banks depending upon where the flow re-enters the channel. Some means of controlling the
overbank return flows must be incorporated into the structure design. One method is simply
to design the structure to be submerged below the top bank elevation, thereby reducing the
potential for a head differential to develop across the structure during overbank flows. If the
structures exerts hydraulic control throughout a wider range of flows including overbank,
then a more direct means of controlling the overbank return flows must be provided. One
method is to ensure that all flows pass only through the structure. This may be accomplished
by building an earthen dike or berm extending from the structure to the valley walls which
prevents any overbank flows from passing around the structure (Forsythe, 1985). Another
means of controlling overbank flows is to provide an auxiliary high flow structure which will
pass the overbank flows to a specified downstream location where the flows can re-enter the
channel without causing significant damage (Hite and Pickering, 1982).
12.2.4 ENVIRONMENTAL CONSIDERATIONS
The key phrase in water resources management today is "sustainable development"
which simply means that projects must work in harmony with the natural system to meet the
needs of the present without compromising the ability of future generations to meet their
needs. Engineers and geomorphologist are responding to this challenge by trying to develop
new and innovative methods for incorporating environmental features into channel projects.
The final siting of a grade control structure is often modified to minimize adverse
environmental impacts to the system.
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