Selection and Design of Channel Rehabilitation Methods
Grade control structures can provide direct environmental benefits to a stream. Cooper and Knight
(1987) conducted a study of fisheries resources below natural scour holes and man-made pools below
grade control structures in north Mississippi. They concluded that although there was greater species
diversity in the natural pools, there was increased growth of game fish and a larger percentage of
harvestable-size fish in the man-made pools. They also observed that the man-made pools provided
greater stability of reproductive habitat. Shields et al. (1990) reported that the physical aquatic habitat
diversity was higher in stabilized reaches of Twentymile Creek, Mississippi than in reaches without grade
control structures. They attributed the higher diversity values to the scour holes and low-flow channels
created by the grade control structures. The use of grade control structures as environmental features is
not limited to the low-gradient sand bed streams of the southeastern United States. Jackson (1974)
documented the use of gabion grade control structures to stabilize a high-gradient trout stream in New
York. She observed that following construction of a series of bed sills, there was a significant increase in
the density of trout. The increase in trout density was attributed to the accumulation of gravel between the
sills which improved the spawning habitat for various species of trout.
Perhaps the most serious negative environmental impact of grade control structures is the
obstruction to fish passage. In some cases, particularly when drop heights are small, fish are able to migrate
upstream past a structure during high flows (Cooper and Knight, 1987). However, in situations where
structures are impassable, and where the migration of fish is an important concern, openings, fish ladders,
or other passageways must be incorporated into the design of the structure to address the fish movement
problems (Nunnally and Shields, 1985). The various methods of accomplishing fish movement through
structures are not discussed here. Interested readers are referred to Nunnally and Shields (1985), Clay
(1961), and Smith (1985) for a more detailed discussion.
The environmental aspects of the project must be an integral component of the design process when
siting grade control structures. A detailed study of all environmental features in the project area should be
conducted early in the design process. This will allow these factors to be incorporated into the initial plan
rather than having to make costly and often less environmentally effective last minute modifications to the
final design. Unfortunately, there is very little published guidance concerning the incorporation of
environmental features into the design of grade control structures. One source of useful information can
be found in the following technical reports published by the Environmental Laboratory of the Corps of
Engineers, WES (Shields and Palermo, 1982; Henderson and Shields, 1984; and Nunnally and Shields,
1985).
6.2.3
SUMMARY
The effectiveness of grade controls in stabilizing incised channels has been documented based on
results from the DEC watersheds. Effectiveness in improving stream ecology has also been briefly
documented through references. Additional benefits in reducing damage to infrastructure, improving flood
control and other factors is discussed. All of these benefits require careful design, construction, and
maintenance. However, the most important factor in enhanced grade control effectiveness may be the initial
planning and goal identification for the incised channel rehabilitation project.
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