Chapter VIII 



COMPUTATION OF TIDES AND CURRENTS IN LONG CANALS 

 WHEN THE FLOW IS NOT FRICTIONLESS 



Paragraphs 



Connecting canals; general consideration 351-352 



Selection of representative entrance tides 353-358 



Primary entrance tides 359-360 



Computation of primary tides and currents 361-374 



First example, canal of uniform cross section 375-384 



Discussion of results 385-391 



Second example, canal whose cross section is not uniform 392-403 



Adjustment for distortions of primary tides and currents 404-419 



Computations for closed canals 420-432 



Midstream currents 432-434 



CONNECTING CANALS 



351. General considerations. — Perhaps the most important apphca- 

 tioii of the principles of tidal hydraulics is in the computation of the 

 currents which may be expected in a projected long open connecting 

 canal because of the tides at the entrances; and in the concurrent de- 

 termmation of the elevation of low and high water through the canal. 

 As has been pointed out, no artificial canal can be so deep that the 

 tidal flow approaches a frictionless condition, if the currents are of 

 any consequence whatever. Frictional resistance to flow has there- 

 fore an important effect upon the tides and currents. The variation 

 in the depth and width of the water prism as the tide rises and falls 

 also may have a sufficient effect to warrant consideration, and the 

 entrance, recovery and velocity heads may be more than negligible. 



352. Accuracy required. — The usual purpose of such computations is 

 to ascertain whether the currents will be strong enough to affect ad- 

 versely the use of the canal for navigation, or to cause serious erosion 

 of the bed and banks. The computations also show the depths to 

 which the successive sections of the channel must be excavated to 

 afford the designed depth for navigation at the selected low-water 

 datum. These purposes are fulfilled if the maximum current ordin- 

 arily to be expected in anj^ part of the canal is reliably determined to 

 say the nearest half of a foot per second, and the elevation of low water 

 in the successive sections to the nearest half foot; but good workman- 

 ship in the calculations usually requires that they check to the nearest 

 tenth of a foot per second, and tenth of a foot of elevation respectively. 



(183) 



