Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/477

 

HARBOURS AND DOCKS

 

I. Harbours.

ALL harbours may be classed either as havens for the protection of ships during storms, or as ports suited for commercial purposes. Of the first mentioned class, or those which are called harbours of refuge, some are natural and some artificial.

The designing of harbours constitutes confessedly one of the most difficult branches of civil engineering. In making such a design the engineer, of course, avails himself of the information which is derived from past experience, and endeavours, to the best of his power, to institute a comparison between the given locality and some existing harbour which he supposes to be similarly situated. Perfect identity, however, in the physical peculiarities of different localities seldom if ever exists, and all that can be done in deriving benefit from past experience is to select the harbour which seems most nearly to resemble the proposed work.

In order the better to understand the nature of the difficulties which beset the marine engineer, let us suppose that he is called upon to design works for the accommoda tion of shipping in a given locality. The questions which immediately press on his attention are (1) What is the cheapest kind of design that is suitable for the place and sufficient for the class of shipping which has to be accom modated? and (2) What are the smallest sizes of materials and thickness of walls that are admissible in its construc tion 1 as on these the cost of the work will materially depend. Before considering how far it is possible to answer such ques tions, let us endeavour to define the varieties of design into which all sorts of harbours may, with propriety, be resolved.

In the first place they may be all classified under two nain heads viz., interior works and exterior ivories.

The interior works are provided for the accommodation and repair of vessels, while the exterior works may be con- eniently enough divided into the following five classes.

1. Harbours of Refuge and Anchorage Breakwaters.—These consist of one or more breakwaters, so arranged as to form a safe roadstead, which shall be easily accessible to the largest vessels in all states of the weather and tide. A breakwater forms a barrier either complete or partial to the progress of the waves, and is intended for sheltering the anchorage ground under its lee. It is not used for com mercial traffic as are piers or quays, and therefore a parapet is not necessarily required for preventing the waves from breaking over the top, although this may be useful as a protection against the wind.

2. Deep-water and Tidal Harbours for Commercial Purposes (fig. 1, a, b).—A harbour for commercial purposes is any arrangement of piers or breakwaters, or of both, which encloses and so tranquillizes a sheet of water, that vessels may be moored at the quay walls or wharves which form the inner sides of the piers. Where the coast line lies open to a very heavy sea it is often found necessary to make a double or compound harbour (fig. 1, b). In such a case the entrance to the inner basin is situated within the shel tered area formed by the outer works.

Fig. 1.

3. Ranted or Curved Piers (fig. 1, c).—Where there is a single pier of this kind vessels lie under the lee of the kant or kants, and the sheltered side of the pier is therefore finished as a quay. As the outer end of any such single pier is surrounded by the open sea, it may be convenient to call it a " free end" in contradistinction to the outer end of a close harbour, where the sea room in the fairway is limited to the breadth of the entrance which ought, for sheltering the basin, to be made as narrow as is consistent with the safe passage of vessels. Both extremities of all single insulated breakwaters are of course free ends, as are also the seaward ends of all single breakwaters which are connected with the land.

4. Straight Piers (fig. 1, d).—A straight pier generally projects at right angles to the coast line, with a free end at its seaward extremity; and, unless when the wind blows right in upon the shore, a straight pier will always afford some shelter on its lee side.

5. Quay or Wharf (fig. 1, e).—A quay wall is usually built parallel to the line of shore. It affords no shelter of any kind, and the only advantage which it possesses is that of enabling vessels to load and unload without their having to " beach," or, where the shores are steep, even to take the ground.

It will be observed that all the kinds of piers or harbours just enumerated differ materially from each other in the amount of shelter which they afford, and are suitable for places having very different degrees of exposure. The first step to be taken, therefore, is to select from the different classes of designs which have been enumerated the one which is best adapted to the physical peculiarities of the situation. The engineer in order to make this selection judiciously must keep in view the essential elements of stability, economy, safety of ingress and egress, and con venient accommodation for the trade of the port. If the place be much exposed, he must therefore arrange the different parts of the work so as to produce a harbour which may be easily taken and left in stormy weather, without endangering the tranquillity of the internal area : for it is the combination of the qualities of an easy and safe entrance and exit, with a good " loose " and a smooth interior, which alone constitutes a good harbour. Lastly, he must fix the width of the piers and height of the parapets.

The local characteristics which at the outset demand our Local consideration are—(1) the geological and other physical cLarac peculiarities of the shore; (2) the exposure ; (3) the force of teiisti the waves due to the exposure; (i) the strength, direction, and range of the tides ; (5) the depth of the bay or sea in which the harbour is to be built ; (6) the proximity of deep water to the works, or in other words, the slope of the fore shore ; and (T) the angle at which the heaviestwaves impinge on the coast line and on the works.

Before considering some of these questions the reader may refer to figs. 2 and 3, on which are marked the techni cal names of different parts of harbour works.

Generation of Waves.

Mistakes as to the level of the highest tides are some- Level >

times made by drawing too hasty conclusions from the P la . ne presence of vegetable life. In the Shetland Islands there actlon may often be seen large blocks of rock (and to these refer

ence will afterwards be made), which during storms have