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 till, on its energy being spent, it recoils back to the sea down the beach. A breaking wave concentrates its transmitted force on a portion of the water forming the undulation, which, consequently, strikes a more powerful blow over a limited area against any structure than the more distributed shock of a simple undulation beating against a vertical wall. Moreover, the recoil of broken waves down a sloping beach or rubble mound produces a greater scour than the simple reflection of an undulation from a vertical wall, especially where the depth is sufficient to provide a cushion of water below the undulation, protecting the toe of the wall from the wash of recoil.

Types of Breakwaters.—There are three distinct types of breakwaters:—(1) A simple rubble or concrete-block mound; (2) a mound for the bottom portion, surmounted on the top by a solid superstructure of masonry or concrete; and (3) an upright-wall breakwater, built up solid from the sea-bottom to the top. The second type forms a sort of combination of the first and third types; and each type presents several varieties. In a few harbours, two different types have been adopted for different situations at the same place; but generally the choice of type is determined by the materials available at the site for the construction of the breakwater, the nature of the sea-bottom and the depth into which the breakwater has to be carried.

1. Rubble and Concrete-Block Mound Breakwaters.—A rubble mound consists merely of a mass of rubble stone, just as it is obtained from a neighbouring quarry, tipped into the sea along a predetermined line, till the mound emerges out of water. The rubble stone is deposited, either from barges, as adopted for the construction of the detached breakwater sheltering Plymouth Bay, or from wagons, having hinged opening flaps at the bottom for dropping their load, run out from the shore along staging erected in the proposed line, according to the method employed for the outer breakwater enclosing Portland Harbour, and the north-east breakwater at Colombo Harbour. The mound thus deposited is gradually consolidated under the action of the sea; and a tolerably stable form is by degrees attained by continued deposits of stone. This system of construction is very wasteful of materials, and can only be resorted to where extensive quarries close at hand are able to furnish readily and cheaply very large quantities of stone, especially where, as at Portland and Table Bay, convict labour has been advantageously utilized in quarrying. When the site is very exposed, the large waves in storms, dashing over a rubble-mound breakwater, carry the stones on the top, if unprotected, over on to the harbour slope, and in recoiling down the outer slope, draw down the stones on the face, so that the top and sea slope of the mound need replenishing with a fresh deposit of stones after severe storms.

1.—Table Bay Breakwater.

Under the action of the breaking and recoiling waves, the mound assumes a very flat slope on the sea side, from a few feet above high-water down to several feet below low-wafer level (fig. 1). The flatness of the sea slope depends on the exposure of the site, and the limited size of the stones covering the outer portion of the mound; and its extent increases with the range of tide, as a large tidal rise exposes a greater length of slope to the action of the waves. This flattening of the sea slope greatly increases the amount of stone required for a rubble-mound breakwater, in proportion to the exposure and the range of tide; and the amount is also affected, but in a proportionately minor degree, by the depth in which the breakwater is situated. In order to avoid the injuries to which an ordinary rubble mound is subjected by waves, certain methods have been devised for protecting the top and sea slope of the mound. For instance, the upper portion of Plymouth breakwater has been covered over by granite paving set in cement, to diminish the displacement of the stones by the waves. Frequently, on the continent of Europe, rubble mounds have been formed of materials so sorted that the smallest stones are placed in the centre of the lower part of the mound, and covered over along the slopes and top by layers of larger stones, increasing in size towards the outer part of the mound, so that the largest stones obtainable are deposited on the outside, and especially on the top and sea slope of the mound. This is, no doubt, theoretically the correct method of construction of rubble mounds exposed to the sea; but it involves a considerable amount of trouble and expense.

2.—Alexandria Breakwater.

Practically the chief point of importance is to cover the outer slope and the top of the mound with the largest stones that can be procured, and where large stones are not readily obtainable concrete blocks furnish a very convenient substitute. These blocks are generally deposited as the outer covering on the top and sea slope of a rubble mound, as for example at the mound breakwaters in deep water sheltering Algiers harbour, and at the French parts of Cette and Bona on the Mediterranean; whilst they furnish the protection of the top and upper part of the sea slope of the rubble-mound extension of Marseilles breakwater down to 20 ft. below sea-level. At Alexandria, concrete blocks compose the outer half of the mound, sheltering the inner half consisting of small rubble (fig. 2); at Biarritz the mound breakwater is formed mainly of concrete blocks, with rubble stone filling the interstices and on the top; whereas at the outer end of the western breakwater at Port Said, protecting the entrance to the Suez Canal, a bottom layer of rubble is surmounted by concrete blocks. These blocks are generally deposited at random; but at Cette (fig. 3), and at the breakwater in deep water at Civita Vecchia, the concrete blocks covering the rubble have been laid in stepped, horizontal courses. This arrangement necessitates more care and better appliances in construction; but, in compensation, the blocks so placed are less exposed to disturbance and injury by the waves.

Concrete blocks possess the great advantages for breakwaters that they can be made wherever sand and shingle can be procured, and of a size only limited by the appliances which are available for handling them. In fact, in places where stone of any kind is difficult to procure at a reasonable cost, as for instance at Port Said, concrete blocks are indispensable for the construction of breakwaters. Large concrete blocks, moreover, by enabling a comparatively steep slope to be formed with them on the sea side of a mound breakwater, reduce considerably the amount of materials required, especially at exposed sites, and also for breakwaters extended into deep water, such as those of Algiers and Marseilles.

3.—Cette Breakwater.

4.—Port Said Western Breakwater.

Occasionally, in the absence of suitable rubble stone, a mound breakwater has been formed entirely with concrete blocks; and of this the main portion of the western breakwater at Port Said furnishes a notable example (fig. 4). Sometimes, in exposed situations, the mounds of the composite type

of breakwaters have been constructed exclusively with concrete blocks, such, for instance, as in the curved breakwater protecting the outer harbour at Leghorn, and in the central breakwater in deep water sheltering the harbour of St Jean de Luz, and directly facing the Bay of Biscay. These large concrete blocks are deposited by cranes from staging, tipped into the sea from a sloping platform on barges, or floated out between pontoons, or slung out from floating derricks. This last method proved so expeditious for the upper blocks at Alexandria, that, in conjunction with the tipping of the lower blocks from the inclined planes on the decks of barges and the deposit of the rubble from hopper barges, provided also with side flaps for the higher portions, the detached breakwater, nearly 2 m. long, sheltering a very spacious harbour, was constructed in two years (1870–1872). Sometimes, when a mound breakwater has been raised out of water, advantage is taken of a calm period of the year and a low tide to form large blocks of concrete within timber framing on the top of the mound, so as to provide a very efficient protection.