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|Title:||Environmental features for flood-control channels|
|Authors:||Environmental and Water Quality Operational Studies (U.S.)|
Shields, F. Douglas.
|Publisher:||Environmental Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: This report presents and documents preliminary findings of an information review performed to develop environmental guidance for flood-control projects that involve modification of natural stream channels by clearing and snagging, alignment, enlargement, and lining. The response of the fluvial system to modification sometimes results in unintentional or unforeseen environmental impacts. In general, channel modification results in a shorter, smoother, more uniform channel with larger cross-sectional area and less natural vegetation. Overbank flooding is eliminated or reduced, and depths and velocities are changed at all flows. Since extreme channel instability has adverse effects on ecological and aesthetic resources, channel straightening should be minimized. The channel cross section should be designed for low as well as high flows; the existing velocity-versus-discharge relationship should be preserved as much as possible at low and intermediate flows to maintain the sediment transport characteristics of the existing channel. Environmental features have been found to have limited effectiveness unless the modified channel is reasonably stable, the project area is protected against further modification, and construction and maintenance work is closely supervised and inspected. The adverse environmental impacts of channel enlargement can be reduced by following the existing channel alignment and excavating from one side only. Floodways may be used to preserve portions of the existing channel and its associated aquatic habitat. Low-flow channels may be constructed inside a larger channel, or the existing channel may be preserved as a low-flow channel. Pools and riffles may be constructed. Water control structures may be placed in the channel to maintain water levels for aquatic habitat and aesthetics and to prevent invasion and blockage. Meander loops may be maintained as small ponds or wetlands. Many of the adverse impacts of channel work can be avoided by preservation of existing valuable vegetation and by prompt revegetation with appropriate species. Aquatic habitat diversity may be restored to a modified channel by placing simple habitat structures in the channel to create vertical relief, nonuniform flow patterns, and stable substrate; these devices should be used with care since they increase hydraulic roughness and tend to offset flood control measures. Biological recovery of some modified streams may be improved by armoring the new channel with biologically desirable coarse bed material. Adverse impacts of channel lining or paving may be addressed by incorporating natural materials such as boulders in the lining, by ponding water in the lined channel, and by constructing low-flow channels, fishways, pools, and spawning channels. Modification of stream channels in urban areas frequently offers many ppportunities for recreational development and beautification.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Technical Report|
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|TR-E-82-7.pdf||25.29 MB||Adobe PDF|