Channelization and Channel Modification Activities and Impacts
make the area more conducive for sedimentation, and thus periodic dredging will be required to maintain
project depth. A stable substrate will no longer be available thus the diversity and suitability of the habitat
will be reduced, with native aquatic organisms displaced. Turbidity generated by dredging operations can
impact nearby fish and shell fish spawning grounds and inhibit plant growth.
4.2.4.3 Remedial Practices
To protect adjacent sensitive areas such as spawning grounds or vegetation, restrictions can be
placed on dredge operations. Restrictions on dredge type and minimum turbidity generated can be
specified in dredging contracts to insure that environmentally sensitive areas are not impacted. Physical
barriers such as silt screens can be used to contain the suspended sediment plume to the immediate area
surrounding the dredge. Specialty dredges designed to minimize turbidity are available.
Dredging induced channel instability is similar to that resulting from channel enlargement and
realignment. Grade control structures and bank stabilization practices may be necessary to address bed
and bank erosion and ultimately stabilize affected reaches.
4.2.4.4 Operation and Maintenance of Dredging and Mining Projects
The river reaches that are maintained through dredging must be periodically surveyed to insure
navigable depth and width. The cost of dredging can be significant. At the mouth of the Mississippi river,
dredging is conducted year round. The cost of a large hydraulic dredge can cost more than
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,500 per
hour of operation. At low water or after a flood event, multiple dredges may be operating continuously
to insure safe navigation. Hydraulic dredges come in a variety of sizes for a variety of applications. Six inch
to eight inch diameter pipeline dredges are generally used for small waterways, canals, or lakes and
reservoirs, and are limited in productivity and power. Small hydraulic dredges can generally be transported
to the site by overland transportation. Costs of dredging include mobilization and de-mobilization, disposal
site creationandpreparation, and general operating expenses.
4.2.5
CONSTRUCTION OF LEVEES
Levees fall into the general category of embankments. Embankments, also known as flood banks,
levees, bunds or stopbanks (Brookes, 1988), are constructed to artificially increase the capacity of a
channel to confine high flows that otherwise would overtop the banks and spread over the floodplain.
Some of the largest river systems in the world have extensive levees. Levees extend more than 1,000 km
along the Nile River and 1,400 km on the Red River in Vietnam. In the United States, levees are key
components of a basin wide a flood control plan implemented to protect communities and agricultural areas
within the floodplain. Levees are used in conjunction with reservoirs, floodways, control structures, and
various channel modification activities to reduce and control the extent and duration of flooding.
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