Encyclopædia Britannica, Ninth Edition/Breakwaters

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From volume IV of the work.
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BREAKWATERS differ from piers in their not being necessarily adapted for commercial purposes. They do not, therefore, require to have roadways for the accommoda tion of traffic, or parapets for keeping water or spray from passing over them. Breakwaters are artificial structures consisting generally of stones or blocks of concrete, built or deposited in deep water. Their object is to tranquillize those portions of the sea which they cover, and which thus become sheltered anchorages. They may be divided into three classes: (1.) Vertical or nearly vertical structures of built masonry for arresting the onshore progress of the waves, and for either reflecting them seawards or deflecting them laterally. (2.) Sloping structures of rubble stones dropped into the sea from timber stages or floating barges, and hence termed pierres perdues, having a sloping face on each side, termed a talus or glacis. These slopes, which, after the blocks have been consolidated, are generally protected above low water by stones set closely together, called pitching, are the angles of repose assumed by the loose blocks under the influence of the waves, and vary in steepness from above high water to below low water, where the force of waves is least. They vary from about 1 foot horizontal to 1 foot vertical to 7 feet horizontal to 1 foot vertical. (3.) What may be termed composite breakwaters are partly sloping and partly vertical, and act by causing the waves to break, and also by partially reflecting them seawards or deflecting them laterally. The new breakwater at Aberdeen and the Dover Admiralty pier, which acts also as a breakwater, are examples of the first class. Plymouth breakwater, which rises with a general sea-slope of from 2 to 5 horizontal to 1 vertical to a height of 23 feet above high water, is an example of the second class. Cherbourg, which slopes from low water to high water, above which level there is a vertical barrier rising to 12 feet above high water, is an example of the third class. Breakwaters, though passive, are nevertheless real agents by which work is done in com bating the waves in one or other of the three modes whicli have been defined. For further information regarding the design of breakwaters and the details of their construction see Harbours.