[1] _Memoirs_ of John Macky (Roxburghe Club, 1895), 121.
[2] _Corr. of Col. N. Hooke_ (Roxburghe, Club, 1870), i. 49.
[3] Note by Sir W. Scott in Sinclair's _Mem. of Insurrection in
Scotland_ (Abbotsford Club, 1858), 185.
BREADALBANE, a large district of Perthshire, Scotland, bordered N. by
Atholl, E. by Strathtay, S. by Strathearn and W. by the districts of
Argyll and Lorne, and occupying some 1020 sq. m. Most of the surface is
mountainous, Ben Lawers (3984 ft.), Ben More (3843), and Ben Lui (3708),
being the principal hills. Loch Tay is the chief lake, and among the
rivers are the Orchy, Dochart, Lochay, Lyon, Almond and the Tay (during
the early part of its course). Population mostly centres in Aberfeldy,
Fortingal, Kenmore and Killin. The soil is not cultivable excepting in
some of the glens and straths. Game is plentiful, the lakes and rivers
afford good sport, and the deer forests and shootings are valuable. The
district has given the titles of earl and marquess to the Campbells of
Glenorchy.
BREAD-FRUIT. This most important food staple of the tropical islands in
the Pacific Ocean is the fruit of _Artocarpus incisa_ (nat. ord.
Moraceae). The tree attains a moderate height, has very large, acutely
lobed, glossy leaves, the male flowers in spikes, and the female flowers
in a dense head, which by consolidation of their fleshy carpels and
receptacles form the fruit. The fruit is globular in shape, about the
size of a melon, with a tuberculated or (in some varieties) nearly
smooth surface. Many varieties of the tree are cultivated, the fruits of
some ripening numerous seeds, which are eaten as chestnuts; but in the
best kinds the seeds are aborted, and it is only these that are highly
prized as vegetables. The tree is a native of the South Sea Islands,
where its fruit occupies the important position that is held by cereals
in temperate latitudes. The fruit, which on distinct varieties ripens at
different periods, affording a nearly constant supply throughout the
year, is gathered for use just before it ripens, when it is found to be
gorged with starchy matter, to which its esculent value is due. It may
be cooked and prepared for use in a great variety of ways, the common
practice in the South Sea Islands being to bake it entire in hot embers,
and scoop out the interior, which when properly cooked should have a
soft smooth consistence, fibrous only towards the heart, with a taste
which has been compared to that of boiled potatoes and sweet milk. Of
this fruit A.R. Wallace, in his _Malay Archipelago_, says: "With meat
and gravy it is a vegetable superior to anything I know either in
temperate or tropical countries. With sugar, milk, butter or treacle it
is a delicious pudding, having a very slight and delicate but
characteristic flavour, which, like that of good bread and potatoes, one
never gets tired of." In the Pacific Islands the fruit is preserved for
use by storing in pits, where the fruits ferment and resolve themselves
into a mass similar in consistency to new cheese, in which state they
emit an offensive odour; but after baking under hot stones they yield a
pleasant and nutritious food. Another and more common method of
preserving the fruit for use consists in cutting it into thin slices,
which are dried in the sun. From such dried slices a flour is prepared
which is useful for the preparation of puddings, bread and biscuits, or
the slices are baked and eaten without grinding. The tree yields other
products of economic value, such as native cloth from the fibrous inner
bark of young trees; the wood is used for canoes and articles of
furniture; and a kind of glue and caulking material are obtained from
the viscid milky juice which exudes from incisions made in the stem.
[Illustration: _Artocarpus incisa_, the Bread-fruit tree.
Fig. 1. Branch reduced about a 6th natural size, with cuneate-ovate
pinnatifid leaves, male flowers in a club-shaped deciduous catkin,
and female flowers in rounded clusters.
Fig. 2. Transverse section of the male spike with numerous flowers.
Fig. 3. Male flowers.
Fig. 4. Single male flower separated, with a perianth in 2 segments
and a single stamen.
Fig. 5. Female flowers.
Fig. 6. Single female flower separated, with ovary, style and bifid
stigma.
Fig. 7. Ovary.
Fig. 8. Ovary laid open to show the ovule.
Fig. 9. A variety of the ovary with 2 loculaments.
Fig. 10. Transverse section of a bilocular ovary.]
The bread-fruit is found throughout the tropical regions of both
hemispheres, and its first introduction into the West Indies is
connected with the famous mutiny of the "Bounty," and the remarkable
history of a small company of the mutineers at Pitcairn Island.
Attention was directed to the fruit in 1688 by Captain Dampier, and
later by Captain Cook, who recommended its transplantation to the West
Indian colonies. In 1787 the "Bounty" was fitted out under command of
Lieutenant William Bligh (q.v.) to proceed to Tahiti to carry plants
thence to the West Indian Islands; and it was after the cargo had been
secured and the vessel was on her way that the mutiny broke out, and
Lieutenant Bligh and some of his crew were turned adrift in a small boat
in the open sea. The mutineers returned with the vessel to Tahiti,
whence a number of them, with a few native men and women, sailed to the
desolate and lone islet of Pitcairn. Lieutenant Bligh ultimately reached
England, and was again commissioned to undertake the work of
transplanting the plants, which in the year 1792-1793 he successfully
accomplished.
A somewhat similar but inferior fruit is produced by an allied species,
the Jack or Jak, _Artocarpus integrifolia_, growing in India, Ceylon and
the Eastern Archipelago. The large fruit is from 12 to 18 in. long by 6
to 8 in. in diameter, and is much eaten by the natives in India. This
tree is chiefly valuable on account of its timber, which has a grain
very similar to mahogany, and although at first light-coloured it
gradually assumes much of the appearance of that wood.
BREAKING BULK, a nautical term for the taking out of a portion of the
cargo of a ship, or the beginning to unload; and used in a legal sense
for taking anything out of a package or parcel, or in any way destroying
its entirety. It was thus important in connexion with the subject of
bailment, involving as it did the curious distinction that where a
bailee received possession of goods in a box or package, and then sold
them as a whole, he was guilty only of a breach of trust, but if he
"broke bulk" or caused a separation of the goods, and sold a part or
all, he was guilty of felony. This distinction was abolished by the
Larceny Act 1861, which enacted that whoever, being a bailee of any
chattel, money or valuable security, should fraudulently take or convert
the same to his own use, or the use of any person other than the owner,
although he should not break bulk or otherwise determine the bailment,
should be guilty of larceny (s. 3).
BREAKWATER. When a harbour (q.v.) is proposed to be established on an
exposed coast, whether for naval or commercial purposes, to provide a
protected approach to a port or river, or to serve as a refuge for
vessels from storms, the necessary shelter, so far as it is not
naturally furnished by a bay or projecting headlands, has to be secured
by the construction of one or more "breakwaters." 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. 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; and the extent of
breakwater exposed to breaking waves depends upon the difference in
level between low and high water. The existence, also, of any drift of
sand or shingle along the shore must be ascertained, and its extent; for
the projection of a solid breakwater out from the shore is certain to
affect this littoral drift, which, if large in amount, may necessitate
important modifications in the design for the harbour.
Winds.
Observations of the force and prevalence of the winds from the different
quarters at the various periods of the year, and the instruments by
which they are recorded, belong to the science of meteorology; but such
records are very valuable to the maritime engineer in indicating from
which directions, open to the sea, the worst storms, and, consequently,
the greatest waves, may be expected, and against which the most
efficient shelter has to be provided. Moreover, it is necessary, for
constructing or repairing a breakwater, to know the period of the year
when the calmest weather may be safely anticipated, and also the stormy
season during which no work should be attempted, and in preparation for
which unfinished works have to be guarded by protective measures. In the
parts of the world subject to periodical winds, such as the monsoons,
the direction and force of the winds vary with remarkable regularity
according to the seasons; and even such uncertain occurrences as
hurricanes and cyclones generally visit the regions in their track at
definite periods of the year, according to the locality. Even in western
Europe, where the winds are extremely variable, violent gales are much
more liable to beat upon the western and northern coasts in the winter
months than at any other period of the year; whilst the calmest weather
may be expected between May and August.
Waves.
The size of waves depends upon the force of the wind, and the distance
along which it blows continuously, in approximately the same direction,
over a large expanse of ocean. The greatest waves are, accordingly,
encountered where the maximum distance in a certain direction from the
nearest land, or, as it is termed, the "fetch," coincides with the line
travelled by the strongest gales. The dimensions, indeed, of waves in
the worst storms depend primarily on the extent of the sea in which they
are raised; though in certain seas they are occasionally greatly
increased by the exceptional velocities attained by hurricanes and
typhoons, which, however, are fortunately restricted to fairly well
defined and limited regions. Waves have been found to attain a maximum
height of about 10 ft. in the Lake of Geneva, 17 ft. in the
Mediterranean Sea, 23 ft. in the Bay of Biscay, and 40 ft. in the
Atlantic Ocean; whilst waves of 50 to 60 ft. in height have been
observed in the Pacific Ocean off the Cape of Good Hope, where the
expanse of sea reaches a maximum, and the exposure to gales is complete.
The length of large waves bears no definite relation to their height,
and is apparently due, in the long waves often observed in exposed
situations, to the combination of several shorter waves in their onward
course, which is naturally dependent on the extent of the exposure. Thus
waves about 560 ft. in length have been met with during severe gales in
the Atlantic Ocean; whilst waves from 600 to 1000 ft. long are regarded
as of common occurrence in the Pacific Ocean during storms.
The rate of transmission of the undulation also varies with the
exposure; for the ordinary velocity of the apparent travel of waves in
storms has been found to amount to about 22 m. an hour in the Atlantic
Ocean, and to attain about 27 m. an hour off Cape Horn. The large waves,
however, observed in mid-ocean do not reach the coast, because their
progress is checked, and their height and length reduced, by
encountering the shelving sea-bottom, which diminishes the depth of
water on approaching the shore; and the actual waves which have to be
arrested by breakwaters depend on the exposure of the site, the
existence of continuous deep water close up to the shore, and the depth
in which the breakwater is situated. On the other hand, the height, and,
consequently, the destructive force of waves, is increased on running up
a funnel-shaped bay, by the increasing concentration of the waves in the
narrowing width, just as the tidal range of a moderate tidal current is
much augmented by its passage up the Bay of Fundy, or up the Bristol
Channel into the Severn estuary, or by filling the shallow enclosed bay
of St Malo. 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.
Southwesterly gales, blowing up the Gulf of Genoa, cause large waves to
roll into the bay, reaching a height of about 21 ft. in the worst
storms.
Where outlying sandbanks stretch in front of a coast, as for instance
the Stroombank in front of Ostend and the adjacent shore, and the
sandbanks opposite Yarmouth sheltering Yarmouth Roads, large waves
cannot approach the land, for they break on the sandbanks outside.
Waves, indeed, always break when, on running up a shoaling beach, they
reach a depth approximately equal to their height; and the largest waves
which can reach a shore protected by intervening sandbanks, are those
which are low enough to pass over the banks without breaking.
The force of the wind, as transmitted by degrees to the sea, is
manifested as a series of progressing undulations without any material
displacement of the body of water, each undulation transmitting its
accumulated force to the next in the direction the wind is blowing, till
at last, on encountering an obstacle to its onward course, each wave, no
longer finding any water to which to communicate its energy, deals a
blow against the obstacle proportionate to its size and rate of
transmission; or on reaching shoal water near the shore, the undulation
is finally transformed into a breaking wave rushing up the sloping
beach. 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.
Rubble mound.
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.
[Illustration: FIG. 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.
Concrete blocks with rubble mound.
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.
[Illustration: FIG. 2.--Alexandria Breakwater.]
[Illustration: FIG. 3.--Cette Breakwater.]
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.
[Illustration: FIG.4.--Port Said Western Breakwater.]
Concrete block mound.
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.
The large masses composing mound breakwaters give them great stability
against the attacks of the sea; and, moreover, the wide base of the
mounds enables them to be deposited on a sandy or silty sea-bottom,
without any fear of settlement or undermining. A mound breakwater,
however, has the disadvantages of requiring a large amount of
material, and of occupying a wide space on the bed of the sea, more
especially where the mound consists of rubble stone and is in deep
water, so that the system, though simple, is costly, and is unsuited
for harbours where the available space to be sheltered is limited.
Nevertheless, a mound breakwater can be rapidly constructed by the
employment of a large number of barges; and by the adoption of large
concrete blocks, the quantity of materials and the space occupied by
the mound can be considerably reduced. 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.
2. _Breakwaters formed of a Mound surmounted by a Superstructure._--The
second type of breakwater consists of a mound, composed of rubble or
concrete blocks, or generally a combination of the two, carried up from
the sea-bottom, on the top of which some form of solid superstructure
is erected. This superstructure reduces considerably the amount of
materials required (which, on account of the slopes of the mound,
increases rapidly with the height) in proportion to the depth at which
the superstructure is founded; and the solid capping on the mound
serves also to protect the top of the mound from the action of the
waves. In the case, however, of a mound breakwater, portions of the
highest waves generally pass over the top of the mound, and also to
some extent expend their force in passing through the interstices
between the blocks; whereas a superstructure presents a solid face to
the impact of the waves. A superstructure, accordingly, must be very
strongly built in proportion to the exposure, and also to the size of
the waves liable to reach it, which depends upon the height and
flatness of the slope of the mound just in front of it on the sea side.
Special care, moreover, has to be taken to prevent the superstructure
from being undermined; for the waves in storms, dashing up against this
nearly vertical, solid obstacle, tend in their recoil down the face to
scour out the materials of the mound at the outer toe of the
superstructure, and thereby undermine it, especially where the
superstructure is founded on the mound near low-water level, and there
is, therefore, no adequate cushion of water above the mound to diminish
the effect of the recoil on the foundation.
The mound constituting the lower portion of the composite type of
breakwater has been formed in the same varied way as simple mound
breakwaters, namely, of rubble, sorted rubble, rubble protected by
concrete blocks, and wholly of concrete blocks. The only differences
introduced in the mound in this case are, that it is not carried up so
high, that the top portion covered by the superstructure needs no
further protection, and that special protection has to be provided on
the slope of the mound adjacent to the outer toe of the
superstructure.
Superstructures.
The forms of the superstructures exhibit considerable variations,
ranging from a few concrete blocks laid in courses on the top of the
mound, or a paving furnishing a quay protected by a narrow parapet
wall on the sea side, up to a large, solid structure, only differing
from an upright-wall breakwater in being founded upon a mound, instead
of on the sea-bottom. Notwithstanding, however, this great variety in
design, these breakwaters may be divided into two distinct classes,
namely, breakwaters having their superstructures founded at or near
low-water level, and breakwaters with superstructures founded some
depth below low water. The object in the first case is to lay the
foundations of the superstructure on the mound at the lowest level
consistent with building a solid structure with blocks set in mortar,
out of water, in the ordinary manner; and, in the second case, to stop
the raising of the mound at such a depth under water as to secure it
from displacement by the waves. In fact, the solidity and facility of
construction of the superstructure were the primary considerations in
the older form of breakwater; whereas the stability of the mound and
the avoidance of the undermining of the superstructure have been
regarded as the most important provisions in the more modern form.
Superstructures at low-water level.
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, 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.
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). In this case the quay
has been rendered accessible for vessels on the harbour side by a quay
wall, formed of concrete blocks deposited one above the other,
providing a vertical face to a depth of about 22-3/4 ft. below
sea-level; and a similar arrangement has been adopted at Trieste, and
in a less effective manner at Civita Vecchia and Naples. At
Marseilles, however, when the breakwater reached great depths, the
quay was abandoned on account of the increased exposure, and the
extension made of a simple rubble mound, protected on the sea side,
from the top down to 20 ft. below sea-level, by large concrete blocks
deposited at random.
[Illustration: FIG. 5.--Marseilles Breakwater, central portion.]
The superstructures at Holyhead and Portland, being built on the old
weak system of a sea wall and a harbour wall, with rubble filling
between, are protected on the sea side by raising the rubble against
them from low water up to high water of spring tides; whereas the
superstructure of Cherbourg breakwater, being built solid and less
exposed, is only protected on the sea side by large rubble and some
concrete blocks, forming an apron raised slightly above low water.
These three breakwaters are provided with a quay sheltered by a raised
wall or promenade on the sea side; but as the mound on the harbour
side is raised up to, or a little above low water, the quay is only
accessible for vessels near high water. This, however, is of
comparatively little importance, since these quays, though very useful
for access to the end of the breakwater in fairly calm weather, are
inaccessible in exposed situations with a rough sea; and quays for the
accommodation of vessels are better provided well within the sheltered
harbour.
The outer portions of the main breakwaters at Genoa and at Naples
(fig. 6), extending into depths of about 75 ft. and 110 ft.
respectively, have been provided with superstructures, similar in
type, but more solid than the superstructure at Marseilles; and the
sorted rubble mounds upon which the superstructures rest are protected
on the sea slope by stepped courses of concrete blocks from a depth of
26 ft. below sea-level, covered over at the top by a masonry apron
forming a prolongation of the superstructure. The outer extension of
the main breakwater at Civita Vecchia furnishes an interesting example
of a composite form of breakwater, in which the rubble mound has been
protected, and greatly reduced in volume and extent in deep water, by
stepped courses of concrete blocks carried up from near the bottom of
the mound (fig. 7).
[Illustration: FIG. 6.--San Vincenzo Breakwater, Naples.]
The breakwaters in front of Havre, constructed in 1896-1907, for
sheltering the altered entrance to the port, were formed of a sorted
rubble mound, protected on the sea slope by concrete blocks, and
raised a little above low water of spring tides, upon which large
blocks of masonry, built on land, were deposited with their upper
surfaces about 18 in. above low water of neap tides. As soon as
settlement of the mound under the action of the sea appeared to have
ceased, these masonry blocks were connected together by filling the
spaces between them with masonry; and a solid masonry superstructure
was built during low tide on this foundation layer, as shown in fig.
8.
[Illustration: FIG. 7.--Civita Vecchia Outer Breakwater.]
The breakwaters constructed for forming harbours on the sea coast of
the United States are almost all rubble-mound breakwaters. The two old
detached breakwaters sheltering Delaware Harbour near the
south-eastern extremity of Delaware Bay, were formed of simple rubble
mounds raised about 13 ft. above low water; but in closing the gap
between them towards the close of the 19th century, the rubble mound
was stopped at low water, and a sort of superstructure, consisting of
stepped courses of large rectangular blocks of stone on the sea and
harbour sides, with tightly packed rubble between them and capped
across the top for a width of 20 ft. with a course of large blocks,
was raised to 14 ft. above low water, resembling, on a small scale,
the upper part of the Civita Vecchia mound (fig. 7). A similar
construction was adopted for the new breakwater formed in 1897-1901
for providing a harbour of refuge at the mouth of Delaware Bay; but in
this instance the mound was made considerably wider at the top, and
had to be protected along the toe of the superstructure on the sea
side by large stones. The same form of superstructure, also, on a
narrower base, was resorted to for a breakwater in deeper water at San
Pedro in California with satisfactory results. When, however, a
breakwater of the Delaware type was in progress for forming a harbour
of refuge in Sandy Bay, Massachusetts, in front of Rockport to the
north of Boston, the upper 13 ft. of the 600 ft. of completed
superstructure were carried away during a severe storm in 1898 leaving
only a portion about 5 ft. in height above low water, the average rise
of tide there being 8-3/5 ft. The design was, accordingly, modified in
1902, by commencing the stepped courses of large stones at 12 ft.
below mean low water on each slope, instead of at low water raising
this kind of superstructure to 22 ft. above low water in place of 18
ft., and capping the stepped courses at the top by large blocks of
stone, 20 ft. long and 5 ft. deep, laid across the breakwater, which
thus presented a marked resemblance to the upper section of the mound
at Civita Vecchia.
Superstructure below low-water level.
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 can be regarded as forming part of
a superstructure; but as the protection afforded by these courses
differs only in the arrangement of the blocks from that obtained by
blocks deposited at random, it appears expedient to restrict this
class to the more solid structures, resembling upright-wall
breakwaters, founded on a mound at some depth below low water As the
main object of this class of breakwater is to keep the mound below the
zone of disturbance by waves in severe storms, it is evident that the
depth at which the superstructure is founded should vary directly with
the exposure of the site, and inversely with the size of the materials
forming the mound.
[Illustration: FIG. 8.--Havre Breakwater.]
The depth at which waves striking against a superstructure may affect
a rubble mound near its toe by the recoil, has been only very
gradually realized. Thus, in 1847, the Alderney breakwater, though
fully exposed to the Atlantic Ocean, was begun with a superstructure
founded at low water of spring tides upon a rubble mound; but within
two years the foundations had to be carried down 12 it. below low
water, and this was adhered to till close to the head, though the
breakwater, completed in 1864, extended 4700 ft. from the shore into a
depth of 130 ft. at low tide, the rise of springs being 17 ft. The
great recoil of the waves in storms from the promenade wall on the sea
side of the superstructure, raised 33 ft. above low water, disturbed
the sea slope of the mound along the outer portion, situated in depths
of 80 to 130 ft. at low water, out to a distance of 90 ft. from the
superstructure and to a depth of 20 ft.; whilst the outer toe of the
superstructure was only preserved from being undermined by frequent
deposits of stone along the sea face.
The south-west breakwater at Colombo Harbour, constructed in
1876-1884, facing the seas raised by the south-west monsoon, extends
into a depth of 39 ft. at low water, where the rise of tide is only 2
ft. at springs, and was built with a superstructure founded upon a
rubble mound at a depth of 20 ft. below low water, but raised only 12
ft. above this level without any parapet, and protected along its sea
face by an apron of concrete in bags. In this case, not only was the
depth of the sea much less than at Alderney, but the small elevation
of the superstructure above low water enabled a portion of the waves
in storms to pass over it without materially impairing the shelter
inside. These circumstances reduced the shock and recoil of the waves;
and the greater depth of the foundations and the protection of the toe
of the superstructure greatly diminished the danger of undermining.
Consequently, the Colombo breakwater has been preserved from the
injuries to which the outer part of the Alderney breakwater succumbed.
Nevertheless, in subsequently constructing the north-west detached
breakwater, less exposed to the south-west monsoon, but in somewhat
deeper water (see COLOMBO), the experience of the action of the sea on
the south-west breakwater led to the laying of the foundations of the
superstructure on the rubble mound at 30-3/4 ft. below low water (fig.
9).
[Illustration: FIG. 9.--Colombo North-West Breakwater.]
The breakwater for sheltering Peterhead Bay, where the rise of springs
is 11-1/4 ft., was begun in 1888, and designed to extend into a depth
of 9-1/2 fathoms at low water (see HARBOUR). It was built as an
upright wall upon the rocky bottom for 1000 ft. from the shore; but
owing to the increase in depth it was decided to construct the outer
portion with a rubble base, surmounted by a superstructure originally
designed to be founded 30 ft. below low water. As, however, during a
storm in October 1898, the recoil of the waves from the breakwater,
which is provided with a promenade wall rising about 35 ft. above low
water, disturbed rubble to a depth of 36-1/2 ft., the superstructure
has been founded 43 ft. below low water on the rubble base; and its
outer toe is protected from being undermined by two rows of concrete
blocks on the rubble mound.
Construction of the superstructure.
Formerly, in constructing a large superstructure upon a rubble mound,
it was a common practice to build a sea wall and a harbour wall
several feet apart, and to fill up the intermediate,. space between
them with rubble, so as economically to form a wide structure on the
top of the mound, and provide an adequate width for a quay along the
top. A sheltering wall was also generally erected on the sea side.
This, for instance, was the system of construction adopted for the
superstructures, founded at low water, of Holyhead breakwater,
Portland inner breakwater, and St Catherine's, Jersey, breakwater.
Alderney breakwater, the Tyne breakwaters and Colombo south-west
breakwater were also commenced with a precisely similar method of
construction. The system, however, possesses a Very serious defect for
exposed situations, namely, that if once the sea can force a small
opening through the sea wall, the scooping out of the rubble filling,
and the overthrow of the thinner harbour wall are rapidly accomplished
if the storm continues or recurs before repairs can be effected.
Experience soon proved at Alderney and Tynemouth the unsuitability of
the system for very exposed situations; and the intermediate rubble
filling was replaced by solid hearting down to a certain depth. At
Colombo, after the first 1326 ft. of the south-west breakwater had
been built with two walls and intermediate rubble for the
superstructure, as the exposure proved greater than had been
anticipated, and a slight displacement of part of the sea wall, 24 ft.
wide, had occurred, the rubble filling was discontinued, and the two
walls were united into a solid superstructure 34 ft. in width.
sloping block system.
A difficulty experienced in constructing a solid superstructure on the
top of a rubble mound consists in the settlement of the mound which
takes place when the weight of the superstructure comes on it, in
spite of the consolidation of the rubble under the action of the sea
for one or two years before the erection of the superstructure on it
is undertaken. When the superstructure is carried out in long
stepped-forward courses, irregular settlement is particularly liable
to occur, as the weight is progressively imposed in an uneven manner
on the yielding rubble, in proportion to the height of the rubble base
and its deficiency in compactness. The open joints between the blocks
laid below low water enable the air to penetrate, on the recoil of the
waves at low tide, into any internal fissures resulting from
settlement; and the following wave, on striking the superstructure,
compresses the air inside, which, on its expansion when the wave
recedes, forces out any unconnected face stones. The hole thus formed
is rapidly enlarged by the sea if the storm continues; and a breach is
eventually formed. The sloping-block system was, accordingly devised
to provide against the dislocation of superstructures by the
inevitable irregular settlement, by forming them of a series of
sloping sections, composed of concrete blocks laid at an angle, free
to settle independently on the mound, as shown in fig. 10. In the
first superstructure thus constructed, in 1869-1874, at the entrance
to Karachi harbour, founded 15 ft. below low water on a rubble mound
and 24 ft. high, the blocks in each section, consisting of two rows of
three superposed blocks laid at an inclination of 76 deg. shorewards,
were entirely unconnected; and, consequently, though the
superstructure offered as little opposition as practicable to the
waves by having its top slightly below high water, the waves in a
storm forcing their way into the vertical joint between the two rows,
threw some of the top 27-ton blocks of the inner row down on the
harbour slope of the mound. This cause of damage was obviated in
effecting the repairs, by connecting the top blocks with the next ones
by stone dowels. The superstructures of the breakwaters forming Madras
harbour, commenced in 1876, were similarly constructed in sloping,
independent sections, 4-1/2 ft. thick, composed of two distinct rows
of four tiers of blocks founded upon a rubble mound 22 ft. below low
water (the rise of tide at springs being 3-1/3 ft.), and raised 3-1/2
ft. above high water. The blocks in each row were connected by a
tenon, projecting at the top of each block, fitting into a mortise in
the block above it. The retention of the vertical joint however,
between the two rows led to the overthrow of the greater part of the
superstructures of the outer arms at Madras, situated in a depth of 45
ft. and facing the Indian Ocean, during a cyclone of 1881. In the
reconstruction of these superstructures, bond was introduced in the
successive tiers of each sloping section; and the blocks of the two
upper tiers were cramped together. Alter settlement on the mound had
ceased, a thick capping of mass concrete was laid all along the top of
the superstructure; and, finally, a mound of concrete blocks was
deposited at random on the mound in front of the sea face of the
superstructure to break the force of the waves and prevent
undermining. A similar wave-breaker, with blocks somewhat specially
arranged, was deposited in front of the sloping concrete-block
superstructure of the breakwater sheltering the Portuguese harbour of
Marmagao on the west coast of India, more particularly with the object
of preventing the undermining of the superstructure founded only 18
ft. below low water of spring tides, on a layer of rubble spread on
the muddy sea-bottom, the settlement in this case being occasioned by
the yielding of the soft clay bed. This breakwater having been
commenced in 1884, subsequently to the failure at Madras, the
superstructure, formed of concrete blocks weighing 28-1/2 to 37-1/2
tons was built in accordance with the design adopted for the
reconstructed outer arms at Madras, with the exceptions that the
separate sections were given a slope of 70 deg. instead of 76 deg.
shorewards to ensure greater stability, that the superstructure was
made 30 ft in width instead of 24 ft., that the top tier of blocks in
each section was secured to the next tier by two dowels, each formed
of a bundle of four rails, penetrating 3-1/2 ft. into each tier, so as
to enable the top courses to be more correctly aligned than with
tenons and mortises, and that the outer side of the continuous
concrete-in-mass capping was raised about 22 ft. above low water (fig.
11). The rise of spring tides at Marmagao is 6 ft.
[Illustration: FIG. 10.--Colombo North-West Breakwater with Titan
Crane.]
At Colombo the superstructures of both the south-west and north-west
breakwaters were built on the sloping-block system in sections 5-1/2
ft. thick, and built at an angle of 68 deg. shorewards (fig. 10); and
the blocks, from 16-1/2 to 31 tons in weight, were laid in bonded
courses across each section, with four tiers of blocks in the
south-west breakwater founded 20 ft. below low water on the rubble
mound, and six tiers of blocks in the north-west breakwater, founded
30-3/4 ft below low water. Five oblong grooves, moreover, were formed
in moulding the blocks, in the adjacent faces of each sloping section,
extending from top to bottom of the sections. These, when settlement
on the mound had ceased, were filled with concrete in bags which not
only connected the tiers of blocks in each section together, but also
joined the several sections to one another, and effectually closed
the transverse joints between the successive sections, which were
further connected together by a continuous capping of concrete-in-mass
along the whole length of the breakwater.
These sloping blocks are laid by powerful overhanging, block-setting
cranes, called Titans (see CRANES), which travel along the completed
portion of the breakwater, and lay the blocks in advance on the mound
levelled by divers, as shown in fig. 10. The earlier Titans, employed
for the sloping-block superstructures at Karachi and Madras, were
constructed to travel only backwards and forwards on the completed
work, with sufficient sideways movement of the little trolley
travelling along the overhanging arm, from which the block is
suspended at the proper angle, to lay the blocks for each side of the
superstructure. In later forms, however, such for instance as the
Titan laying the 14-ton blocks at Peterhead breakwater in horizontal
courses, the overhanging arm is supported centrally on a ring of
rollers, placed on the top of the truck on which the Titan travels, so
that it can revolve and deposit blocks at the side of the
superstructure for protecting the mound, as well as in advance of the
finished work. These Titans possess the important advantage over the
timber staging formerly employed for such breakwaters, that, in
exposed situations, they can be moved back into shelter on the
approach of a storm, or for the winter or stormy months, instead of,
as in the case of staging, remaining out exposed to the danger of
being carried away during stormy weather, or necessitating loss of
time in erection at the beginning of the working season.
[Illustration: FIG. 11.--Marmagao Breakwater.]
Though composite breakwaters are still occasionally constructed with a
superstructure founded on a rubble mound at, or above, low-water
level, these breakwaters are now almost always constructed with the
superstructure founded at some depth below low water, even at harbours
on the continent of Europe, where formerly broad quays founded at
sea-level, protected by a parapet wall and outer concrete blocks, were
the regular form of superstructure adopted. The breakwater for the
extension of the harbour at Naples provides an interesting example of
this change of design. A solid superstructure, formed of large
concrete blocks capped with masonry, about 50 ft. wide at the base, is
laid on a high rubble mound at a depth of 31 ft. below mean sea-level,
and provides a quay on the top, 24-1/2 ft. wide, protected on the sea
side by a promenade wall, 10 ft. high and 12-1/2 ft. wide at the top,
raised 19-2/3 ft. above sea-level (fig. 12). In view of the increased
depth at which superstructures are now founded upon rubble mounds,
causing the breakwaters to approximate more and more to the
upright-wall type, it might seem at first sight that the rubble base
might be dispensed with, and the superstructure founded directly on
the bed of the sea. Two circumstances, however, still render the
composite form of breakwater indispensable in certain cases: (1) the
great depth into which breakwaters have sometimes to extend, reaching
about 56 ft. below low water at Peterhead, and 102 ft. below mean
sea-level at Naples; and (2) the necessity, where the sea-bottom is
soft or liable: to be eroded by scour, of interposing a wide base
between the upright superstructure and the bed of the sea.
[Illustration: FIG. 12.--Naples Harbor Extension Breakwater.]
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 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. The ample depth in which
superstructures are founded, the due protection afforded to their
outer toe, the adoption of the sloping-block system for their
construction, and the dispensing in most cases with a high sheltering
wall on the sea side of the superstructure, render modern
superstructures as stable as upright-wall breakwaters of similar
height. Nevertheless, superstructures require to be given a greater
thickness than similar upright walls, because the greater depth of
water in which such composite breakwaters are built causes them to be
exposed to larger waves under similar conditions.
The superstructures of composite breakwaters erected by the United
States for harbours on the shores of Lake Superior were formerly in
some cases composed of timber cribs floated into position and sunk by
filling them with rubble stone. On account of the cheapness of timber
several years ago in those regions, this simple mode of construction
was also economical, even though the rapid decay of the timber in the
portions of the cribs where it was alternately wet and dry involved
its renewal about every fifteen years on the average. Owing, however,
to the fact that the price of timber has increased considerably,
whilst that of Portland cement has been reduced, durable concrete
superstructures are beginning to be substituted for the rapidly
decaying cribwork structures.
With the exception perhaps of the Alderney breakwater, which, owing to
its exceptional exposure and the unparalleled depth into which it
extended, had its superstructure so often breached by the sea that,
owing to the cost of maintenance, the inner portion only has been kept
in repair, the composite breakwater of Bilbao harbour has probably
proved the most difficult to construct on account of its great
exposure. The original design consisted of a wide rubble mound up to
about 16-1/2 ft. below low water, a mound of large concrete blocks up
to low water of equinoctial spring tides, and a solid masonry
superstructure well protected at its outer toe by a projection of
masonry, and raised several feet above high water, forming a quay
sheltered by a promenade wall. The rise of equinoctial spring tides at
the mouth of the river Nervion is 14-3/4 ft. In carrying out the work,
however, the superstructure built in the summer months was for the
most part destroyed by the following winter storms; and, accordingly,
the superstructure was eventually constructed on a widened rubble
base, so as to be sheltered to some extent by the outlying
concrete-block mound already deposited, a system subsequently adopted
in rebuilding the damaged portion of the North Pier at Tynemouth under
shelter of the ruins of the previous work. The modified superstructure
of the Bilbao breakwater was founded on the extended rubble mound at a
depth of 16-1/4 ft. below low water, and formed of iron caissons
partially filled with concrete and floated out, sunk in position, and
filled up with concrete blocks and concrete. It thus consists of a
continuous row of concrete blocks, each of them being 42-2/3 ft. in
width across the breakwater, 23 ft. in length along the line of the
breakwater, 23 ft. high, and weighing 1400 tons. These caisson blocks,
raised 6-3/4 ft. above low water, form the base of the superstructure,
upon which the upper part was built of concrete blocks on each face
with mass concrete filling between them, forming a continuous quay, 24
ft. wide, raised 8 ft. above high tide, and slightly sheltered by a
curved parapet block only 5 ft. high. The outer toe of the caisson
blocks is protected from being undermined by two tiers of large
concrete blocks laid flat on the rubble mound. This superstructure has
successfully resisted the attacks of the Atlantic waves rolling into
the bay. At this breakwater and at Tynemouth advantage has been taken
of the protection unintentionally provided by previous failures, by
which the waves are broken before reaching the superstructure and pier
respectively; but instead of introducing a wave-breaker of concrete
blocks, for a protection to the superstructure, as arranged at
Marmagao (fig. 11) and the outer arms at Madras, it would appear
preferable to increase the width of the solid superstructure, if
necessary, as carried out at Naples (fig. 12). and to dispense with a
parapet and keep the superstructure low, as being unsuitable for a
quay in exposed situations, according to the plan adopted at Colombo
(fig. 9).
3. _Upright-Wall Breakwaters._--The third type of breakwater consists
of a solid structure founded directly on the sea-bottom, in the form
of an upright wall, with only a moderate batter on each face. This
form of breakwater is strictly limited to sites where the bed of the
sea consists of rock, chalk, boulders, or other hard bottom not
subject to erosion by scour, and where the depth does not exceed about
40 to 50 ft. If a solid breakwater were erected on a soft yielding
bottom, it would be exposed to dislocation from irregular settlement;
and such a structure, by obstructing or diverting the existing
currents, tends to create a scour along its base; whilst the waves in
recoiling from its sea face are very liable to produce erosion of the
sea-bottom along its outer toe. Moreover, when the foundations for an
upright-wall breakwater have to be levelled by divers, and the blocks
laid under water by their help, the extension of such a breakwater
into a considerable depth is undesirable on account of the increased
pressure imposed upon diving operations.
The Admiralty pier at Dover was begun about the middle of the 19th
century, and furnishes an early and notable example of an upright-wall
breakwater resting upon a hard chalk bottom; and it was subsequently
extended to a depth of about 42 ft. at low tide, in connexion with
the works for forming a closed naval harbour at Dover. 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. The blocks, composed of six parts of sand
and stones to one part of Portland cement, moulded in frames, and left
to set thoroughly in the block-yard before being used, are all joggled
together, and above low-water level are bedded in cement and the
joints filled with cement grout. The blocks were laid by Goliath
travelling cranes running on temporary staging supported at intervals
of 50-1/4 ft. by clusters of iron piles carried down into the chalk
bottom. On each line of staging there were four Goliaths, preceded by
a stage-erecting machine. The front Goliath was used for working a
grab for excavating the surface layer of chalk, which was finally
levelled by divers, the second for carrying the diving-bell, the third
for laying the blocks below low water, and the fourth for setting the
blocks above low water. This succession of Goliaths enabled more rapid
progress to be made than with a single Titan at the end of a
breakwater; but it involved a considerable increase in the cost of the
plant, owing to the temporary staging required. The foundations were
carried down from 4 to 6 ft. into the chalk bottom, the deepest being
53 ft. below low water of spring tides, and the average 47 ft. With a
rise of tide at springs of 18-3/4 ft., the average depth is thus
approximately 66 ft. at high tide, necessitating a pressure of 29 lb.
on the square inch, which is the limit at which men can work without
inconvenience in the diving-bells. The breakwaters are raised about 11
ft. above high water of springs. The detached southern breakwater was
finished off at this level; but the extended western breakwater, or
Admiralty pier, is provided with a promenade parapet on its exposed
side, rising 13 ft. above the quay; and the eastern breakwater also
has a parapet on its exposed eastern side, raised, however, only 9 ft.
above its quay. The breakwaters are protected from scour along their
outer toe by an apron of concrete blocks, extending 25 ft. out from
their sea face.
[Illustration: Dover Breakwater.
FIG. 13. South Breakwater.