Surface Armor for Erosion Protection
Advantages are: The most common rigid armors will withstand high
velocities, have low hydraulic roughness, and prevent infiltration of water into
the channel bank. They are practically immune to vandalism, damage from
debris, corrosion, and many other destructive agents. The most common rigid
armors are easily traversed by pedestrians.
Disadvantages are: A rigid armor requires careful design and quality control
during construction, and unfavorable weather conditions can cause
construction delays. Chemical soil stabilization, clay, and ice have a limited
range of effectiveness.
Provision for draining groundwater and preventing the buildup of excess
positive pore water pressures, in the form of a filter or subsurface drains, must
usually be provided for impermeable armors, which may significantly increase
the cost of the project.
Most rigid armors are difficult or impossible to construct underwater,
although this difficulty can be alleviated for concrete by using one of the
commercially available fabric mattresses (see "Concrete" below). Asphalt has
been placed underwater in some cases (see "Asphalt" below).
Rigid armor, being inflexible, is susceptible to breaching if the bank material
subsides or heaves. Increased wave runup on a smooth rigid armor may be
a concern for some projects.
Some of these materials have little to recommend them environmentally, being
biologically sterile and perhaps unacceptable aesthetically, depending on the
surroundings.
Typical applications are: Rigid armor in the form of concrete, asphalt, or
grouted riprap is often considered for use in situations where high velocities
or extreme turbulence make adjustable armor ineffective or very expensive.
Typical uses are in conjunction with hydraulic structures or in artificial
channels on steep slopes.
Rigid armor may be the preferred alternative in flood control or drainage
channels where low boundary roughness is mandatory, or in water supply
channels where prevention of water loss due to infiltration into the bank is
important. It is suitable for bank slopes which must be easily traversed by
pedestrians or recreational users, if the slope is not too steep for safety.
Rigid armor is sometimes the least costly alternative, typically where
adjustable armor is not available locally, especially if a geotechnical analysis
of the bank material indicates that elaborate subsurface drainage work is not
necessary.
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