Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/483

H A R B O U R S 4(51 therefore to depend on the relation subsisting between the height of the waves at the place and the height and con figuration of the rock itself above and below low water, and perhaps also on the configuration of the bottom of the sea near the rock. 1 While the rock at Dim Heartach from its height above the sea forms a protection against the smaller waves, it operates as a dangerous conductor to the larger waves, enabling them to exert a powerful action at a much higher level than if the rock had been lower. It is of great importance that these facts should be kept in view, and that the Eddystone should not be regarded as a safe model for imitation at all rocks which are exposed to a heavy sea. The impact of the waves against the outside of a sea wall or pier gives rise to four distinct forces, namely (1) the direct horizontal force, which tends to shake loose, or drive in, the blocks of which the masonry consists ; (2) the vertical force acting upwards on any projecting stone or protuberance, as well as against the lying beds of the stones ; (3) the vertical force acting downwards, which results either from the receding wave striking upon the toe of a talus wall, or from its passing over the parapet and falling upon the pitching behind, so as to plough it up; (4) the " backdraught," which tends by reaction from the wall to remove the soft bottom, and in this way to undermine the lower courses or to suck loose stones out of the work. It may be concluded from the above that the points which require to be most attended to are the contour and quality of masonry of the wall itself ; the parapet, which if not of proper form, or of insufficient height, leads to damage in the pitching behind it ; and lastly, the foundation- courses, in the design and construction of which, if similar precautions be not attended to, underwashing of the bottom may take place, so as to leave the lowest courses without protection. Within the masonry, as well as without, the waves exert force in the following different ways : (1) by the propaga tion of vibrations produced by the shock of the waves on the outer or sea-wall, through the body of the pier to the inner or quay-wall; (2) by the direct communication of the impulses through the particles of the fluid occupying the interstices of the hearting, so as to act against the back joints of the face stones of the inner quay; (3) by the sudden condensations and expansions of the air in the hearting, so as to loosen, and at last to blow out, the face stones of the quay ; combined with (4) the hydrostatic pressure of the water, which is forced through the sea-wall, and, from want of free exit, is retained and acts as a head at the back of the quay, and which, however small in quantity, will, as in a Bramah press, act with equal force upon all surfaces exposed to its pressure, however large those surfaces may be. The last three causes are probably the most efficient agents in the work of destruction. From a comparison of different piers in the German Ocean where we have most examples, and from careful consideration of the other data, it appears that vertical piers, fully exposed to the ordinary waves of that sea, should be not less than from 35 to 45 feet broad at the level of high water. Of course this remark does not apply to anomalous cases where waves are exceptionally high, but, as already stated, to works exposed to the ordinary waves. The positions in which the piers are to be placed depend on the nature and configuration of the shore and of the 1 Mi D. A. Stevenson has lately shown that a funnel-shaped de pression in the sea-bottom extends from the ocean in the direction of, and nearly as far as, the Dhu Heartach rock, a geological formation which must certainly admit the heaviest class of waves to come nearer to the rock (Min. of Proc. of the Inst. of Civil Engineers, vol.Klvi.). bottom. After a correct plan, with soundings, has been Desigi obtained, the next step is to lay down contour lines of the of different depths which make the limits of the deep and shoal |J water at once obvious to the eye. The lines of the piers may i iar } JO , then be sketched so as, without disregarding other con ditions, to keep the works as much as possible on the shoal ground, while they at the same time enclose the greatest possible area of deep water. The entrance should be fixed seaward of every part of Entrai the works ; and its direction, unless where the internal area is small and the sea very heavy, should if possible be made to coincide with that of the heaviest waves, so that they may run along with and guide vessels into the harbour. The outer pier should in such a case be extended suf ficiently far seaward of the end of the inner pier head to give it a free end, as in fig. 7, thus allowing a ship plenty of sea room to shape an easy course before taking the entrance to the sheltered basin of the harbour. There should be sufficient distance landward of a harbour mouth to allow a vessel, having full weigh on her, to shorten sail. For this Minard recommends that, where vessels of ordinary size require to alter their course in order to reach the inner basin, no circle with a less radius than 200 yards in smooth water should, if possible, be adopted, and it should not be forgotten that a vessel requires a larger radius in turning where the water is shoal than where it is deep. If the beach consist of travelling detritus, the entrance must be so devised as not to form a trap for the passing shingle. This danger is most to be dreaded on the coasts where heavy waves strike obliquely upon the shore. Finally, there must be a good "loose," so that vessels on leaving the harbour shall be able to shape their course free of rocks or a lee-shore; and Minard very properly remarks that the mouths of harbours should be designed rather to suit the entering, which is far more important, than ihe exit of vessels. Preference should generally be given to a pier of hori zontal convex outline, or a polygonal form, than to one long straight pier. The principal objection to a straight pier does not, however, extend to cases where the heaviest waves strike upon it obliquely, and roll landwards along the sea-wall. Especial care, however, must in all cases be taken that a sea-work nowhere presents to the sea a surface of concave horizontal outline, or, what is still worse, abrupt faces which form a re-entrant angle ; for the waves will then act with an almost explosive violence, the breaking of a free wave being a very different thing from the breaking of a wave confined by a barrier of masonry. While the first may be compared to the harmless ignition of a loose heap of gun powder, the other resembles the dangerous explosion pro duced by the discharge of a cannon. Piers hori- zonta cur- vatur pre- ferabl to loi straig

piers.