BREAKWATER (anything that breaks the force of the waves). A barrier intended for the protection of shipping in harbors and anchor ages. It sometimes happens that in front of a semicircular bay a small island is so situated as to form a natural breakwater. This is to some extent the case with the ports of Portsmouth and Southampton. England. which are protected from the south by the Isle of Wight. In most places, however, bays and harbors are without such nat• ural protections, and it is often found necessary to construct artificial breakwaters to prevent the entrance or formation of waves from the sea to disturb the shipping anchored within. It is with the design and construction of these arti ficial breakwaters alone that we shall deal here.
Breakwaters may be divided into two main group,: (1) Those which give shelter and pro tection to commercial harbors or their entrances. and (2) those which shelter an anchorage or roadstead for vessels in transit which have occa sion to escape from the violence of passing storms. Such anchorages are usually termed harbors of refuge. (See IIARBORS.) Each of these two eias-es may be technically further according to their particular type of con struction. To discuss in the case of each of these types its theory of action and conditions of application, involves technicalities which are suitable only for special engineering treatises, but a brief definition of each is possible. The types are as follows: (I) Attached breakwaters are those which project from a cape or headland: (2) insular breakwaters •are those which are wholly detached from the mainland; 131 per manent breakwaters are those which are rigidly supported on the bottom of the sea (the ity of breakwaters are insular and permanent); (4) floating breakwaters are those which are upheld by the water, being simply anchored to the sea bottom; these are seldom employed; (5) reaction breakwaters are those so construct ed that the outer slope breaks the surface of the waves, while the inner slope regulates the cur rents; (6) composite breakwaters are those which consist of an upright wall resting on a comparatively wide and flat mound of loose stones. Examples of each of these types will be described below.
The design and position of a breakwater in all eases are determined by the direction and amount of wave-force which it will have to resist. Gen erally speaking. the height and force of the im pinging wave's in any locality depends upon the force and direction of the winds, the depth of the water, the fetch or distance over which the waves move, and the angle of incidence at which they strike tile breakwater. Some very notable instances of wave-force are on record. In the Wick Bay Breakwater, in England. n concrete block 45 feet wide. 26 feet long. and 21 feet high was completely turned around on its base and finally tipped off its foundation. This block weighed 1330 tons and formed the end of the breakwater. It was replaced by a larger block weighing 2600 tons, which was in turn carried away by the waves. Experiments made by Thomas Stevenson. an English engineer, by means of marine dynamometers to record the pressures, showed pressures of 6000 pound, per square foot in the North Atlantic Ocean. and of
3000 pounds per square foot in the German Ocean. At the time that the beacon at the mouth of .the River Loire. in France, was destroyed. AI. Le Fernie calculated that the •ave-force im pressed upon the work must have exceeded 4S00 pounds. and approximated 6000 pounds, per square foot. To determine the wave-pressure on time breakwater in Lake Ontario at Oswego. N. V., Mr. Wm. P. Judson, civil engineer. attached dy namometers to its face at the water's surface. at 8 feet below the surface, and at S feet above the surface. The pressures recorded were: At S feet below the surface, 10 pounds per square foot : at the surface, 600 pounds per square foot: and at S feet above the surface. 10041 pounds per square foot. The foregoing are isolated instances of wave-pressures, and cannot be assumed to be of general application; they serve simply to con vey some idea of the force- imposed upon break waters by action of waves. in closing this refer ence to wa ve-pro—nres. it should be carefully noted that the angle at which the waves strike the breakwater has much to do with the pres sures exerted. For example, by reducing the angle at which the ware,: struck the Wick Bay Breakwater from 90° to :40°. the structure with stood safely the action of the wave. which pre viously had repeatedly destroyed it. From this brief consideration of the wave-force acting upon a breakwater, it is evident that its design from the standpoint of stability must depend upon conditions arising from the location of the In considering typical examples of break waters, one of the largest and most expensive is the Cherbourg Breakwater. which was built to protect the ereseent-shaped bay forming the great French naval port of Cherbourg. Its con struetion was begun in 1786-8S by sinking 18 timber frustums of cones having a diameter of 142 feet at the base and 113 feet at the top. and a vertical height of 65 feet, which allowed them to project from 6 feet to 10 feet out of water when sunk. These timber frameworks were sub merged at irregular intervals by filling them with stone. so that they formed a row nearly miles long. About 3,703,703 cubic yards of loose stone were deposited in and around these cones. This original construction cost about $4,512,200, but the experiment proved unsuccessful, as the cones were battered into pieces by the waves, leaving only a series of mounds of loose stone, the tops of which were eventually below water level. The work remained practically in this con dition until 1830, when it was decided to make the loose stone mounds continuous by filling be tween them, and to construct on this ridge a masonry wall rising from low-water level, 36 feet 3 inches wide at the bottom and 29 feet 3 inches high. and 8 feet 3 inches wide at the top. It was built of rubble masonry, with a dressed stone facing. Since the construction of this original wall considerable work has been done in the way of strengthening and raising the rubble base and wall.