Channelization and Channel Modification Activities and Impacts
A gradual build-up of sediment occurs in the slack-water areas between dikes during high flows.
At low flows, the shoals may be out of water, thus allowing vegetation such as willows to colonize the area.
During high flow events, the increased vegetation effectively increases the roughness thus further
encouraging sediment deposition. This results in a decrease in aquatic habitat and an increase in terrestrial
habitat. Brookes (1988) reports a study by Morris et al. (1968) that reported that the construction of pile
dikes on the Missouri River in Nebraska reduced the width from 720 to 240 meters and reduced benthic
habitat by approximately 67 percent. Habitat diversity may be reduced by stabilizing the stream with dikes.
4.2.8.3 Remedial Measures
Although a reduction in sedimentation in dike fields can be achieved by varying the length and height
of dikes, constriction gaps or notches in dikes are presently the most widely used environmental restoration
method (Shields, 1983). Notched dikes are used to mitigate the loss of aquatic habitat due to
sedimentation on the downstream side of dikes. Stone is removed from the dike to a specific width and
depth to create a gap allowing flow to pass through the dike. The flow through the gap induces scour that
removes sediment deposits and restores aquatic habitat. The notch width, shape, and depth design can
varied to provide varying degrees of habitat restoration. Notch openings should be adequate to provide
the necessary effect of creating habitat without causing excessive erosion or deposition.
The Missouri River Division of the Corps of Engineers has used notched dikes to restore aquatic
habitat on the Missouri River (Shields and Palermo, 1982). Small gaps in the Missouri River dikes were
observed to produce small chutes and submerged bars behind the dikes, whereas large openings created
open-water habitat.
4.2.8.4 Operation and Maintenance of Dikes
Dikes, like other in-stream structures, require inspection to insure proper operation.
4.2.9
GRADE CONTROL
The most common method of establishing grade control is the construction of in-channel grade
control structures. There are basically two types of grade control structures. One type of structure is
designed to provide a hard point in the streambed that is capable of resisting the erosive forces of the
degradational zone. This is somewhat analogous to locally increasing the size of the bed material. Lanes's
relation would illustrate the situation by QS+ α Qs D50+ , where the increased slope (S+) of the
degradational reach would be offset be an increase in the bed material size (D50+). This is referred to as
a bed control structure. Sills are placed across the channel at or just above the bed elevation to control
scour. Materials such as concrete rubble, stone, or locally available non-erodible materials can be used.
The sill acts as a hard point in the channel that resists erosion, thus stabilizing the bed. Channels may be
completely stabilized by lining the channel with non-erodible material such as concrete or stone. This is a
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