The shorter the radius of curvature of a bend, the deeper the channel will be adjacent to the
concave bank. The deeper the channel is, the greater the possibility of undermining bank
protection work in the bend and the greater the cost of maintaining the structure. Therefore,
sharp curvature of bends should be avoided to obtain the most economical control of the river.
6.3.3 Countermeasures for Channel Instability
Countermeasures can be used to control both lateral and vertical channel instability and
include river training structures and revetment armoring. River training structures are those
which modify the flow (flow control). River training structures are distinctive in that they alter
hydraulics to mitigate undesirable erosional and/or depositional conditions at a particular
location or in a river reach. River training structures can be constructed of various material
types and are not distinguished by their construction material, but rather, but their orientation to
flow. River training structures are described as transverse, longitudinal or areal depending on
their orientation to the stream flow.
To protect against lateral channel instability, flow control structures are used to:
Direct flow from one bend into the next bend downstream
Flair out sharp bends to a larger radius of curvature to provide a more desirable channel
alignment
Close off secondary channels and old bendways
Concentrate flow on a limited width within a wider channel
To protect against vertical channel instability, flow control structures are used for:
Limiting or halting long-term degradation
Establishing a desired channel bed elevation in a bridge reach
Arresting the migration of a head cut or nickpoint through a bridge reach
Revetments are structures parallel to the current and are used to armor the channel bank from
erosive/hydraulic forces. They are usually applied in a blanket type fashion for areal coverage.
Revetments can be classified as either rigid or flexible/articulating. Rigid revetments are
typically impermeable and do not have the ability to conform to changes in the supporting
surface. These countermeasures often fail due to undermining. Flexible/ articulating
revetments can conform to changes in the supporting surface and adjust to settlement. These
countermeasures often fail by removal and displacement of the armor material.
Rock riprap is probably the most widely used revetment material to stabilize river banks and
protect the side slopes of embankments and river training devices. In the final report to
Congress, the U.S. Army Corps of Engineers (1981) concluded that rock will likely continue to
be the first choice of bank protection materials where material of sufficient size is available and
affordable, because of durability, and other advantages. Because of its wide use and
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