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engineering. It often is accomplished by constructing jetties to im- 

 pound tlie littoral drift and to concentrate and direct the currents 

 over the sea bar. In the design of these works consideration must be 

 given to the cross section of the channel that can be maintained by 

 the tidal flow, and to the effect of a contraction or enlargement of the 

 channel on the tidal ranges in the basin and the consequent currents 

 through the inlet. A mathematical analysis of the relation between 

 the capacit}" of an inlet channel, the tides in a basin of given surface 

 area, and the currents in the inlet, is necessarily based on the assump- 

 tion that the inlet channel is of determinable length and regular cross 

 section, and the basin so deep and of such limited area that its tides 

 have the same timing and the same amplitude at all points. The 

 approach channels of a natural inlet depart so far from these ideal 

 conditions that the computation of the tidal currents in them is as 

 uncertain as is the computation of the currents in an irregular shoal 

 reach of an upland stream. Even a channel between parallel jetties 

 is apt to liaA^e an unpredictably irregular cross section. Furthermore, 

 the tides at stations on a wide and comparatively shallow basin do not 

 rise and fall simultaneously, but become progressively later the more 

 distant the station from the entrance. Space will not therefore be 

 taken for a mathematical analysis of inlet tides. Their outstanding 

 characteristics may be inferred from elementary hydraulic relations. 



481. It is fairly evident that the frictional resistance in the con- 

 stricted channels through an inlet must reduce the amplitude of the 

 tides in the basin, and delay the rise and fall of these tides, so that high 

 and low water in the basin are later than in the sea off the inlet. If 

 the constricted channels of the inlet are relatively short, the currents 

 must become excessive before the friction head can be sufficient to 

 have any material effect upon the tides in the basin. vStable short 

 inlets through erodible material therefore are usually so large that the 

 tidal range inside the inlet is practically the same as that outside. If 

 the improvement of such an inlet is so designed that the discharge, 

 determined from a cubature of the recorded tides in the basin, will 

 not produce excessive currents, no apprehension need be felt that the 

 improvement will have any material effect upon the tides in the basin, 

 or reduce the tidal discharge. Again, the straightening and deepen- 

 ing of inlet approach channels which have become so filled and pro- 

 longed as to throttle the tidal range in the basin may be expected to 

 increase the currents in these channels, and increase the tides in the 

 basin, until the channels have been given a sufficient capacity to nearly 

 equalize the tidal range in the basin and in the sea. In either case 

 the maximum cross section of a self-maintaining channel is determined 

 by the volume of the unimpaired tidal prism in the basm. 



