Channelization and Channel Modification Activities and Impacts
used extensively on the Lower Mississippi River to maintain navigation channels, and can be used in
conjunction with other measures such as floodways, cutoffs, bank protection and levees to aid in flood
control, maintain navigation, and stabilize river systems. Additional applications include cutting off side
channels and chutes, concentrate a braided river into a single channel, realigning a river reach, and
streambank protection.
A variety of materials can be used to construct dikes. Stone dikes and pile dikes are the most
common type in use, but soft dikes consisting of sand filled geotextile containers have been used
successfully on the lower Mississippi River. Dikes may be constructed either parallel or perpendicular to
the flow. Spur dikes, which are sometimes referred to as transverse or cross dikes, are the most common
types of dikes used on major streams (Shields and Palermo, 1982). Dikes are generally constructed in
groups perpendicular to the flow, extending outward from the bank toward the center of the channel.
Spacing between dikes in a dike field is generally a function of the location of the next dike downstream
(Peterson, 1986). Longitudinal dikes extend downstream and parallel to the flow. The primary purpose
is for reducing the curvature of sharp bends and provides erosion protection for the adjacent bank.
L-head dikes consist of both a section perpendicular to the flow extending from the bank, and a
section parallel to the flow extending downstream from the end of the perpendicular section. L-head dikes
are designed to reduce sedimentation behind the dike and can be used to reduced sedimentation in specific
areas such as harbor entrances.
4.2.8.1 Hydraulic Effects
Dikes are designed and constructed to confine flows in a narrow channel and induce an increase
in sediment transport through the channel. Depending on design, dikes can affect the flow in a number of
ways. For example, spur dikes, which extend perpendicular to the flow, are used to constrict the flow and
concentrate the flow within the constricted reach. Longitudinal dikes are arranged downstream and parallel
to the flow, and are used to reduce the curvature of sharp bends, develop stable channel alignments, and
provide erosion protection for the adjacent bank. Because of the increased velocities, localized scour and
undercutting occurs at the end of the transverse dike. Incorporation of design criteria such as improved
profile slope and dike angle can reduce the effects of scour. At low water, sediment deposition occurs in
the slack water between dikes.
4.2.8.2 Environmental Effects
Shields and Palermo (1982) report work by Thackston and Sneed (1980) and Johnson et al.
(1974) which identified three areas of environmental impacts due to dike fields: 1) impacts associated with
dike construction, 2) changes in water surface area and aquatic habitats, and 3) increased water-level
fluctuation. Because the majority of dike construction occurs in depositional zones near the bank, some
benthic habitat is lost during construction. Additionally, construction techniques may temporarily increase
localized turbidity. Dikes increase the habitat diversity. The areas between the stones and downstream
of the dike provide feeding and resting areas for fish. Slack water between dikes provides additional
aquatic habitat unless excessive sedimentation occurs.
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