in its rotary character. This is illustrated in figure 7 for the 

 current at Frying Pan Shoals Lightship under different wind conditions. 

 This lightship is stationed off the coast of North Carolina about 20 

 miles southeasterly from Cape Fear. The hourly velocity and direc- 



Ju(y l^-ZO, 1920 

 L+3, H-3 



Jan 29 -Feb 2, 1920 



L-l "Z-Z 



L-3 H+3 



Scale of Knots 

 I I 



0.0 02 OA 0£ 08 



Figure 7.— EfEect of nontidal current on rotary tidal current, Frying Pan Shoals Lightship. 



tion of the current here is referred to the times of high and low water 

 at Charleston, S. C. 



Observations made at this lightship show the tidal current here to 

 be rotary clockwise, the average velocity at strengths of flood and 

 ebb being about a third of a knot and setting northwest and south- 

 east, respectively. During the 5-day period January 29-February 2, 

 1920, the wind was blowing steadily from the northeast with a veloc- 

 ity of about 30 miles per hour, and the current was observed to be 

 setting at all times southwesterly with a velocity varying from a little 

 less than one-half a knot to a little more than three-quarters of a 

 knot. Apparently the current here at this time was altogether non- 

 tidal. But if the hourly velocity and direction of the current during 

 this period is plotted, the rotary character of the current is immedi- 

 ately apparent. The right-hand diagram of figure 7 represents the 

 current conditions during this 5-day period, the velocity and direc- 

 tion of the current at the different hours being given by the length 

 and direction of the lines drawn from the point P. 



Now, although the current at all times during this period set south- 

 westerly, the diagram reveals clearly the existence of a rotary cur- 



429061°— 42 3 



