257 



503. Because the strengths of the flood and of the ebb occur at 

 dift'erent heights of the tide, the areas of the cross sections of the 

 channel are not the same at both and their velocities would differ 

 somewhat even if the flow were wholly tidal. For the purpose of 

 applying the correction, the tidal parts of flood and ebb strengths are 

 considered to be equal. The tidal current strength at the station 

 during the period of the observations is then taken as one-half of the 

 arithmetic sum of the mean observed flood and ebb strengths, and the 

 nontidal current as one-half of their algebraic sum, with the flood cur- 

 rent positive and the ebb negative. These tidal current strengths are 

 corrected to tlieh long-term values by applying the factor derived 

 from the comparative tidal ranges at the reference station. The cor- 

 rected average flood strength is then derived by adding, algebraically, 

 the nontidal current to the corrected tidal current strength; and the 

 corrected ebb strength by the algebraical subtraction of the nontidal 

 current. 



504. At stations in tidal straits, in which the flow is largely frictional 

 and determined almost entirely by the surface head between the 

 entrances, the average tidal current strength derived from a short 

 series of observations is multiplied by the square root of the ratio of 

 the established mean range at a suitable tidal station in the water- 

 way to the average observed range during the period of the current 

 observations. 



505. Average polar curves of rotary currents. — The rotary currents at 

 offshore stations usually are weak and irregular. To prepare an average 

 current curve at a station where the tides and currents are of the 

 semidiurnal type, such as that shown in figure 82, the directions and 

 velocities of all currents observed within half an hour before or after 

 a predicted time of high water at the reference station are summed 

 and averaged to give the average direction and velocity at the time 

 of liigh water at the reference station; those observed between half 

 an hour and an hour and a half after high water, to give the average 

 direction and velocity 1 hour after high water at the reference station ; 

 and so on. The reference times usually extend from 2 hours before to 

 3 hours after both high and low water at the reference station. Cur- 

 rents of the mixed type, such as those shown in figures 83 and 84, are 

 similarly grouped at the nearest hours at, before and after, higher 

 liigh, higher low, lower high and lower low water at the reference 

 station. 



506. Any average constant current at the station may be determined 

 by resolving either the original observations or their hourly compila- 

 tions into south-north and west-east components. The algebraic 

 average value of these components in each direction quite evidently 

 is the component of the constant current in that direction. The 

 summation of the component velocities to derive these averages and 



