Channelization and Channel Modification Activities and Impacts
proportionate reduction in flow, then the bed and banks of the main stream may erode and enlarge the
crossection.
4.2
CHANNEL MODIFICATION ACTIVITIES AND ASSOCIATED IMPACTS
The previous section described the general types of channel modification projects and related
impacts. This section will address specific activities that are implemented to achieve project goals or
address project environmental concerns. As previously described, activities that alter channel geometry
may create stability problems. Each of the following activities is implemented to achieve some level of
benefit to the surrounding community or region. Both the benefits and adverse impacts for each activity
are described.
4.2.1
SNAGGING AND CLEARING
Snagging and clearing activities are implemented to increase discharge capacity of channels for
flood control and drainage purposes and to prevent hazards to navigation or bridges. The increased flow
resistance due to the presence of vegetation and debris may increase the frequency and duration of
overbank flows. The goal of the practice is to remove sufficient vegetation, debris, logs, sediment
blockages, large rocks, and other obstructions from the channel and adjacent banks to decrease flow
resistance. These obstructions retard flow by reducing the effective cross-sectional area of the channel,
increasing the channel roughness, and trapping additional debris, particularly during high flows (Shields and
Palermo, 1982). Various methods are used for removing channel debris and obstructions.
For flood control on small streams, conventional practice has been to remove all obstructions from
the channel and to clear all significant vegetation within a specified width on both sides of the channel
(Nunnally and Shields, 1985). For small streams, clearing of the channel is accomplished with heavy
equipment such as bulldozers. On navigable streams, a floating plant may be utilized for the clearing
operation. Comprehensive guidelines and practices for removing obstructions from streams are presented
by the Stream Renovation Guidelines Committee, The Wildlife Society and American Fisheries Society
(1983). This guidance is intended to aid in correcting stream flow problems caused by obstructions in an
environmentally sound manner and to maintain natural stream characteristics.
4.2.1.1 Hydraulic Effects
The extent of the effect of clearing and snagging operations on channel discharge capacity is related
to the degree of blockage prior to clearing. Potential stability and sedimentation responses to clearing and
snagging are associated mainly with increased velocities, increased transport capacity, and with removal
of vegetation that may have acted locally as erosion protection. Effects on stability may be adverse in some
locations and beneficial in others. The qualitative effect on stability was demonstrated using Lanes Balance
as described in Chapter 3.
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