234 



tion does not affect greatly the tides and currents inside of the entrance. 

 In short, long natural tidal channels, other than tidal straits, are 

 usually of estuary form; and if they are not too deep, their tides and 

 currents ordinarily have the general characteristics of those of an 

 ideal estuary. 



449. Effect of local contractions and enlargements upon the range and 

 rate of the tide. — Variations in the consistency of the bed and banks of a 

 natural estuary result in local contractions and enlargements of the 

 cross section, so that the strength of the current is no more uniform 

 than is the current velocity in a natural upland stream whose bed and 

 banks are of similar material. The consequent variation in the 

 amplitude of the surface slope produces variations both m the tidal 

 range and in the rate at which the tide advances up the channel. The 

 nature of these variations may perhaps be developed most readily from 

 a diagram. 



In figure 79, CPo is the generating radius 

 of the primary tide at the downstream, or 

 initial, end of a short section of a tidal 

 estuary. If the currents were of uniform 

 strength, and the tides of constant range 

 throughout the estuary, the generating 

 radius of the tide at the upstream end of 

 the section would be CPi, equal in length, 

 but lagging behind CPq by an angle deter- 

 mined by the rate of progress of the tide, 

 ■\gH. The surface head in the section 

 would then be CH, equal and parallel to 



Figure 79 



PqPi (par. 244); and the generatmg radius 

 of the primary current would be CB, making an angle of —0 — 90° 

 with CH (par. 248). Upland inflow disregarded, this cm-rent is due 

 wholly to the discharge at the section produced by the filling and 

 emptying of the tidal prism upstream therefrom. The phase of the 

 current CB has therefore a fixed relation to the phase of the tide CPi . 

 If, because of a local contraction at the section, the discharge pro- 

 duces a current of increased amplitude CB\ the amplitude of the head 

 is increased to CH', but the angular lag, 4>, of the current with respect 

 to the head is decreased, so that the angle H'CH is greater than B'CB. 

 The generating radius of the tide at the upstream end of the section 

 becomes CP'i. The angle PiCP'i is nearly or quite equal to BCB' , 

 and the angle PiPJP'i is equal to HCH' . It is apparent from the 

 figure that the increase in the current results in a decrease in the tidal 

 range and an increase in time of the tide at the upstream end, with a 

 consequent decrease in the rate at which the tide travels through the 

 section. The decrease in the tidal range at the upstream end tends to re- 

 duce the discharge at the section, and checks the increase in the current. 



