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EAST

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Originally appearing in Volume V23, Page 382 of the 1911 Encyclopedia Britannica.
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EAST PIC INOPJETTIES. MEAN See also:

LOW See also:TIDE. I o \ ~'~sb - ' Tore o 2 M(LE5. The shoaling, however, in the See also:jetty channel necessitated its reduction in width by mattresses and spurs from moo ft. to 600 ft., and also dredging to maintain the stipulated central See also:depth of 30 ft., and 26 ft. depth for a width of 200 ft., out to deep See also:water; whilst the See also:outer channel was deflected to the east and narrowed by the See also:alluvium carried. westwards by the littoral current and also deposited in front of the jetty outlet. Accordingly, dredging has been increasingly needed to straighten the channel outside and maintain its depth and width; and since the See also:United States See also:engineers took in See also:hand its See also:maintenance iii 1901, the available depth of the outlet channel has been increased from 26 ft. up to 28 ft. by extensive suction dredging. In See also:order to provide for the increasing requirements of See also:sea-going vessels, the dredging of a channel 35 ft. deep and moo ft. wide, cut from the large See also:south-See also:west pass outlet to deep water in the gulf, was begun at the end of 1903; and jetties of See also:fascine mattresses weighted with See also:stone and See also:concrete blocks have been carried out about 4 and 3 m. respectively from the See also:shore on each See also:side of the outlet for maintaining the dredged channel 3 (fig. 15). These See also:works differ FIepee. o. 1 1 1 Iaooe.F.T. 1 I 1 I t from the See also:prior improvement of the south pass in the See also:adoption mainly of suction dredging for the formation of the channel in See also:place of scour alone, so that it will be unnecessary to restrict the width of the jetty channel to secure the desired depth; whilst as the See also:discharge through the south-west pass is rather more than three times the discharge through the south pass, and the See also:bar is See also:double the distance seawards of the outlet, the slightly converging jetties, in continuation of the south-west pass, are placed about 3400 ft. apart at their outer ends, and have been given about twice the length of the south pass jetties. As soon as the dredging of the channel has been completed (which depends on the appropriations granted by See also:Congress) the south pass will be abandoned, and the south-west pass will See also:form the navigable approach. Dredging will be required for preserving the depth of the outlet of the south-west pass; and when the large See also:volume of See also:sand and other alluvium discharged by the pass accumulates in front sufficiently to begin forming a bar farther out, an See also:extension of the jetties will be necessary to maintain the elongated channel See also:free from See also:drift, and extend the scour, especially in See also:flood-See also:time.

Improvement of Tidal See also:

Rivers for See also:Navigation. Whereas the See also:size of tideless rivers depends wholly on their fresh-water discharge, the See also:condition of tidal rivers is due to the configuration of their outlet, the rise of tide at their mouth, the distance the tide can penetrate inland, and the space available for its reception. Accordingly, tidal rivers sometimes, even when possessing a comparatively small fresh-water discharge, develop under favourable conditions into Iarge rivers in their See also:lower tidal portion, having a much better natural navigable channel at high tide than the largest deltaic rivers, as shown by a comparison of the See also:Thames, the See also:Humber and the See also:Elbe with the See also:Danube, the See also:Nile and the See also:Mississippi. Tidal water is, indeed, unlimited in volume; but, unlike the drainage See also:waters which must be discharged into the sea, it only flows up rivers where there is a channel and space available for its 3 See also:Report of the See also:Chief of Engineers for 1906, pp. 382 and 1296 and charts. ' L. F. See also:Vernon-See also:Harcourt, Rivers and Canals, 2nd ed. pp. 187-90, See also:plate 5, See also:figs. 1 and 9. 2 Ibid. plate 5, figs. 2, 3, 4 and to.

reception. Consequently, it is possible to exclude the tide by injudicious works, such as the sluices which were erected See also:

long ago across the fen rivers to secure the low-lying lands from the inroads of the sea; the tidal influx is also liable to be reduced by See also:accretion in an See also:estuary resulting from training works. The See also:great aim, on the contrary, of all tidal See also:river improvement should be to facilitate to the utmost the flow of the flood-tide up a river, to remove all obstructions from the channel so as to render the scouring efficiency of the flood and ebb tides as great as possible, and by making the tidal flow extend as far up the river as possible to reduce to a minimum the See also:period of slack tide when See also:deposit takes place. Tidal Flow in a River.—The progress of the flood-tide up a river and the corresponding ebb are very clearly shown by a See also:diagram giving a See also:series of simultaneous tidal lines obtained from simultaneous observations of the height of the river See also:Hugli during a high See also:spring-tide in the dry See also:season, taken at intervals at several stations along the river, and exhibiting on a very distorted See also:scale the actual water-level of the river at these periods (fig. 16). The steep form assumed '` P~ vPP PQJ JP P, 4.P~ yJP ,''' J4, a ' P 0'9 UT .F~— `F PJ~ 49 'tJ 4P +a9 FT. -1a 152 See also:MILES 115 75 59 53 35 17 by the foremost See also:part of the flood-tide lines from the entrance to beyond See also:Chinsura, attaining a maximum in the neighbourhood of Konnagar and Chinsura, indicates the existence of a See also:bore, caused by the sand-See also:banks in the channel obstructing the advance of the flood-tide, till it has risen sufficiently in height to See also:rush up the river as a steep, breaking See also:wave, overcoming all obstacles and producing a sudden reversal of the flow and abrupt rise of the water-level, as observed on the See also:Severn, the See also:Seine, the See also:Amazon and other rivers. A bore indicates defects in the tidal condition and the navigable channel, which can only be reduced by lowering the obstructions and by the regulation of the river. No tidal river of even moderate length is ever completely filled by tidal water; for the tide begins to fall at its mouth before the flood-tide has produced high water at the tidal limit, as most clearly shown in the See also:case of a long tidal river by the Hugli tidal diagram. Every improvement of the channel, however, expedites and increases the filling of the river, whilst the volume of water admitted at each tide is further augmented by the additional capacity provided by the greater efflux of the ebb, as indicated by the lowering of the low-water See also:line. Deepening Tidal Rivers by Dredging.—The improvement of tidal rivers mainly by dredging is specially applicable to small rivers which possess a sufficient navigable width, like the See also:Clyde and the See also:Tyne; for such rivers can be considerably deepened by an amount of dredging which would be quite inadequate for producing a similar increase in depth in a large, wide river, with shifting channels. Both the Clyde below See also:Glasgow and the Tyne below See also:Newcastle were originally insignificant rivers, almost dry in places at low water of spring-tides; and the earliest works on both rivers consisted mainly in regulating their flow and increasing their scour by jetties and training works.

They have, however, been brought to their See also:

present excellent navigable condition almost wholly, since 1840 on the Clyde and 1861 on the Tyne, by continuous systematic dredging, rendered financially practicable by the growing importance of their sea-going See also:traffic. The Clyde has been given a minimum depth of about 22 ft. at low water of spring-tides up to Glasgow, and can admit vessels of 27 to 28 ft. See also:draught. In the Tyne (figs. 17 and 18), it was decided in 1902 to provide a minimum dredging depth in the river channel at low water of 25 ft. from the sea to the docks, of 20 ft. thence to Newcastle and of 18 ft. up to Scotswood, the rise of spring-tides increasing these depths by 15 ft. In 1906 it was determined to make the channel 30 ft. deep at low water of spring-tides from the sea to the docks, and in 1908 to deepen it between the docks and Newcastle See also:swing See also:bridge from 20 to 25 ft., and also between the swing bridge and Derwenthaugh from 18 to 25 ft. The natural scour of these rivers has been so much reduced by such an exceptional enlargement of their channels that a considerable amount of dredging will always be required to preserve the depth attained.

End of Article: EAST

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