219 



stages at that entrance, more water will flow through the canal from 

 that entrance than will flow back at low tide when the direction of the 

 flow is reversed ; provided at least that no adverse constant component 

 of the head is produced 

 by a difference in the 

 elevation of mean tide 

 at the two entrances. 

 The more intricate 

 tides and currents ui 

 a longer canal and 

 differences in the ele- 

 vation of mean tide 

 at the entrances may 

 produce a preponder- 

 ance of flow which is 

 not necessarily from the entrance having the larger tidal range. 



Ml 



10 



6-1 



Sta O 



Figure 74.- 



40 80 I20 I60 



-Instantaneous profiles (adjusted). 



200 



CLOSED CANALS 



420. A computation of the currents and tides in a projected closed 

 canal seldom is necessary, as usually it may be taken for granted that 

 the currents in such a canal will not be troublesome; but should the 

 occasion arise, the primary tides and currents may be computed by 

 a procedure paralleling that applied in the preceding paragraphs to 

 connecting canals. Aside from a practical application, the develop- 

 ment of the effect of frictional resistance upon the primary tides and 

 currents in a long closed canal of uniform dimensions will cast some 

 light upon the characteristics of tidal flow in closed channels in general. 



421. Computation jor closed canals oj moderate length. — If a projected 

 canal is so short that the instantaneous profiles will not depart widely 

 from horizontal lines, the computations may be started by determining 

 the currents that would be produced in successive subsections of the 

 canal if the primary tides in each subsection had the same amplitude 

 and phase as at the entrance. The surface heads in the subsections 

 are then computed, corrected tides derived therefrom, the currents 

 recomputed, and the computations repeated until further corrections 

 become negligible. 



422. Since the discharge at the head of the canal is zero, the dis- 

 charge, Q, at a velocity station at the middle of any subsection is, 

 from ec^uation (278): 



Q = MB sin {<it^^) = l.a UA sin (at+a) (293) 



in wliich M is the area of the cross section at the velocity station, B 

 the amplitude and jS the initial phase of the primary current at the 

 station; and ZallA sin (af+a) is the summation, from the head of the 



192750—40 15 



