221 



^=0.000,005,148 CV=0.823 

 fo/^=0.000,004,37X20,000=0.0874. 



425. Fii'st computation of the primary currents and heads. — The com- 

 putations are started by taking the equation of the tide throughout 

 the canal as: 



y=3 cos at. 



At all storage stations therefore: 



A cos a=3 A sin a=0. 



The computations are conveniently made in the form previously 

 used for connecting canals, and are shown on figure 76, facing page 

 222. The value of {Bjm) cos /3 at station 10, is the storage increment 

 for the half subsection, to 10; that at station 30 is obtained by 

 adding the storage increment between stations 10 and 30, and so on. 

 The subsection velocities and heads are then computed, but since the 

 tide at the entrance to a closed canal is alone fixed, the computed 

 coordinate heads are not subject to adjustment. 



426. Recomputation oj currents and heads. — The currents and heads 

 are next recomputed as shown in figure 76 from the tides established 

 by the heads determined in the initial computation. The component 

 tides, A sin a and A cos a, at stations 40, 20, and are obtained 

 by successively subtracting, algebraically, the component heads, 

 H sin H° and H cos H°, found in the first computation, from the 

 component tides at station 60. 



In the final computation the current at the entrance, station 60, is 

 determined by adding to the component currents at station 50, the 

 storage increments from stations 50 to 60. The component tides at 

 the storage station, station 55, are interpolated. 



427. Results of computation. — The amplitudes and initial phases of 

 the tides at the ends of the subsections, derived from the final com- 

 putation, are: 



Station: a a 



60 3.00 



40 3.12 -3°50' 



20 3.19 -5°10' 



3.21 -5°20' 



The tidal range therefore increases from 6.0 feet at the entrance to 

 6.42 feet at the head of the canal. High water at the head of the 

 canal is 5°.33/28°.98 = 0.18 hours = ll minutes later than at the 

 entrance. The strength of the current at all sections is nearly at 

 midtide, and decreases from 1.66 feet per second at the entrance to 

 zero at the head of the canal. The currents are so weak that the 

 tides and currents approach the condition of frictionless flow. 



428. Computations for a longer canal. — ^The procedure which has 

 been described is applicable only to a comparatively short canal. As 



