In the construction of harbors, the great desiderata are sufficient depth of water and perfect, security for the vessels likely to frequient them, together with the -possible facilities for ingress during any weather; while, the chief obstacles to be sur mounted are the action of the waves upon the protecting piers and lireakwaters, and the formation of sand-banks and bars.
The harbors may be classified under the following heads: 1. Harbors of refuge and anchorage breakwaters; 2. Deep water and tidal harbors for commercial purposes; 3. Ranted or curved piers; 4. Straight piers; 5. Quays or wharves.
These different works are obviously suited for different localities, and for contend ing with different exposures. The last-mentioned is clearly suited for the most shel tered situations only, and the engineer must consider, when designing a harbor, which of all those will be most economical and effective. In judging of this, the geological features of the coast must be carefully considered. A good chart furnishes valuable evidence as to the forces to which harbor-works will be exposed. Among those may be noted the line of maximum, e.?posure, or fetch or reach of open sea in front of the harbor. Mr. Thomas Stevenson, civil engineer, has proved by observations that the wares increase in the ratio of the square root of their distances from the windward shore as measured along the line of exposure. and he gives the following simple forinula: Where is = height of wave in feet during a strong gale, and d = length of exposure in miles for distances of say 10 miles and upwards: 14=1.5 yd.
The heights so obtained will be increased when they pass into confined channels, and decreased when they pass into expanding channels. The greatest measured height of the waves was by Scoresby in the Atlantic ocean, where he found billows of 43 ft. in height from hollow to crest, and 36 ft. was not an uncommon height. At Wick, Caith ness-shire, waves of about 40 ft. strike the breakwater.
The greatest recorded forces exerted by the waves are the following• A mass of 13 tons was broken or quarried out of, its position in situ on the sherries of Whalsey, in Zetland, at a level of 74 ft. above the sea; but the most astonishing feat of which we have any knowledge was at Wick breakwater, where, in, the winter of 1872, a mass of masonry, concreted together as a monolith, and bound with iron bars 4i- inches in diameter, and weighing no less than 1350 tons, was torn from its seat in the work, and thrown toleeward, where it still lies in an unbroken state.
Mr. Thomas Stevenson, by means of au instrument called the marine dynamometer, -has ascertained numerically the force which is exerted by the waves in the Atlantic and German oceans, and has found that the mean of observations during winter was more than three times that exerted during summer, the maximum force recorded being 3i tons per square foot.
Various local causes materially affect the height and therefore the force of the waves. In some cases, where a strong current sets off the coast, as at Siamburgh Dead roost, in Zetland. it causes a dangerous breaking sea, and while this roost or race continues to
rage, the coast under lee is comparatively sheltered; but when the force of the tide is exhausted, and the roost disappears, a heavy sea rolls in upon the shore.
It is this encounter between the ground-swell of the ocean and the current of title or land water, which causes miniature races at the mouths of rivers. Another most material clement in the question of exposure is the depth of water in front of the har bor; for if that depth be insufficient to admit of the propagation of the waves, they break or spend themselves before they reach the piers. Thus, Mr. Leslie found at Arbroath harbor that the works were not so severely tried by the heaviest waves as by others of lesser size which were not tripped up and broken by the outlying rocks. In the same way at the river Able, the harbor within the bar is more disturbed by ordinary waves than during great storms. It thus appears that the largest waves are not always so destructive as smaller ones. Mr, Scott Russell has stated the law, that waves break whenever they come to water as deep as their own height; so that 10 ft. waves should break in 10 ft. water, and 20 ft. waves in 20 ft. water. There seem, however. to be some waves which break on reaching water whose depth is equal to twice their own height. Proofs of the depth to which the surface undulations extend have been given by sir George Airy, sir John Coode. rapt. Calver, and Mr. John Murray, e.n. The late Dr, Rankine has shown that the crest and trough of the sea are not, as was generally believed, equidistant from the level of stagnant water. When / is the length of the wave, II its height .from crest, lAygt (P) Crest above still water = 2 Trough below still water — There is much difference of opinion among engineers as to the best profile or cross section of breakwaters for deep-water harbors. It is asserted by col. Jones and others, that in deep water the waves are purely oscillatory, having uo power of translation, and therefore incapable of exerting any force against the masonry. This, however, is incor rect, and calculated to lead to dangerous consequences. Were there no wind propelling the waves, and no current to interfere with their character, such a result might be true. This, however, is not the case, and all sea-works, in whatever depth of water they be placed, will assuredly have to withstand impulsive action. Besides, it must be kept in view, that in order to reduce the expense of construction, it is essential, Nvhere the bottom is soft, to make the foundation a pile of loose rubble, or concrete blocks. It follows, from what has already been said, that the rubble, by shoaling the water is front of the work, will cause the waves to become waves of translation before they reach the vertical superstructure, which, assuming the waves to have been simply oscil latory, would have reflected them without breaking, and therefore without their having exerted an impulsive force upon the masonry.