Breakwater

“ Well-known examples of breakwaters formed of a rubble mound surmounted by a superstructure founded at or near low water or sea-level, are furnished by Cherbourg and Holyhead breakwaters, the inner breakwater at Portland, and the breakwaters at Marseilles, Genoa, Civita Vecchia, Naples, Super-structures at low-water level. Trieste and other Mediterranean ports. The very exposed breakwater at Alderney was commenced on this principle about the middle of the 19th century; and the outer breakwaters at Leghorn and St Jean de Luz have superstructures founded at low water on concrete-block mounds. ”

“ A breakwater, we are told on the highest authority, is an obstruction of wood, stone, or other material, as a boom or raft of wood, sunken vessels, &c., placed before the entrance of a port or harbour, or any projection from the land into the sea, as a mole, pier, or jetty, so situated as to break the force of the waves and prevent damage to shipping lying at anchor within them. ”

“ The entire frame of the breakwater, the double apparatus crushed and mingled confusedly, came in a whirl of foam, rushing upon the stone barricade like chaos upon a mountain, where it stopped. Here the fragments lay together, a mass of beams penetrable by the waves, but still breaking their force. The conquered barrier struggled nobly against destruction. The waves had shattered it, and in their turn were shattered against it. Though overthrown, it still remained in some degree effective. The rock which barred its passage, an immovable obstacle held it fast. ”

“ Reduced to a single float, the breakwater might remain under the waves too long, but, owing to the two others, it rights itself, warps around, and always presents the spur of its sharp roof to the wave. ”

“ From the opposite shore of the bay stretched the immensely long arm of the new breakwater, on top of which crawled toy-like engines and work-trains. What looked like a spacious, sluggish river extended straight inland toward the distant ramparts of the hills. ”

“ Vessels of 1000 tons can anchor to the lee of the breakwater in perfect safety, can also come alongside the wharf attached to the breakwater, and load and unload with perfect ease and great dispatch, even when there is a heavy sea running and breaking over the breakwater. All this has not been obtained without some trouble, for at times the angry seas have knocked about the concrete blocks as if they were of wood, and on one occasion threw down 100 feet in length of the mole, distributing the blocks over the bottom to forty feet from the line of works. ”

“ 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. ”

“ It is therefore essential, before constructing a breakwater, to investigate most carefully the force, periods and duration of the winds from the quarters to which the work will be exposed, the distance of any sheltering land from the site in the most stormy direction, the slope of the beach and the depth of the sea in the neighbourhood of the shore, and the protection, if any, afforded by outlying shoals or sandbanks. In a tidal sea, the height required for a breakwater is affected by the amount of tidal range ”

“ This form of breakwater, with its long outer slope exposed to breaking waves, particularly where the tidal range is considerable, is, indeed, more subject to frequent small injuries than the other types, but they are readily repaired; and a mound is not generally liable to the serious breaches which occasionally are formed in solid superstructures and upright walls in exceptional storms. ”

“ Those who have witnessed, for example, at La Rocquaine in Guernsey, or at Bourg-d’Eau in France, the effect produced by a few posts fixed in the rock, will understand the power of these simple preparations. This sort of breakwater is a combination of what is called in France épi with what is known in England as “a dam.” The breakwater is the chevaux-de-frise of fortifi [Pg 256] cations against tempests. Man can only struggle against the sea by taking advantage of this principle of dividing its forces. ”

“ The shock of the waves is received several thousand times in the course of twenty four hours, and hence the sum of impulse which the breakwater resists in one stormy day amounts to many thousands of millions of tons. The breakwater is entirely an artificial construction. If then man could accumulate and control the forces which he is able effectually to resist, he might be said to be physically speaking, omnipotent. ”

“ Three hours later it appeared again. The return tide had washed it back, and a little later a larger wave than usual carried it on to the rough stones of the unfinished Breakwater, where it held fast. The water receded, and the four unhappy voyagers crept out on to the rocks, to be rescued half an hour later by a pilot boat. Such was one of the unexpected services rendered by the Breakwater at Plymouth; but its expected benefits, worthily accomplished, have been too numerous to record.
John Lea
"The weight of the two birds had the desired effect." ”

“ The worst results occur where the littoral drift is almost wholly in one direction, so that the projection of a solid breakwater out from the shore causes a very large accretion on the side facing the exposed quarter; whilst owing to the arrest of the travel of sand, erosion of the beach occurs beyond the second breakwater enclosing the harbour on its comparatively sheltered side. ”

“ In large structures, however, uniformity of design may be desirable for each straight length of breakwater; though where two or more breakwaters or outer arms enclose a harbour, the design should obviously be modified to suit the depth and exposure. At Colombo harbour, the superstructure of the less exposed north-west breakwater has been made slightly narrower than that of the south-west breakwater; and a simple rubble mound shelters the harbour from the moderate north-east monsoon. ”

“ The long, detached breakwater sheltering the series of basins formed by wide projecting jetties along the sea coast at Marseilles (see Dock) , is a typical instance of a breakwater where a quay has been formed on the top of a sorted rubble mound, sheltered on the sea side by a high wall, or narrow superstructure, founded at sea-level, and protected on the sea slope of the mound from undermining by large concrete blocks deposited at random (fig. 5) . ”

“ This breakwater, the Prince of Wales pier of the commercial harbour, and the eastern breakwater and detached south breakwater for the naval harbour, were all founded on a levelled bottom, carried down to the hard chalk underlying the surface layer, by means of men in diving-bells. The extension of the Admiralty pier and the other breakwaters of Dover harbour consist of bonded courses of concrete blocks, from 26 to 40 tons in weight, as shown in figs. 13 and 14, the outer blocks above low water being formed on their exposed side with a facing of granite rubble. ”

“ Why not build a breakwater? Because the water is so deep that the cost of a breakwater long enough to give effective protection would be enormous. There is a more sheltered haven some miles to the north, but as all the business offices and warehouses are here, not to speak of the labouring population and their houses, the idea of moving the city and railway terminus has not been seriously considered. ”

“ The wind and tide had the effect also to set her bodily down the river. Her course, from the time of her starting until the collision, was the usual course of ferry boats on leaving their slips under like circumstances. The course of the Breakwater from the vicinity of the Battery was along the New York docks. As she neared the Cortlandt street ferry slip she approached closer to the docks, and from that time continued on a course about 400 feet therefrom.
' (8) The Pavonia sounded the usual long, single whistle to warn approaching vessels as she commenced to move. ”

“ The rough breakwater is formed by rubble-stones, brought from the mountain in waggons peculiarly constructed for the express purpose (the design of which, I understand, was furnished by the resident engineer, and proves the versatility of his genius) , and dropped perpendicularly through the staging into the sea, and is then left to be dealt with by the sea, which arranges the deposit in a manner best suited to form a consolidated mass ”

“ This tidal channel would be protected by a breakwater running from the Leasowe embankment to a point in the Rock Channel west of the Dove Spit. An outer breakwater would also run in an easterly and south-easterly direction for a distance of 5000 feet, sheltering the greater part of the Rock Channel, which is to be dredged for upwards of a mile to a depth of 30 feet below low-water mark. The scheme does not appear to be either difficult or costly, but as it is objected to by the Corporation of Liverpool and by the Mersey Harbour Board, it may not come to maturity. ”

“ This effect is intensified when the bay faces the direction of the strongest winds. Thus at Wick a mass of masonry weighing 1350 tons, placed at the head of the breakwater projecting half-way across the bay and facing the entrance, was moved by the waves during a violent storm; and a portion of Peterhead breakwater, weighing 3300 tons, was shifted 2 in. in 1898, indicating a wave-stroke of 2 tons per sq. ft. ”

“ 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. ”

“ Following then the Larig over rocks and rough stones, among which it chafes and foams, we reach a sort of barrier of stones laid together by the hand of nature with the regularity of an artificial breakwater. As we pass over this barrier, a hollow rumbling is heard beneath; for the stream, at least at ordinary times, finds its way in many rills deep down among the stones. ”

“ What was more reasonable than to hope that in the influential circles in whose hands the reins are placed we should be greeted with joy as 73the breakwater against the onrush of the destroying billows, as the dam which is to be carefully repaired and made secure at a time when a freshet is expected?... ”

“ The former is more advantageously situated for the interior trade, but unfortunately has no docks for shipping, and is exposed to the prevailing southeast gales.
Great sums of money have been spent in the construction of a breakwater, which it was fondly hoped would form a refuge for ships during the heavy storms. But before it could be finished it proved itself useless, for the sand would “silt up” on the lee side, until it threatened to form a wide strip of beach between the landing place and the sea. ”

“ To prevent the recoil of the sea and the scooping away of the base and the foreshore in much exposed situations, and consequent slips and subsidences, breakwaters which simply act as wave screens, and not as wind screens, are sometimes kept a little below high-water mark so that the heads of the waves may have free action, although their onward motion is prevented. On the contrary, light open work, such as a lattice screen, although it may somewhat lessen wave action, does not prevent it passing through. ”

“ The breakwater at Sandy Bay just referred to, and the one at Civita Vecchia, which it somewhat resembles, approximate to that class of breakwater which has a superstructure founded below low-water level, so far as stepped courses of blocks Superstructure below low-water level. can be regarded as forming part of a superstructure ”

“ These breakwaters, having to prevent the waves that beat upon the coast from reaching the site which they are designed to protect, must be made sufficiently strong to withstand the shocks of the waves during the worst storms to which they are exposed. ”

“ With a large harbour and the adoption of a detached breakwater, it is possible to gain the advantage of two entrances facing different quarters, as effected at Dover and Colombo, which enables vessels to select their entrance according to the state of the wind and weather; where there is a large tidal rise they reduce the current through the entrances, and they may, under favourable conditions, create a circulation of the water in the harbour, tending to check the deposit of silt. ”

“ The injuries to which composite breakwaters appear to have been specially subject must be attributed to the greater exposure and depth of the sites in which they have been frequently constructed, as compared with rubble mounds or upright walls. The latter types, indeed, are not well suited for erection in deep water, in the first case, on account of the very large quantity of materials required 480 for a high mound with flat slopes, and in the second, owing to the increased pressure of air under which divers have to work in laying blocks for an upright wall in deep water. ”