106 



U. S. COAST AND GEODETIC SURVEY 



logical conditions, which change from month to month, leave their impress 

 on the tides. For accurate results the direct determination of the tidal datum 

 planes and other tidal quantities should be based on a series of observations that 

 cover a period of a year or preferably three years. Values derived from shorter 

 series must be corrected to a mean value. 



Two methods may be employed for correcting the results of short series to a 

 mean value. One method makes use of tabular values, determined both from 

 theory and observation, for correcting for the different variations. The other 

 method makes use of direct comparison with simultaneous observations at some 

 near-by port for which mean values have been determined from a series of 

 considerable length. 



II. TIDAL CURRENTS, GENERAL CHARACTERISTICS 

 DEFINITIONS 



Tidal currents are the horizontal movements of the water that accompany the 

 rising and falling of the tide. The horizontal movement of the tidal current 

 and the vertical movement of the tide are intimately related parts of the same 

 phenomenon brought about by the tide-producing forces of sun and moon. 

 Tidal currents, like the tides, are therefore periodic. 



Knots 



zar 



FiG. C— Velocity and direction curves for the current, Hudson Eiver, July 22, 1922 



It is the periodicity of the tidal current that chiefly distinguishes it from other 

 kinds of currents, which are known by the general name of nontidal currents. 

 These latter currents are brought about by causes that are independent of the 

 tides, such as winds, fresh-water run-off, and differences in density and tempera- 

 ture. Currents of this class do not exhibit the periodicity of tidal currents. 



Tidal and nontidal currents occur together in the open sea and in inshore tidal 

 waters, the actual currents experienced at any point being the resultant of the 

 two classes of currents. In some places tidal currents predominate and in others 

 nontidal currents predominate. Tidal currents generally attain considerable 

 velocity in narrow entrances to bays, in constricted parts of rivers, and in pas- 

 sages from one body of water to another. Along the coast and farther offshore 

 tidal currents are generally of moderate velocity; and in the open sea, calculation 

 based on the theory of wave motion, gives a tidal current of less than one-tenth 

 of a knot. 



