Channelization and Channel Modification Activities and Impacts
potentially increases streambank erosion. Low flows in enlarged channels may not have the pools
necessary for aquatic organisms to thrive. Because of the low velocities in enlarged channels, vegetation
may invade the channel and create a future channel maintenance problem.
Material excavated from the enlargement operations may be used to construct levees as a
management tool for providing additional flood protection. In Louisiana, material excavated from channels
was used to prevent saltwater intrusion into a brackish coastal marsh (Scott, 1972). Levees will reduce
overbank flows, thus potentially interfering with groundwater recharge and floodplain plant diversity.
Shields and Palermo (1982) list the following environmental consequences that should be considered when
enlarging a stream:
1)
Placement of excavated or dredged material;
2)
Cross-sectional shape and uniformity;
3)
Changes in substrate and substrate diversity;
4)
Removal of channel armor;
5)
High and low flow depths and velocities in the modified channel;
6)
Increased peak flows downstream; and
7)
Changes in stream-floodplain-groundwater interactions.
4.2.2.3 Remedial Practices
A method of enlargement that can reduce instability problems is the use of side berm cuts to form
a two-stage channel (USACE, EM 1110-2-1418, 1994). Although it has the disadvantage of using more
adjacent land than simply enlarging the channel, it is more effective in conveying bed material because higher
velocities are maintained at moderate discharges. The level of the berms should correspond to the channel
forming discharge under modified conditions. The side berm design is described by Nunnally and Shields
(1985) as a high flow channel.
Before any environmental improvement projects are undertaken, the system stability must be
addressed. The key to successful project implementation is to design a stable channel before enlarging
operations take place. A complete analysis of the hydrologic, hydraulic, and sediment transport
requirements of the enlarged channel should be evaluated before channel modifications commence.
Anticipated stability problems can then be addressed and resolved to prevent problems upstream and
downstream of the affected reach. A systematic approach to channel rehabilitation is presented in Chapter
2 of this manual. Placing environmental enhancements such as artificial structures in an unstable reach of
the channel can result in a total loss of the structures or inefficient or ineffective operation.
Efforts to reduce environmental impacts should be incorporated into the design of channel
enlargement projects. Consideration should be given to reproducing or improving the habitat diversity of
the existing stream, or preserving a part of the natural stream. In-stream diversity can be improved in post-
construction channels by use of artificial structures. The purpose of artificial structures is to restore habitat
and habitat diversity conducive to the growth and re-population of desirable species. In enlarged channels
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