Channelization and Channel Modification Activities and Impacts
4.2.4.1 Hydraulic Effects
Continuous dredging causes a river bed to degrade until the balance between sediment load
supplied to the river reach and the sediment transport capacity is restored (Brookes, 1988). Deepening
the river channel will lower tributary base levels, thus increasing tributary slopes. Channel instability within
the tributary will result in degradation of the channel bed, increased sediment transport, and ultimately
deposition of sediment within the river. Channel deepening also reduces the sediment transport capability
of the river, thus deepened sections act as sediment traps and encourage sediment deposition. A study
reported by Griggs and Paris (1982) described increased sediment deposition due to channel deepening.
Within 10 years of completion of the U.S. Army Corps of Engineers flood channel on the San Lorenzo
River at Santa Cruz in California, 350,000 cubic meters of sediment had been deposited. This reduced
the carrying capacity of the river from the designated 100-year flood to a 25-30 year flood. The channel
had been deepened by some 0.9 to 2.1 meters below the original bed elevation.
Mining operations that remove sand and gravel from the channel bed result in a localized lowering
of the bed. This has the effect of increasing the slope upstream of the mining operation that in turn increases
the sediment transport capability of the river. Bed degradation advances upstream with sediment
aggradation occurring downstream. If sand and gravel mining is performed at many locations along a river,
the rate of sediment removed may exceed the rate of replenishment. This can result in a significant lowering
of the bed that increases the potential for undermining foundations and bridge piers during major floods.
Frequent mining operations can also remove the coarser fractions of sediment that are important for
armoring the bed and stabilizing the banks along the river. From Lane's balance described earlier, a
reduction of sediment grain size can result in degradation as the channel flattens the slope in order to satisfy
the increased transport requirements.
4.2.4.2 Environmental Effects
Dredging operations may increase turbidity at the point of dredging. Suspended sediment plumes
can migrate to sensitive areas such as fish and shell fish spawning grounds. Generally, hydraulic dredging
operations with plain suction intakes operating in coarse sediment environment produce very little turbidity.
Cutterhead dredging operations do tend to resuspend sediments around the rotating cutterhead, particularly
when working in fine sediments. The turbidity generated can be minimized by reducing the speed of the
cutterhead and the swing rate of the dredge ladder (suction line). Mechanical dredges have the highest
probability of re-suspending sediments. Sediments are resuspended by leaking buckets and through the
uplift of sediments from the excavation area when the bucket or dipper is raised. Environmental dredge
buckets are available that have a positive pressure seal to prevent leakage.
The major impact of dredging on biological communities is the removal and subsequent changing
of the substrate. For maintenance dredging in major river systems that have a continually moving and
shifting bed, this is a minor concern. For new work dredging in channels that have historically had a stable
substrate, the impacts can be severe and permanent. Not only is the substrate removed, the deepening will
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