201 



on the diagram the discharge is in the positive direction at both 

 entrances, and the outflow at station 200 largely exceeds the inflow 

 at station 0. The tidal prism in the canal is therefore emptying 

 through the further entrance. During the interval BC it is emptying 

 through both entrances; and dm'ing the brief period CA' through 



^ 



R 



1 1 — 1 i r 



o 



^ I i I I 1 i 

 6 \e 



1 I 



Lvjhctp Ho ur^s 



Figure 64.— Discharges and storage, canal 200,000 feet long. 



the initial entrance. The filling of the tidal prism during the interval 

 A' A" follows a similar sequence. 



It is apparent from the figure that, in this case, the discharges and 

 velocities at the further end of the canal are caused principally by the 

 fining and emptying of the tidal prism. Because the filhng and empty- 

 ing is largely through this entrance, the currents are there the strong- 

 est, although the tidal range is the least. 



390. The strength and timing of the currents in a connecting canal 

 depend upon the relative timing of the tides at the entrances, as well 

 as upon the amphtudes of these tides. Thus if in the example devel- 

 oped in the preceding paragraphs, high water at the further entrance 

 is taken 2 lunar hours (60°) after, instead of 2 lunar hours before, that 

 at the initial entrance, the equations of the current are found to be: 



At the initial entrance: ■y = 3.04 sin (a^+108°30'). 

 At the further entrance: zj = 2.30 sin (a^ + 52°200. 



The change in the timing of the tide produces therefore the strongest 

 currents at the initial entrance, where the tidal range is the greatest, 

 instead of at the further entrance, where the range is the least. A 

 diagram of these velocities, and consequent discharges and the storage 

 and release of water in the tidal prism of the canal, is shown in figure 65, 

 page 202. 



In this case the interval, ^5, during which the prism is emptying 

 because of the greater outflow through the further entrance, station 

 200, is shorter than the interval CA' , during wliich the outflow 



