Selection and Design of Channel Rehabilitation Methods
Rigid armor is an erosion-resistant material which has little or no flexibility to conform to bank
irregularities occurring after construction. Typically, the armor is placed directly on the bank slope in a fluid
or chemically reactive state, then hardens.
The most common rigid armors are: asphalt; concrete; grouted riprap (or other grouted armor
material); and soil-cement. Materials which have a more restricted use, but which can be classified as rigid
armors, are chemical soil stabilizers, and clay.
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.
Advantages: 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: A rigid armor requires careful design and quality control during construction, and
unfavorable weather conditions can cause construction delays. Chemical soil stabilization, and clay 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.
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 offer environmentally, being biologically sterile and perhaps
unacceptable aesthetically, depending on the surroundings.
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