202 



through the mitial entrance is the greater; and the interval BC, 

 during which the prism is emptying through both entrances, is rela- 

 tively long. The tidal prism fills and empties through both entrances, 

 but somewhat more water enters and leaves through the initial 

 entrance than through the other. 



(/? 



Q_ 



^ 



^0 



> 





o 



.'2 -^0-" 



C K 



<>^ 



.^j 



tP. 



-<p; 



T 1 1 i 1 T- 



6 



\a 



1 — I — i — r- 



Luhan Hour;s 



Figure 65.— Discharges and storage, with different timing of entrance tides. 



391. The longer a canal the less is the net slope set up by given 

 tides at the entrances, and the more are the currents determined by 

 the storage and release of water in the canal prism. It may be 

 expected that in a very long canal the tidal prism will fill and empty 

 from both ends during most of the tidal cycle, and that the currents 

 will be stronger at the two entrances than in the interior of the canal. 

 Thus a computation of the primary currents in a canal 360,000 feet 

 (68.5) miles in length, and 30 feet in mean depth at mean tide, with 

 the same entrance tides as in the first example, i. e., 



2/0= cos 4m2f 



^1 = 2 cos (m2^+60°) 



and with the same coefficient of roughness, shows the strength of the 

 current decreasing from 2.3 feet per second at the initial entrance to 

 1.0 foot per second at a point 100,000 feet from that entrance; thence 

 increasing to 3.8 feet per second at the further entrance. The equa- 

 tions of the entrance tides are: 



Initial entrance: v=2.3 sin (m2f+149°). 

 Further entrance: v=3.8 sin (nio^+lO^SO')- 



The diagram of the entrance velocities, discharges, and channel 

 storage, figure 66, shows that in this case the entrance currents are 

 due principally to the filhng and emptying of the tidal prism through 

 both ends of the canal. 



