Channelization and Channel Modification Activities and Impacts
4.2.3.3 Remedial Practices
The environmental impacts of channel realignment should be included in project design
considerations. An estimation and evaluation of the losses of aquatic and riparian habitat should be
considered if cutoffs will be formed during channel realignment. Flow should be maintained, if possible,
through the old meanders to prevent them from filling with sediment. The upstream migration of channel
degradation due to increased slopes resulting from shortening the channel is the most significant impact on
channel stability. It must be addressed before habitat restoration practices are applied. To mitigate bed
and bank erosion, grade control structures and bank stabilization techniques are implemented.
4.2.3.4 Operation and Maintenance of Channel Realignment Projects
Realigned or straightened channels should be periodically inspected for signs of instability. Grade
control and bank stabilization projects incorporated into the project should be inspected and maintained
to insure proper function.
4.2.4
DREDGING AND MINING
Dredging is the process by that sediments are removed from channels for the purpose of maintaining
existing navigation (maintenance dredging) or deepening existing channels for deep draft navigation (new
work dredging). Dredging is also utilized in bays and harbors located along rivers or at the river outlets that
continuously shoal with fine sediments. Additionally, dredging operations are used for mining sand and
gravel from rivers. Generally, two different types of dredging operations are used for riverine dredging.
Hydraulic dredging operations consist of a floating plant that removes and transports sediments from the
channel bed using large centrifugal pumps. The pump suction line extends to the channel bed where the
sediment is hydraulically entrained, passed through the pump, and discharged to disposal. Disposal areas
can either be within banks or located at inland confined sites. For loosely flowing coarse sediments, a plain
suction head is used to entrain the sediments. For more consolidated sediments, a rotating cutterhead is
employed to loosen the material and feed the suction line. In some riverine environments, hopper dredges
are used. The hopper dredges are deep-draft seagoing vessels used primarily for maintenance dredging
in harbors or river outlets. Hopper dredges make successive passes over the problem area, deepening
progressively on each pass. The pumped material is stored in hoppers in the dredge, and when fully
loaded, the dredge travels to a designated dump site in the ocean. It is only effective for dredging loose,
unconsolidated material.
Mechanical dredging operations are generally conducted in shallow areas containing loose or
consolidated sediments. The operation involves excavating sediment with either a barge mounted power
shovel (dipper) or a clamshell bucket operation. Bucket capacities range from 1 to 12 cubic yards. The
material is excavated and loaded into an adjacent barge that is towed to disposal.
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