Channelization and Channel Modification Activities and Impacts
In some cases, it may be justified to restore the former sinuosity to the modified stream. This action
is taken assuming that the engineering function for which the channel was originally modified is either no
longer required or will not be impacted, and that no major watershed changes have occurred since initial
straightening that would disrupt the equilibrium of the restored channel. In Southern Denmark, a new
channel was constructed to replace an 800 meter section of severely degraded channel (Brookes, 1987).
The original sinuosity was determined from historical maps, comparison of other neighboring streams, and
field reconnaissance of the watershed. Native grasses and woody vegetation were planted for stabilization,
with riprap used for bend stabilization before vegetation became established. The new sinuous channel
restored morphologic and hydrologic diversity, with colonization by a number of flora and fauna. In West
Germany, Glitz (1983) described the restoration of the sinuosity of the Wandse river in Hamburg-Rahlstedt,
a lowland river about 1.5 meters in width. A partial restoration was performed assuming that the stream
would eventually adjust naturally. A survey conducted two years later indicated that pool and riffle
formations were limited, probably due to the low energy of the stream.
Management practices may be implemented to preserve the morphological and ecological aspects
of the channel without modifying the existing channel to accomplish engineering goals. The concept of
floodplain corridors provides sufficient land area on both sides of the stream to allow for natural migration
of bends and general channel shifting across the floodplain. This allows the natural formation of habitat
enhancement features such as pools, riffles, and point bars. Future watershed planning and management
activities are possible with the channel confined to a fixed position on the floodplain.
4.2.11.1 Hydraulic Effects
The use of channel restoration techniques to enhance stream ecology is growing. Many of the
restorative methods have a limited influence on hydraulics of the channel. The use of artificial structures and
sills to create a pool and riffle habitat do not have a significant impact on stream hydraulics, particularly at
high flows for which the structures are inundated and no longer function as intended (Brookes, 1988).
However, the use of in-channel vegetation can significantly increase the roughness and consequently reduce
the discharge capacity of the stream. Wilson (1973) determined that vegetation such as willows and shrubs
can reduce the discharge capacity up to 50 percent after only one year of growth. The use of vegetation
within stream channels for purposes such as restoration or bank protection requires a thorough hydraulic
and sediment transport analysis during the project design phase. Low energy stream systems with
moderate flows and low sediment transport are more amenable to vegetative projects. The survival of
vegetation in high energy channels with high peak flows and substantial sediment transport is questionable.
To insure a successful project, a multi-disciplined team consisting of biologists and hydraulic engineers is
recommended.
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