In contrast to bulkheads and revetments, breakwaters are placed out in the water, rather than
directly on shore, to dissipate energy of approaching waves and form a low-energy shadow zone on their
landward side (Figure 35). Even a small decrease in wave height significantly reduces the ability of
waves to transport sediment. Sand moving along the shore, therefore, is trapped behind the structure and
accumulates. In the meantime, downdrift beaches are deprived of their normal sand supply and may
suffer increased erosion. For this reason, the area behind any such structure should be partially filled (to
perhaps 50 to 75 percent capacity) with sand after construction to insure an uninterrupted supply of sand
to downdrift beaches.
Breakwaters are either fixed or floating. The effectiveness of fixed breakwaters in dissipating
wave energy depends on their height and porosity (amount of voids). Floating breakwaters function at or
near the water's surface and must be firmly anchored to the seafloor to prevent their displacement. They
are generally effective in sheltered waters where wind-generated waves with short periods (less than 5
that may be less than the structure width, while their energy is concentrated near the surface and not
distributed down through the water column as with long-period waves.