Like bulkheads, revetments protect only the land immediately behind them and provide no
protection to adjacent areas. Erosion may continue on adjacent shores and may be accelerated near the
revetment by wave reflection from the structure, although not as seriously as with vertical-faced
bulkheads. Also, a downdrift shore may experience increased erosion if formerly supplied with material
eroded from the now protected area. If a beach is to be retained adjacent to a revetment, additional
structures such as groins or breakwaters may be required.
Of the revetment's three components, the primary one, which determines the characteristics of the
other two, is the armor layer, which must be stable against movement by waves. The second component,
the underlying filter layer, supports the armor against settlement, allows groundwater drainage through
the structure, and prevents the soil beneath from being washed through the armor by waves or
groundwater seepage. The third component, toe protection, prevents settlement or removal of the
revetment's seaward edge.
Overtopping by green water (not white spray) which may erode the top of the revetment can be
limited by a structure height greater than the expected runup height, or by protecting the land at the top of
the revetment with an overtopping apron. Flanking, a potential problem with revetments, can be
prevented by tying each end into adjacent shore protection structures or the existing bank. As the bank
retreats, however, the ends must periodically be extended to maintain contact.
The armor layer of a revetment maintains its position under wave action either through the
weight of, or interlocking between, the individual units. Revetments are either flexible, semi-rigid, or
rigid. Flexible armor retains its protective qualities even with severe distortion, such as when the
underlying soil settles or scour causes the toe of the revetment to sink. Quarrystone, riprap, and gabions
are examples of flexible armor. A semi-rigid armor layer, such as interlocking concrete blocks, can
tolerate minor distortion, but the blocks may be displaced if moved too far to remain locked to
surrounding units. Once one unit is completely displaced, such revetments have little reserve strength and
generally continue to lose units (unravel) until 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.
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