Sea-wall.
（
Hydraulic Engineering.) A revetment along a line of coast, or the bank of a watercourse.
A sea-wall may form one or more sides of a harbor, as in the case of the Heptastadium of
Alexandria, the moles of the Pireus and Rhodes, of Civita Vecchia,
Ostia, and Antium, as well as those of many modern ports, especially in
Europe, whose small rivers afford but narrow refuges.
The Roman embankments of
Essex and
Lincolnshire, in England, were steep mounds of earth, defended by rows of
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piles, the intervals between the rows being filled up with chalk.
Modern embankments on the estuary of the
Thames consist of three strata.
The rise of tide is 10 feet, and the lower or main portion of the work is made with a base of 5, to 1 vertical The top width of this portion is 20 feet, and on it rests the
outburst bank, 5 feet high and 8 feet wide at top; having a slope of 1 1/2 to 1; thirdly, the
swash bank, 2 1/2 feet high, and the same width at top.
The embankments are
pitched with stone on a solid facing of clay, in positions much exposed, while in others the surface is covered with clay and gravel where it is washed by the water, the upper slope being turfed or protected by a growth of couch grass, ray grass or lucerne.
Shore defenses are of four kinds: —
1. Artificial constructions which break the force of the waves before they reach the shore.
2. Those which consolidate and elevate the shore itself, so as to resist the action of the waves.
3. Those which make or assist the accumulation of sand or shingle upon the shore.
4. Permanent break waters, which act as islands in the offing and exclude the waves.
Of the first description
were the cones of Cessart, placed off
Cherbourg.
Being destroyed, a break water of coursed masonry on a bank of
pierre perdue was substituted.
Of the same description also are the moored crates of timber, through whose interstices the water passes, the timbers breaking the force of the waves.
See breakwater.
Of the second description are the dikes of
Holland and of
France.
These are
sea-walls, and belong to this article
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Sea-walls. |
Of the third description are groins or timber erections, which are common on the sandy coasts of
England and
France.
See groin.
The works of Bremontier in the
Landes of
Bordeaux are also of this character, though they are rather those of the planter than the constructor.
His devices were eminently worthy of the engineer on the principle of Leupold's maxim, —
“Artis est naturam imitare.”
Of the fourth description are the solid breakwaters of
Plymouth,
Cherbourg, Cette, at the mouth of the
Delaware,
Buffalo, and elsewhere.
Sea-walls are made in many places to protect harbors or to save the land from encroachment.
When faced with coursed masonry, they may have the form shown at
B.
a is an earthen embankment;
b, a solid wall or core of puddle;
c, a facingwall of masonry:
d, the high-water level of the sea or bay; and
e, the natural bed. The form of the front wall must be adapted to resist the action of the waves, and the embankment must have an internal slope, according to the nature of the materials of which it is composed; for ordinary materials, a base of 1 1/2 to a perpendicular hight of 1 will insure the necessary stability and firmness.
A,
Plymouth breakwater.
B, sea-dike with facing wall and core.
C D. inclosure of Zuid Plas, near
Rotterdam, Holland.
E, polder bank,
Holland.
F, Havre sea-wall.
If the entire embankment be formed of loose stones, with occasional facing only of laid masonry, as in the case of the celebrated breakwater at
Plymouth, a form of less steepness must be adopted for the sea-face of the embankment.
A,
Fig. 4800, is a section of the
Plymouth (England) breakwater.
The line
a a shows the level of high-water spring-tides;
b b, low-water spring-tides;
c c, original bottom, varying from 40 to 45 feet below low-water mark;
d, the foreshore;
e, seaslope;
f, top, 45 feet wide.
The mass of the work is composed of limestone, from the Overton quarries, distant 4 miles from the spot.
The stone is raised in blocks varying from 1/4 to 10 tons and upward in weight, which are promiscuously thrown into the sea, care being taken that the greater number of the large blocks are thrown upon the outer or sea slope, and that the whole are so mixed together as to render the mass as solid as possible, the rubbish of the quarry and screenings of lime being flung in occasionally to assist the consolidation of the materials.
The form of the outer slope, below low-water line, has been effected by the action of the sea, and is ascertained to be at from 3 to 4 feet of base to 1 of perpendicular altitude.
From low water upward the work has been set artificially and inclined at 5 to 1.
The inner slope next the land is nearly 2 feet base to 1 altitude.
The foreshore, shown at
d, which is from 30 to 70 feet wide at different parts of the work, rises from the toe of the slope to a hight of 5 feet above low water at its outer extremity, and serves to break the waves before they reach the main work; thus diminishing their force, and, at the same time, preventing the recoil of the wave from undermining the base of the slope.
No other country in the world has so stern a contest to wage with the waters for its existence as
Holland.
Many thousands of square miles of tillable land have been reclaimed, and are only maintained by vigilance in the preservation of the
dunes or banks of sand which oppose the waters of the ocean and the large estuaries and rivers of the country.
The conformation of these is indicated by the illustrations, one of which (
D) shows a sea-wall, and the other (
C) one of the subordinate embankments, whereby the interior water-courses are lifted above the level of the country they traverse.
The materials of the
Dutch sea-walls or
dikes vary with the facilities, situation, materials, exposure, slope, etc.
Piling; double sheet-piling, with interposed puddle or rubble; earthen embankments, with or without walls of puddle, to render them water-proof; bundles of reeds, fascines, or gabions, laid in cross tiers with strata of soil, and secured by stakes or masonry.
Another form of sea-wall
E is extensively used in the banks of the polders of
Holland.
It consists of a double row of sheetpiling inclosing a
puddle dike of retentive soil.
The foot is protected by an apron of rubble.
At
Havre an embankment of earthwork is formed behind a vertical inclosure of sheet-piling, as shown at
F. The piles of the face are fastened by diagonal ties to the rear row of piles.
The foot is protected by rubble or coursed masonry, according to the character of the ground, the exposure to currents, etc.
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Sea-wall. |
In
Fig. 4801,
A B illustrates the sea-wall now being built along
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the water-front of New York City.
Beyond this, piers will project into the stream.
In the execution of the work the old wooden docks are removed, and the bed of the river dredged until solid bottom is reached.
Piles are then driven, for the width of 27 feet, sawed off to a uniform depth below low water, and the interstices between them filled up to within two feet of their tops with stone thrown over from barges.
Upon this a layer of concrete is spread by divers to the level of the tops of the piles.
The concrete is composed of 1 part of
Portland cement, 2 sand, 4 crushed stone; and, when it has set, blocks of the same are laid upon it to form the wall.
These blocks are molded; the larger blocks designed for the front of the wall average 36 tons in weight, and batter 2 1/2 inches to the foot on the outer or water side; square, smaller blocks are used for backing.
The upper part of the wall has a granite coping whose exposed portions are dressed.
The sides of the large concrete blocks are grooved to receive a chain by which it is lowered from a derrick; the
setting is done by divers.
The wall extends into the river to various distances, — at Christopher Street 250 feet, — beyond the present front; the intermediate space will be filled in with rubbish and paved.
The work, when completed, is designed to extend around nearly the whole circumference of
Manhattan Island.
At present (November, 1874) operations are in progress at the
Battery, at the foot of Canal Street, and at the foot of Christopher Street; all on the
North River side.
See floating-der-rick; wharf.
