For a deepwater site or one with an extreme
range between high and low tides, a rubble-mound
breakwater or jetty may by topped with a cap struc-
ture to form the composite type shown in figure
10-23. In this case, the cap structure consists of a
series of precast concrete boxes called caissons,
each of which is floated over its final location and
sunk into place by filling with rock. A single-piece
concrete cap is then cast in place on the top of each
caisson. Breakwaters and jetties are sometimes built
entirely of caissons. A typical caisson breakwater/
jetty is shown in figure 10-24. A jetty may also be
constructed to serve as a wharfage structure. If so,
it is still called a jetty.
ALONGSHORE
Alongshore structures include seawalls, groins, and
bulkheads. Their main purpose is to stabilize a shore-
line.
Seawalls
Seawalls vary widely in details of design and
materials, depending on the severity of the exposure,
the value of the property to be protected, and other
considerations. Basically, though, they consist of
some form of barrier designed to break up or reflect
the waves and a deep, tight cutoff wall to preclude
washing out of the sand or soil behind and under the
barrier. The cutoff wall is generally constructed of
timber, steel, or concrete sheet piling. Figure 10-25
Figure 10-25.-Riprap seawall.
10-13
