254 



of the variation in the currents at different points in a cross section of 

 a tidal waterway, the harmonic constants at individual current sta- 

 tions are not of such general interest, but a summary description of 

 some of the processes employed in their computation may not be out 

 of place. 



497. Harmonic analysis of reversing currents.- — Current measure- 

 ments at a station in tidal waters usually must be made by a party of 

 some size, from a suitable boat, anchored accurately in position. 

 Consequently current measurements generally are fewer than the tidal 

 observations used for harmonic analysis. If hourly current obser- 

 vations for a 29-day period are available, the harmonic constants for 

 the groups of the principal lunar and solar components, M and S, 

 including their overtides, and the N2, K, and O] components, usually 

 are determined directly from the observations by precisely the process 

 used in determining the tidal harmonic constants. These compo- 

 nents are sufficient for current prediction. The overcurrents, such 

 as M4 and Me, generally are proportionally larger than the correspond- 

 ing overtides. If a longer series of observations is available, other 

 components may be included. If hourly observations for a 29-day 

 period are not available, harmonic analyses are made both of the 

 currents at the station during the limited period of the observations, 

 and of the concurrent tides at a standard tide station. The mean 

 value of the amplitude of each current component and of its over- 

 currents is then found by multiplying the amplitude computed from 

 the observations by the ratio of the established mean amplitude of the 

 corresponding component of the tide at the base station to the tidal 

 amplitude computed from the concurrent observations. The epoch 

 of each current component is found by applying the differences be- 

 tween the initial phases of the current at the current station and the 

 tide at the base station, computed from the concurrent short-term 

 observations, to the established epoch of the tide at the base station, 

 corrected for the difference in the longitudes of the two stations. 

 The predicted hourly heights of the tide at the base station, instead 

 of the recorded heights, usually are preferred for this comparison, 

 since accidental meteorological disturbances of the tide may not 

 produce corresponding changes in the current at another station. 



498. Prediction oj currents in tidal straits. — It was shown in para- 

 graph 256 that if a channel is so short that its currents are but little 

 modified by the storage and release of water in its tidal prism, and if 

 the fluctuating surface head between the entrances produces such 

 strong currents that the flow is essentially frictional, or "hydraulic," 

 the current lags behind the head by but a small angle, and the square 

 of the successive strengths of the current is closely proportional to the 

 nearly concurrent maximum surface heads. The amplitude and phase 

 of each component of the surface head in the strait may be determined, 



