Breakwater

Breakwater, a barrier in front of a harbour or anchorage, mainly for the protection of shipping. It may be of natural or of artificial formation, or advantage may be taken in its construction of natural partial barriers that exist. Breakwaters are of most importance where the harbours are much used, and where the position is exposed to heavy storms, as at Plymouth, Holyhead, Portland, or Cherbourg. The material employed varies with the locality. If good stone can be quarried in the neighbourhood, the breakwater is generally built of that material. Thus at Plymouth large blocks of limestone were quarried near, shipped, and dropped down as rubble in the required position; and at Holyhead the stone was cut from the Holyhead mountain, and run out to the sea on timber staging. In places where stone cannot be readily obtained, blocks of concrete have been satisfactorily employed instead. Usually the stone available is first deposited irregularly in a long mound as rubble, with a base of considerably greater width than the top. This mound is faced with masonry or concrete, to diminish the effect of the action of the waves. In some cases little more than a firmly built paving exists above the facing, as in the breakwaters at Plymouth and Cherbourg; but the rubble is often surmounted by a masonry wall, as at Portland and Holyhead. Where the breakwater is composed of concrete blocks, it is usual to build it up from the bottom as a wall with outwardly sloping faces, like that at Dover, where no stone is available in the neighbourhood. The depth of water on the site of the structure varies considerably in different cases, but rarely exceeds 100 feet; at Portland the water is about 50 feet deep at low water spring tide, at Cherbourg about 60 feet. At Alderney the depth at the outer end of the breakwater is 130 feet, but the difficulties of building and maintaining this outer portion have been so great that the original design of 1847 has not yet been carried out.