ructure, 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 Triest