Figure 8-31.Caisson breakwater or jetty.
70 feet. The width of its base depends on the width of
the cap, the height of the structure, and the slopes of the
inner and outer faces. For a deepwater site or for one
with an extra-high tide range, a rubble-mound break-
water may be topped with a concrete cap structure, as
shown in figure 8-30. A structure of this type is called
a composite breakwater or jetty. In figure 8-30, the cap
structure is made of a series of precast concrete boxes
called caissons, each of which is floated over its place
of location and then sunk into position. A monolithic
(single-piece) concrete cap is then cast along the tops of
the caissons. Sometimes, breakwaters and jetties are
built entirely of caissons, as shown in figure 8-31.
A GROIN is a structure similar to a breakwater or
jetty, but it serves a third purpose. A groin is used in a
situation where a shoreline is subject to along shore
erosion, caused by wave or current action parallel or
oblique to the shoreline. The groin is run out from the
shoreline (usually there is a succession of groins at
intervals) to check the along shore wave action or deflect
it away from the shore.
A MOLE is a breakwater that is paved on the top
for use as a wharfage structure. To serve this purpose,
it must have a vertical face on the inner side, or
harborside. A jetty may be similarly constructed and
used, but it is still called a jetty.
These structures are constructed parallel with the
shoreline to protect it from erosion or other wave
damage. They are covered as follows.
A SEAWALL is a vertical or sloping wall that
offers protection to a section of the shoreline against
erosion and slippage caused by tide and wave action. A
seawall is usually a self-sufficient type of structure, such
as a gravity type retaining wall. Seawalls are classified
according to the types of construction. A seawall may
be made of riprap or solid concrete. Several types of
seawall structures are shown in figure 8-32.
Figure 8-32.Various types of seawalls.