223 



corresponding amplitude of the tide at the head of the canal is, by- 

 interpolation from the tabulated data, 3.34 feot; and its initial phase 

 is — 5°30' — 8°20' = — 13°50'. The primary tides and currents at 

 any other station on an 80,000 foot canal could be determined in a 

 similar manner by establishing their relation to the amplitude of the 

 tide at station 60. 



Perhaps a better method for computing the tides and currents in a 

 very long closed canal is to determine those that would be produced 

 in the successive subsections by tides of several amplitudes at the 

 head of the canal. If the subsections are 20 stations in length, the 

 current produced at station 10 by a tide of given amplitude at the 

 head of the canal is derived from the velocity increments from the 

 tide at station 5, and the coordinate heads, tides and currents at 

 station 20 computed therefrom. The coordinate amplitudes of the 

 tide at station 25 can then be set forward with fair assurance and 

 the currents at station 30 determined by adding the velocity incre- 

 ments due to the tide at station 25 to the coordinate currents at 

 station 20 ; and so on to the station at the entrance. The amplitudes 

 and phases of the tide and current at any station on the canal can 

 then be plotted against the several computed amplitudes of the 

 tide at the entrance, and those corresponding to an entrance tide of 

 a given amplitude taken off these diagrams. 



430. Characteristics of the tides and currents in a long closed canal of 

 uniform cross section. — The primary tides and currents in a canal 

 140,000 feet (about 26.5 miles) in length, of uniform cross section, 16 

 feet in mean depth, produced by an entrance tide of 3 foot amplitude, 

 as computed by the step by step process just outlined, are as follows: 



Primary tides and currents in closed canal 14-0,000 feet long, with a mean depth of 



16 feet at mean tide 



[C=ioo] 



The angular lag, 4>, of the current increases from 15° in the entrance 

 subsection, station 120 to 140, to 90° at the head of the canal. To- 

 ward the entrance the flow becomes largely frictional, while near the 

 head it is essentially frictionless. 



