A Treatise On The Principles And Practice Of Harbour Engineering

HARBOUR DESIGN 23 being aided therein by the littoral current. The heavier particles are rolled along and partake of a zig-zag movement, as shown in fig. 14. It is generally agreed that the action is practically confined to the region between high and low water marks. The trend of littoral drift is therefore attributable, in the first instance, to the wind which governs the pré- dominant direction of the waves. To illustrate in some way, however imperfectly, the general effect of wind and flow upon a coast-line, with the modifications brought about by intru- sive structures, figs. 15-22 have been Fig. 14. —Track of Shore Drift. prepared. A simple case only has been taken ; the action, as can well be imagined, is often much more complex. The supposition made is that of a coast-line with the dominant current flowing parallel thereto (from right to left) coincident with the direction of the prevailing wind. Fig. 15 and those which servi Fig. 15. the tendency towards shoaling in various parts, which is brought about by the construction of harbour works of typical kinds. The straight pier or breakwater at right angles to the coast-line (fig. 15) induces an accretion of sand and shingle along each of its sides. The windward accumulation is the more pronounced, the leeward deposit being reduced by eddying round the outer end of the pier. The returned pier (fig. 16) serves to increase the leeward deposit, there being a circular motion of the water round the pier- head with a tendency to scour at that point, while the slacker water inside leads to settlement of suspended material. An example of this is to be found in the harbour of Salina Cruz on the Pacific Coast (fig. 18), where the initiation of a break- water of this type brought the low-water line forward, tempo- rarily, at anyrate, to the 26 feet contour of six years previously. Much the same effect is apparent with double piers (fig. 17), the accretion being emphasised by reason of the additional extent of quiescent area. Evidence in support of this is forth- coming from Madras, where the harbour entrance is slowly but surely silting up in this way. Apparently the only practicable means of remedying the evil due to solid structures is that of substituting openwork for the portion of the jetty which immediately joins the land. It has even been suggested that the most logical method is that of “ Island Harbours,” formed in deep water out