transported sediments accumulate.
The breakwater reflects or dissipates the
incident wave energy and alters the wave direction and height by diffraction,
thereby modifying the local longshore transport.
Depending on the length and
spacing of the breakwaters in combination with several other parameters, sedi-
ments will accumulate landward of the structure and sometimes a bar or spit
that connects the structure to the shore (tombolo) will form.
Offshore breakwaters are generally designed to be one of two types:
shore connected (tombolo formation) and detached.
A shore-connected break-
water is generally long enough and/or close enough to shore that incoming
littoral material or beach fill will be reshaped by waves in the form of a
tombolo.
Detached breakwaters, on the other hand, are usually shorter and
farther from shore; wave conditions in the lees of these breakwaters are
therefore too severe to permit tombolo formation, yet reduced enough to cause
some sediment accumulation.
Shore-connected breakwaters offer the advantage
that inspection and maintenance are performed more easily.
Detached break-
waters are usually preferred, however, since they do not create a total
barrier to littoral transport and have a lesser impact on neighboring shore-
lines. The resulting beach is more suitable for bathing and other water
sports. Swimmers are also less likely to climb--and possibly injure them-
selves-- o n detached breakwaters.
DESIGN CONSIDERATIONS: The design of an offshore breakwater, especially a se-
ries of segmented offshore breakwaters, is a complex problem involving a number
of design parameters (Figure 2).
These are discussed in detail in the Shore
Protection Manual (Coastal Engineering Research Center 1984). Some parameters
are more important than others, and often one can be tailored at the expense
A thorough understanding of
of another without jeopardizing design success.
littoral processes, bathymetry, and wave conditions is necessary in order to
achieve a workable design.
Generally, the most important parameters are length of the structure
relative to the distance offshore and, in the case of a segmented breakwater,
spacing between breakwaters. Breakwater orientation should usually be parallel
to the shoreline. A breakwater that is placed too close to shore in an area
of abundant sand supply will likely create a tombolo where it may not be
desired. On the other hand, if a breakwater is too far offshore or if several
segmented breakwaters are too far apart, they may not provide the desired de-
gree of protection.
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