PIERS, of Harbours and of Landing-Places. In the artificial ban hours formed upon the shores of bays or roadsteads, it ia often necessary to execute a defensive mass of stone-work, or occasionally of timber, either for the purpose of securing the tranquillity of the harbour, or of providing the means for landing goods and passengers from ships which require a considerable depth of water. These works are known by the names of piers or of jetties, and their construction involves some of the moat interesting questions of the science of marine hydraulic engineer ing, on account of the mechanical dillieulties of their execution, and of the influence which every change in the outline of a sea-shore must exercise upon the relative conditions of the tides, currents, and alluvial deposits which take place on that shore. It would seem, from these remarks, that the most important points to be discussed in designing any work of these descriptions are—I, the directions to be given to them in plan; 2, the technical methods of their execution ; and 3, the precautions to be taken to prevent, or the methods to be adopted for removing, any accumulation of alluvial matters caused by the erection of the piers.
I. Before entering upon the consideration of the details of piers or jetties, it may be observed that their execution is only desirable when the port they are intended to serve is situated in a Lay which is natu rally sheltered from the extreme violence of the sea. There are, no doubt, many harbours formed upon exposed coasts where the piers are made to inclose, at once from the sea, an outer harbour, leading to still water inner basins; as, for instance, at Ramsgate, Dieppe, &c. But however the piers of such harbours may be placed, there Must always be so great a danger of a ship missing the entrance in a storm, and the agitation of the waves in the outer harbour, at such periods, must pro duce such an amount of injury to the lock-gates of the inner harbour, that it may be laid down as a general rule that no safe harbour, no real harbour of refuge, can ever be formed on an open shore.
Assuming, however, that the position selected for the erection of the piers of a harbour is properly sheltered from the violence of the open sea, the direction of the piers themselves must be such as to allow the current of the flood-tide to assist in carrying a ship into the fair way of the channel; and the general practice appears to be in favour of their being placed so as to form an angle of from 12° to 20° to the direction of the prevailing winds, although local circumstances may, and often do, require that the latter law should be modified. If the
sea should be exposed to the action of the wind to such an extent as to be very rough in stormy weather, the piers must not be built in straight, or even in long, circular, or elliptical lines ; because in that case the agitation of the open sea would be transmitted to the inner harbour. But when the outlines of the pima, in plan, are made poly gonal, the agitation of the sea may be reflected from side to side, and thus reach the inner harbour in perhaps a more dangerous manner than even in the case of the straight walls, because the waves will be more irregular, and will interfere greatly with the steering of the ship. It would appear that the system adopted in the construction of the jetties of Dieppe and of Havre,'in both of which ports wooden break waters are inserted in the line of the piers (the latter being polygonal in plan), is the one most fitted for harbours in exposed situations ; whereas, in sheltered positions, either straight or curved piersanay be resorted to.
One of the most important functions piers are designed to per form being to facilitate the movements of the shipping resorting to a harbour, it is advisable in designing them to dispose their plans so as rather to facilitate the entry than the departure of vessels; because a ship in harbour can always wait for a favourable condition of circum stances, whilst a ship entering may be compelled to do so under stress of weather. Under these conditions, the entrances are disposed so as to allow the current of the flood-tide, or the inshore currents, if no marked tides should exist, to carry the ship into the fair-way channel, without exercising any influence likely to carry the ship against the jetty walls. As to the length to be given to the piers, it must be regulated, firstly, by the distance of the deep-water line from the line of the shore ; secondly, by the set of the currents; and thirdly, by the means used in the particular port for hauling out the vessels. This latter practical detail of navigation will be found especially to affect the respective lengths of the piers, when two are used ; because when the outgoing vessels are hauled by men, or warped by ropes, the pier which serves for those operations must be made sufficiently long to allow the vessel to make her first tack without any danger of her falling off against the opposite pier. As a general rule, it is preferable to make the pier under the prevailing wind longer than the other one; but local circumstances may at times render it necessary to modify the application of this law.