Channelization and Channel Modification Activities and Impacts
with shallow depths and uniform unvarying substrates, artificial structures can reproduce the diversity of the
natural channel by creating alternating pool and riffle areas. Examples of artificial structures include
randomly placed boulders, small check dams, artificial riffles, bank covers, and current deflectors (Shields
and Palermo, 1982). Care must be taken to avoid creating additional channel instability problems due to
increased roughness or scouring when using artificial structures. Single bank construction is the preferred
technique for lessening environmental impacts of channel enlargement (Nunnally, 1985). The existing
channel alignment is followed with enlargement confined to one side. Vegetation on the opposite bank is
left undisturbed. The disturbed bank is revegetated to reduce erosion and sedimentation in the channel.
Erosion of the stream bank can be addressed with bank protection works. Concrete lined
channels have been employed, but are typically much more expensive than stone covers and further reduce
the in-stream and riparian habitat.
4.2.2.4 Operation and Maintenance of Channel Enlargement Projects
As with snagging and clearing projects, channel enlargement projects require periodic inspections.
Effects of channel instability due to the alteration of channel geometry may need to be addressed upstream
and downstream of the project. Channel stability should be monitored for signs of aggradation,
degradation, and planform changes. Re-growth of vegetation may occur during periods of low flow that
may require periodic maintenance.
4.2.3
CHANNEL REALIGNMENT
Channel alignment is often performed in conjunction with clearing and snagging. It is the process
of taking a sinuous channel and straightening it for the purpose of flood control, infrastructure protection,
or navigation. Additionally, channel realignment activities are implemented to reduce loss of land by
meander migration. Channel realignment can be implemented in varying degrees. An improved stream
alignment can be accomplished by removing shoaling areas such as point bars. For flood control
applications, the channel may be straightened to increase the slope and reduce flow resistance, thus
increasing the capacity of the stream to convey floodwaters. This practice may involve cutting off large
meanders of the river, thus actually shortening the river. The resulting cutoff generally results in slope
adjustments for the affected reach. In some environments, streams with stable meanders, flat slopes and
erosion resistant boundaries can withstand considerable realignment without serious impacts on system
stability (Brice, 1981). In other systems, it can lead to serious problems of channel degradation, bank
erosion, and tributary incision. Lane (1947) describes the response of an alluvial channel to a single cutoff.
The channel upstream of the cutoff will degrade as the channel slope flattens to re-establish an equilibrium
slope at a lower elevation. The reach downstream of the cutoff aggrades due to the increased sediment
supply from the degrading reach. A comprehensive description of the impacts of man-made cutoffs on the
Lower Mississippi River is provided by Biedenharn (1995).
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