complete failure occurs. Rigid structures may be damaged and fail completely if subjected to differential
settlement or loss of support by underlying soil. Grout-filled mattresses of synthetic fabric and
reinforced concrete slabs are examples of rigid structures.
Detailed information about specific revetment configurations is contained in APPENDIX B.
BREAKWATERS
In contrast to bulkheads and revetments, breakwaters are placed out in the water, rather than
directly on shore, to intercept energy of approaching waves and form a low-energy shadow zone on their
landward side (Figure 9). Even a small decrease in wave height 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 significant
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 depends on their
height and porosity (amount of voids), distance from shore, length parallel to shore, and spacing (where
there is more than one structure).
Floating breakwaters are constructed of buoyant materials, such as logs, hollow concrete boxes,
and scrap rubber tires. The latter are most popular because of their ready, no-cost availability and
durability. Floating breakwaters are generally effective in sheltered waters where short-period (less than
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