TYPES OF HARBORS

26. Introductory. Harbors are first of all refuges for ships, — places where they may lie safely at anchor, be loaded and unloaded. They require, therefore, (1) sufficient depth of water leading from the open sea to the anchorage; (2) protection from waves. Without these features they are not adapted to man's use; but a thoroughly satisfactory natural harbor requires, in addition, (3) an anchorage ground large enough for many ships; (4) a fairly straight channel without strong currents; (5) relative freedom from shifting sand; (6) exemption from closure by ice. Commercial harbors are, however, more than this; they are terminal ports where inland and ocean commerce meet. They must have, therefore, (7) convenient access to the interior by available routes, or a location at the crossing of great highways of trade.

The absence of any one of these conditions may affect the value of a harbor otherwise satisfactory. For instance, Vladivostock, although possessing many characteristics of a good harbor, is closed by ice during a large part of the year. On the other hand, Valetta has a larger harbor than Malta requires, and Trincomalee, Ceylon, though an excellent natural harbor, is available only for naval purposes because of the absence of trade and back-country.

27. The places most suitable for harbors on the coast lines of the world are (1) drowned valleys, (2) barrier beaches, (3) rivers, (4) atolls, (5) craters.

28. Drowned-valley harbors. Drowned-valley harbors owe their existence to the fact that the coast line has sunk, allowing sea water to enter a river valley. If such a coast line is bordered by a plain, the sinking of the shore produces harbors like those of Norfolk, Baltimore, and Buenos Ayres. Depression of a more uneven land surface produces harbors like those of Rio de Janeiro, New York, those on the Maine coast, and Vancouver and Seattle on the Pacific coast. The drowning of a valley in a mountainous region produces fiords in which harbors may be located, as at Trondhjem on the Norwegian coast. Occasionally a range of hills or mountains is depressed to such an extent that sea water has access to a valley back of the range, and a former mountain pass becomes a harbor gateway, as at San Francisco.

Drowned-valley harbors have a depth of water determined by the amount of sinking the coast line has undergone. They are well protected from waves and have the advantage of easy access to the interior of the country, either by the rivers themselves or by railroads

along the valleys. The outlet to the sea, however, may be unsatisfactory; tides and currents may be difficult to control, and the drifting sands at the river mouths demand constant attention. The harbors of New York and San Francisco are examples of this type.

The present distribution of salt water in the neighborhood of New York is in accordance with the previous topography (fig. 3). North River and East River were valleys on the old land, the former being traceable for a hundred miles out into the Atlantic. The flatter depressions of the ancient land surface are now represented by Newark Bay, Upper New York Bay, and Raritan Bay, and the minor elevations in the old land surface are still visible as islands. The lower Hudson has sunk so much that a good depth of water is furnished at New York. The access to this deep water from the Atlantic, however, is impeded by drifting sand. The shore currents of wave and tide run westward on the south side of Long Island and northward on the east side of New Jersey; Coney Island and Sandy Hook are being extended, and sand is thus carried directly into the mouth of the harbor. Quantities of shore drift are piled up about the harbor's mouth in the form of shoals, and if these shore currents alone were at work, the bay would be completely walled in by embankments of sand. In spite of the tidal wash and the sinking coast (§ 22) the sand is slowly filling the harbor entrance. The outlet to the sea is kept open by the waters from Hudson River and from Long Island Sound, together with the scour from the tides which enter the bay directly, and so effective are these forces in offsetting the drifting sands that a natural depth of 18 to 20 feet is maintained, a condition found in few other ports. Ships drawing less than 18 feet of water pass directly into the bay through South Channel and Swash Channel (see fig. 3), but most of the shipping enters the harbor through two channels, largely artificial. The entrance for larger ships has long been through Gedney Channel and its continuation, Main Channel, which in 1883 had a depth of 23 feet at mean low water and was later deepened until a depth of 30 feet at low water is maintained for the 22 miles from the Battery to the Atlantic. This depth is not sufficient for modern commerce, and, furthermore, the channel was so narrow that ships over 600 feet in length could take the sharp turns only with great danger. The Ambrose Channel, with a depth of 40 feet and a width of 2000 feet, is therefore being constructed. This channel, which required 7 miles of dredging, was buoyed for navigation in September, 1907, and at the present time about four fifths of its length has been cut to a depth of 37 feet.

The position of New York and the character of its harbor accounts for its commercial supremacy. Access to the interior of the continent is easy, the tributary country is large, and the harbor has 100 miles of water front available for anchorage. There is already 90 miles of frontage along the docks and piers on the New Jersey and New York shores, which take care of two thirds of the foreign commerce of the United States.

The drowning of the coast at San Francisco (fig. 4) has admitted the sea to the Sacramento valley, which, combined with the San Joaquin, makes the great valley of California (§ 68). The shoaling bottoms bordering on the eastern part of the bay make the protected edge of San Francisco peninsula the natural location for piers

29. Barrier harbors. Wave and tidal currents, which have developed barrier beaches (§ 24), bars, spits, and hooks along a large part of the world's coast line, form inlets through these barriers and thereby furnish access to quiet-water anchorage back of the ridges of sand. Such are the harbors at Provincetown (§ 25), at Galveston and other points on the Gulf coast, at many small cities on the Carolina, Florida, and New Jersey coasts, and such also are the harbors at Danzig, Germany, and the African port of Durban. A constant movement of sand along the shore and back and forth through the inlets makes the maintenance of a permanent channel in a harbor of this type a matter of great difficulty. Inlets change their position in spite of engineering works, and attempts to make harbors though barrier beaches often result in failure after the expenditure of millions of dollars. On