Building

BUILDING.) Depth of Water.—The deepening and widening of the Suez canal in recent years has enabled ships of the largest class to trade with eastern ports. The canal has now (1928) been deep ened throughout to 391f t., and it is practicable for ships drawing 33ft. to pass through it. Further improvements in progress in 1928 will provide for a draught of 35ft. and there is no reasonable limitation to the capacity of the canal. Concurrently with the deepening and widening of the Suez canal the more important ports of the Far East have been developed with the object of ac commodating the largest ship capable of passing through it. The opening in 1914 of the Panama canal, having a navigable depth of 4of t., has also had an important bearing on the development of harbours in the Far East and on the Pacific coasts of America. The general result is that, whereas in 190o a navigable depth of 3of t. was considered ample for practically all requirements, a depth of at least 35ft. is now regarded as essential in harbours of the first class and, in special cases, including the ports used by transat lantic liners, depths exceeding 4oft. are aimed at.

Effect of Depth on Cost of Works.

The cost of construct ing port works in general shows an increase approximately pro portional to the ratios of the cubes of the draughts of the largest ships for which they are designed. (See L. H. Savile, 13th Inter national Congress of Navigation, London, 1923.) It is thus ob vious that the problem of providing port accommodation for shipping of the largest class is one of considerable importance from the point of view of finance, and may become one of balance between the relative economy of ships of large draught and cargo capacity, and the capital cost of port construction.

Sites for Docks.

Low-lying land adjoining a tidal river or estuary frequently provides suitable sites for docks. The posi tion, being more or less inland, is sheltered; the low level reduces the excavation required for forming the docks, and enables the excavated materials to be utilized in raising the ground at the sides for quays ; and the river furnishes a sheltered approach channel. Notable instances are the docks of the ports of London, Liverpool, South Wales, Southampton, Hull, Belfast, St. Nazaire, Rotterdam, Antwerp and Hamburg (fig. 12) . Sometimes docks are partially formed on foreshores reclaimed from estuaries, as at Hull, Grimsby, Cardiff, Rosyth Dockyard, Liverpool, Leith and Havre ; whilst at Bristol, a curved portion of the river Avon was appropriated for a dock, and a straight cut made for the river. By carrying docks across sharp bends of tidal rivers, upper and lower entrances can be provided ; and of this the London, Surrey Commercial, West India, and Victoria and Albert docks are ex amples on the Thames, and Chatham dockyard on the Medway. The enlargement of the port of Antwerp is a recent instance. A series of enclosed docks, 'extending from Kruisschans, on the Scheldt 7m. below the city, to the old dock system higher up the river at Antwerp, cuts off a bend of the river and was opened for traffic in 1927.

Occasionally, when a tidal river has a shallow entrance, docks, formed on its foreshore adjoining the sea-coast, are provided with a sheltered entrance direct from the sea as in the Havre docks at the outlet of the Seine. Many old ports were first established on sandy coasts where a creek, maintained by the influx and efflux of the tide from low-lying spaces near the shore, afforded some shelter and an outlet to the sea across the beach. Some of these, such as Calais, Dunkirk and Ostend, have had their access improved by parallel jetties and dredging; and docks have been readily formed in the low-lying land only separated by sand dunes from the sea.

(See HARBOURS.) In sheltered places on the sea-coast, docks are sometimes con structed on low-lying land bordering the shore, with direct access to the sea, as at Barrow, Hartlepool, Swansea and Bombay. In the Mediterranean open basins have been formed in the sea, by establishing quays along the foreshore, from which wide, solid jetties, lined with quay walls, are carried into the sea at intervals at right angles to the shore. Such basins are sheltered by an out lying breakwater parallel to the coast, and are reached at each end through the openings left between the projecting jetties and the breakwater, as at Marseilles and Trieste, and at the extensions at Genoa (see HARBOURS). In some of these ports additional ac commodation has been obtained by constructing wide quays along the inner face of the breakwater (q.v.) . Where, however, the basins are formed within the partial protection of a bay, as in the old ports of Genoa and Naples, the requisite additional shelter has been provided by converging breakwaters across the opening of the bay, and an entrance to the port is left between the break waters.

The two deep arms of the sea at New York, known as the Hudson and East rivers, are so protected by Staten Island and Long Island that it has been only necessary to form open basins by projecting jetties or piers into them from the west and east shores of Manhattan island, and from the New Jersey and Brook lyn shores, at intervals, to provide adequate accommodation for Atlantic liners and the sea-going trade of New York (Plate I., figs. 4, 5). Somewhat similar conditions obtain in many of the great natural harbours in other parts of the world, as, for instance, in Sydney harbour and at San Francisco.

Dock Extensions.

In designing dock works, it is expedient to make provision, as far as possible, for future extensions as the trade of the port increases. Generally this can be effected along side tidal rivers and estuaries by utilizing sites lower down the river, or reclaiming unoccupied f oreshores of an estuary, as adopted f or extensions of the ports of Liverpool, Hull and Havre. At ports on the sea-coast of tideless seas, it is only necessary to extend the outlying breakwater parallel to the shore line, and f orm additional basins under its shelter, as at Marseilles and Genoa (f or plans see HARBOURS). Quays also along rivers furnish val uable opportunities of extending the accommodation of ports. Ports, however, established inland, like Manchester, though ex tremely serviceable in converting an inland city into a seaport, are at the disadvantage of having to acquire very valuable land for any extensions that may be required; but, nevertheless, some com pensation is afforded by the complete shelter in which the exten sions can be carried out, when compared with Liverpool, where the additions to the docks can only be effected by costly reclam ation works along the foreshore to the north, in increasingly exposed situations.

Venice being situated upon an island of limited area in a lagoon has secured the extension of its dock facilities by the construction of an entirely new port on the adjoining mainland. New York, in view of the congestion of traffic at the piers in the upper areas of the harbour, is developing large districts such as Staten island and Jamaica bay nearer the sea entrance.