TIDAL CURRENTS 15 



MEAN VALUES 



In the nonharmonic analysis of current observations it is customary 

 to refer the times of slack and strength of current to the times of high 

 and low water of the tide at some suitable place, generally nearby. 

 In this method of analysis the time of current determined is in effect 

 reduced to approximate mean value, since the changes in the tidal 

 current from day to day may be taken to approximate the correspond- 

 ing changes in the tide; but the velocity of the current as determined 

 from a short series of observations must be reduced to a mean value. 



In the ordinary tidal movement of the progressive or stationary 

 wave types the change in the strength of the current from day to day 

 may be taken approximately the same as the variation in the range 

 of the tide. Hence, the velocity of the current from a short series of 

 observations may be corrected to a mean value by multiplying by a 

 factor which is the ratio of the mean range of the tide to the range 

 for the period of the observations. 



It is to be noted that in this method of reducing to a mean value, 

 any nontidal currents must first be eliminated, and the factor applied 

 to the tidal current alone. This may be done by taking the strengths 

 of the tidal current as the half sum of the flood and ebb strengths for 

 the period in question. 



In some places the current, while exhibiting the characteristic 

 features of the tidal current, is in reality a hydraulic current due to 

 differences in head at the ends of a strait connecting two independent 

 tidal bodies of water. East River and Harlem River in New York 

 Harbor and Seymour Narrows in British Columbia are examples of 

 such straits, and the currents sweep mg through these waterways are 

 not tidal currents in the true sense, but hydraulic currents. The 

 velocities of such currents vary as the square root of the head, and 

 hence in reducing the velocities of such currents to a mean value the 

 factor to be used is the square root of the factor used for ordinary 

 tidal currents. 



