Channelization and Channel Modification Activities and Impacts
failure differs from erosion in which a relatively large section of bank fails and slides into the channel.
Streambank failure is often considered to be a geotechnical process. A geotechnical failure involves the
movement of a relatively large and possibly intact segment of soil. There are two distinct classes of bank
failure: the slow moving creep and the catastrophic shear failure. The slow moving creep failure occurs
over long periods of time, whereas the catastrophic shear failure occurs instantaneously.
Channel instability can ultimately result in system-wide bank instability. As channel degradation
proceeds through a system, the channel bank heights and angles are increased, which reduces the bank
stability with respect to mass failures under gravity. If degradation continues, eventually the banks become
unstable and fall. Bank failures may no longer be localized in bendways, but rather may also be occurring
along both banks in straight reaches on a system-wide basis. Fluctuating flows through channels and
localized runoff can also contribute to accelerated erosion of the banks.
System-wide instability is treated with channel stabilizing methods described above. Localized
bank erosion and failure is treated with a variety of methods designed to either directly or indirectly protect
the bank (Shields and Palermo, 1982). Bank stabilization projects address local problems such as meander
migration and constricted reaches and are not a remedy for system instability. Direct bank protection
methods are placed in contact with the bank to prevent erosion. Indirect protection methods are designed
to deflect flows from the affected area or reduce turbulence and encourage sediment deposition. Example
of direct methods are stone riprap, trench fill revetment, concrete paving, articulated concrete mattresses,
and vegetation. From an environmental viewpoint, vegetation is the preferred treatment when hydraulic
conditions allow its use. Woody vegetation is usually restricted to banks, but grass linings may be used if
properly maintained and not exposed to excessive velocities (Nunnally and Shields, 1985). Indirect
methods include dikes, fences, and jacks. More detailed information concerning design and placement of
channel and bank stabilization methods is provided in the WES Stream Investigation and Streambank
Stabilization Handbook (Biedenharn et al., 1997).
The primary purpose of reservoir construction is usually flood control or water supply, but
reservoirs may also be designed specifically to induce channel stability and subsequently stabilize banks.
The effect of reservoirs is to reduce peak discharges and sediment supply to the downstream channel. A
reduction in peak discharge often reduces bank instability by inducing deposition at the channel margin in
the form of berms. In effect the channel adapts to a lower effective or dominant discharge by shrinking.
Bank failure upstream of reservoir impoundments will be decreased by the reduction in flow velocities and
bank shear stresses for the length of the channel affected by the impoundment.
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