Channelization and Channel Modification Activities and Impacts
more expensive alternative, but it may be necessary in urban areas where land costs are high, thus narrow
channels with steep side slopes are desirable.
The second type of grade control structure is designed to function by reducing the energy slope
along the degradational zone to the point that the stream is no longer capable of scouring the bed (QS- α
Qs D50 ), which requires establishing a hydraulic control at the structure. Examples of hydraulic control
structures are weirs and drop structures. Weirs are placed across the channel to control the water level
thus controlling the stream energy gradient. For large discharges or significant changes in bed elevation,
drop structures are employed. Drop structures are designed to limit and stabilize channel bed slope by
means of a vertical drop.
4.2.9.1 Hydraulic Effects
The function of hydraulic grade control structures is to reduce the energy slope along the
degradational zone, thus reducing the ability of the river to scour the bed. This results in a backwater above
the structure and a subsequent lowering of the velocity. These areas typically are more conducive to
sedimentation, thus the affected reach is transformed from degradational (erosive) to aggradational
(depositional). This sediment trapping affect along with the desired affect of reducing bed erosion will
deprive downstream reaches of sediment, thus possible affecting downstream stability. Grade control
structures can affect the flood potential of the stream. Hydraulic grade control structures are often designed
to be hydraulically submerged at flows less than bankfull so that the frequency of overbank flooding is not
affected. However, if the structure exerts control through a wider range of flows including overbank, then
the frequency and duration of overbank flows may be impacted. Another factor that must be considered
when siting grade control structures is the safe return of overbank flows into the channel. This is particularly
a problem when the flows are out of bank upstream of the structure but still within bank downstream. The
resulting head differential can cause damage to the structure as well as severe erosion of the channel banks
depending upon where the flow re-enters the channel.
4.2.9.2 Environmental Effects
Grade control structures can provide direct environmental benefits to a stream. A study was
conducted by Cooper and Knight (1987) on fisheries resources below natural scour holes and man-made
pools below grade control structures in north Mississippi. The study results conclude that although there
was a greater species diversity in natural pools, there was increased growth of game fish and a larger
percentage of harvestable-size fish in the man-made pools. 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. 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 most serious negative environmental
impact of grade control structures is the obstruction to fish passage. In cases where drop heights are small,
fish are able to migrate upstream past a structure during high flows (Cooper and Knight, 1987). However,
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