Channelization and Channel Modification Activities and Impacts
4.2.2
CHANNEL ENLARGEMENT
Channel enlargement activities are generally implemented when a larger increase in channel flow
capacity is required. Snagging and clearing operations are undertaken when decreasing flow resistance can
achieve the desired effect on flow capacity. Channel clean out involves changing the channel width, depth,
or both to support both flood control and navigation efforts. In small non-navigable streams, the channel
is generally accessed from the bank, with dragline operations used to increase channel width and depth.
For navigable streams or rivers, a floating dredge plant, either hydraulic or mechanical, is used. The degree
of excavation can range from removal of a few shoals to an order of magnitude change in channel geometry.
The design of the new channel geometry is based on the desired flow rates, sediment transport
characteristics, and bank stability.
4.2.2.1 Hydraulic Effects
Channel enlargement operations result in a significant change in flow capacity, and potentially
impact channel stability. These channel modifications typically increase the cross sectional area (channel
width and depth) and decrease channel roughness due to removal of debris and vegetation, thus increasing
flow capacity. The concept of Lanes Balance presented earlier in chapter 3 indicates that for equilibrium
the supply of sediment must equal the flow capacity. For high flow events, the probable result of increasing
flowrate with the same sized sediment in the channel is a degradation of the bed and increased bank
erosion. Both upstream and downstream reaches are affected. The increased velocity in the enlarged
reach will result in scour from the bed and banks upstream, with the sediment delivered and deposited
downstream. For nominal flows that characterize the majority of the flow events, widening the river results
in an over-designed channel with an increased flow area. This results in reduced velocities, thus decreasing
the sediment transport capacity that results in sediment deposition. In severe cases of over-widened
streams, channel bars or braided flow can occur at low discharges (Brookes, 1988). Deepening the
channel can lower tributary base levels, thus increasing tributary slopes. According to Lane's Balance, if
the slope is steepened, the sediment transport rate must increase for stability. This results in an upstream
migration of degradation of the channel bed often referred to as headcutting. Material excavated from the
channel and associated banks during cleanout operations can be used to build berms along the banks for
additional flood protection, but may further confine flows, thus exacerbating stability problems.
4.2.2.2 Environmental Effects
Like snagging and clearing, removing material from the banks and the channel decreases habitat
diversity, thus negatively impacting the aquatic community. Typically, an enlarged channel will have a
uniform cross section, which destroys pools and riffles associated with natural channels. The associated
loss of habitat diversity can manifest itself by a reduction in species diversity or composition, a reduction
in size, distribution, and condition of the population, or unnatural seasonal variations in populations (Gorman
and Karr, 1978). The uniform geometry along with the banks denuded of vegetation gives the appearance
of a uniform, linear ditch that has very little aesthetic value. When channel clean out operations are
conducted from the bank, riparian vegetation can be damaged or removed that reduces habitats and
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