A Treatise On The Principles And Practice Of Harbour Engineering

26 HARBOUR ENGINEERING extended to a point sufficiently seaward to bring the fluvial deposits within range of powerful submarine currents, there is less fear of the deposit becoming permanently localised to any serious extent. At the same time, the jetties may need extending ultimately on account of the advancement of the coast-line. With dredging employed as an auxiliary, the silting up of outlet channels may, of course, be held in check as long as and to whatever extent may be deemed expedient. We revert to this part of the subject later on (Chapter IX.). In considering the diagrams employed in illustration of the foregoing principles and hypotheses, it is to be distinetly understood that the shoaling indicated does not necessarily appear above water level, even at lowest water, nor indeed does it, in a number of cases, manifest itself to any pronounced extent. Certain tendencies only are outlined, which become more or less marked, according to the absence or presence of counteracting influences. It is the province of the engineer to secure or provide these counteracting influences by natural or artificial means, so as to maintain a state of Pig. 22. is so involved, and its ramifications so individual treatment. equilibrium in so far as it is possible to do so. Some of the means em- ployed for the purpose will be dis- cussed in a later chapter, but we cannot hope to deal exhaustively with a problem which admits of in- numerable variations in accordance with local peculiarities. The sub- ject of littoral drift and its comple- mentary phenomenon, coast erosion, extensive, as to require special and As a forcible illustration of the complex nature of currents set up by artificial works, a series of illustrations (figs. 23-28) are given, showing the vagaries in the tidal flow exhibited at Dover Harbour since the formation of the new breakwaters. Beforetime, the set of the tide was almost parallel to the shore line, running from N.E. to S.W. from 4| hours after to 1| hours before high water, and from S.W. to N.E. from 2 hours before to 4 hours after high water. At high water of spring tides the rate of the east-going stream was about 4 knots, and at low water the west-going stream had a velocity of 2| knots. Both direction and rate of flow are now completely altered in every respect.1 The currents at the eutrance to Langston Harbour are indicated in fig. 9. Harbour Areas and Entrance Widths.—We turn now to the subject of wave power, regarding it simply as it affects the general question 1 When these diagrams (issued by the Admiralty) were prepared, the western portion of the Island breakwater was still ineomplete and the site occupied by open constructional piling. No doubt further changes will manifest themselves with the progress of the solid work.