246 



was opened. The computations of the tides and currents in the canal 

 would then be extended through the approach channel. The co- 

 ordinate amplitudes of the tides at the storage stations in the approach 

 channel used in the initial com.putation would be determined from, the 

 prim.ary tides at these stations derived from the observations. 



477. Computation of the effect of dams or other works decreasing the 

 tidal prism. — The effect of a projected dam, or of other works which 

 would decrease materially the area of the tidal prism, upon the cur- 

 rents in an estuary or other channel, is definitely ascertained by mak- 

 ing a cubature of the channel with the prism unimpaired, and a 

 cubature from the same tides with the reduced prism. While some 

 increase in the tidal ranges below the dam. is to be anticipated, the 

 counterbalancing effect of the increase ordinarily is not sufficient to 

 warrant consideration. Similarly any question that might arise on 

 the effect of the excavation of a considerable tidal basin in the upper 

 reaches of an estuary may be settled by comparative cubatures. 



TIDAL INLETS 



478. Prevalence of inlets. — The littoral drift of sand and shingle 

 along the seacoast tends to build up beaches across the entrances to 

 the identations of the shore line. This process has formed the coastal 

 sounds and lagoons which are the prevailing feature of the coast line 

 of the United States from Maine to the Rio Grande, and which are 

 foimd occasionally on the Pacific and even the Alaskan coasts as well. 

 Inlets into most of these sounds are preserved by the currents set up 

 in these channels by the filling and emptying of the tidal prism. The 

 entrances to nearly all tidal estuaries are similarly contracted by 

 littoral drift, sometimes sufficiently to produce typical inlet channels. 



479. Typical shape of inlet channels. — The material carried by 

 littoral drift into an inlet channel is removed by the currents through 

 the inlet. At an inlet into a coastal sound, it is deposited in fan- 

 shaped bars in the approaches both from the sea and from the sound. 

 A typical natural inlet channel has a deep, narrow gorge through the 

 barrier beach, from which it spreads in both directions with diminish- 

 ing depth. The sea approach to the gorge often is through ill-defined 

 and shifting channels between sand bars. In the sheltered waters of 

 the sound, the bars may even build up into islands. If the basin is 

 small and shallow the approaches m.ay become so prolonged and con- 

 stricted that the currents are no longer sufficient to cope with the 

 encroaching littoral drift, and the inlet closes. The entrance to a 

 large estuary, in which the ebb currents predominate, is often en- 

 circled by a crescent-shaped bar, well out to sea. 



480. Hydraulics of inlet channels. — The improvement of tidal inlets, 

 to afford stable and adequate channels for navigation across their 

 ocean bars, or for other purposes, has an important place in harbor 



