Channelization and Channel Modification Activities and Impacts
4.2.6.2 Environmental Effects
It is environmentally beneficial to use diversion channels as an alternative to modifying the main
channel to convey flood flows. The original stream substrate and meanders are maintained, as well as in-
stream cover and riparian vegetation. If it is designed only for periodic flood flows, the diversion channel
can have multiple benefits such as an urban greenbelt, recreation, pasture for grazing, and a wildlife food
source (Little, 1973). If the invert of the diversion channel is too low, it will convey both low and high
flows, thus continually staying wet. This will inhibit grass growth and increase the possibility of erosion of
the substrate. If adjacent low-lying areas or old abandoned river courses are used for diversion purposes,
some terrestrial habitat may be lost or converted to a wetland habitat.
4.2.6.3 Remedial Practices
The diversion system must be carefully designed and constructed to prevent channel instability in
the main channel and the diversion channel. Channel design must take into account the design flows and
sediment transport to insure bed and bank stability. The hydraulic design of diversion channels can be
accomplished with standard hydrology and hydraulics analysis techniques, while determinations of sediment
transport through the diversion are much more difficult. Because the floodway invert is higher than that of
the main channel, there is a tendency for the channel to become unstable and degrade. Grade control
structures may be necessary on the downstream end of the floodway to prevent upstream migration of bed
degradation, and on any perched tributaries that are hydraulically connected to the diversion channel.
4.2.6.4 Operation and Maintenance of Floodway Projects
Diversion channels that have a seasonal covering of grass will require maintenance, and should be
designed with sideslopes conducive to mechanical mowing. Efforts should be taken to insure that the
channel invert is constructed above the seasonal high water table to prevent excessive growth of aquatic
vegetation that interferes with maintenance.
4.2.7
DAMS
Impoundments are constructed for multiple uses. In canalization projects, dams are constructed
along with locks for navigation purposes. Dams and associated reservoirs are built on rivers primarily for
flood control, with secondary functions such as recreation, water supply, and power generation. Sediment
retention dams are utilized as flow control to reduce sediment loading to downstream areas (USACE,
GDM-54, 1990a). One or more dams are constructed in the upper watershed to trap sediments and thus
reduce bed material load downstream. Additionally, dams reduce the sediment load by changing the flow
duration curve for the stream. Controlled releases through the dam reduce the flood peaks and
subsequently reduce the sediment load downstream. Peterson (1986) describes the social and
environmental impacts of dams on a number of river basin projects. The beneficial uses for which a dollar
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