Selection and Design of Channel Rehabilitation Methods
Concentration ' 164,104,428 ( S 1.73
(6.6)
with the units of concentration as mg/l, and S is the energy slope. The coefficient of determination (R2 ) is
0.89. As shown in Figure 6.16, the sediment concentration for S = 0.0009 is approximately 1000 mg/l,
while for S = 0.004 the concentration is approximately 10,000 mg/l. Figure 6.17 shows the slope-area
curve from GDM No. 54, and has values of sediment concentration taken from Figure 6.16 for selected
drainage areas. Therefore, using the slope-area curve for stable channel design would require the designer
to accept 712 mg/l at 90 square miles, 2,849 mg/l at 10 square miles, and extrapolating the relationship,
7,870 mg/l at 2 square miles. Therefore, investigation of an empirical spacing procedure has shown the
effectiveness of grade control structures in northern Mississippi.
6.2.2.1 Downstream Channel Response
Since grade control structures affect the sediment delivery to downstream reaches, it is necessary
to consider the potential impacts to the downstream channel when grade control structures are planned.
Bed control structures reduce the downstream sediment loading by preventing the erosion of the bed and
banks, while hydraulic control structures have the added effect of trapping sediments. The ultimate
response of the channel to the reduction in sediment supply will vary from site to site. In some instances
the effects of grade control structures on sediment loading may be so small that downstream degradational
problems may not be encountered. However, in some situations such as when a series of hydraulic control
structures is planned, the cumulative effects of sediment trapping may become significant. In these
instances, it may be necessary to modify the plan to reduce the amount of sediment being trapped or to
consider placing additional grade control structures in the downstream reach to protect against the induced
degradation. Therefore, following the hydraulic spacing of a series of grade control structures using a
thorough investigation of providing a balance between supply and transport of water and sediment, the
designer must utilize a long-term sediment routing model such as HEC-6 (Thomas, 1996), to investigate
downstream channel response.
An improved structure spacing procedure would be to select an energy slope based on the desired
sediment transport concentration. The sediment transport concentration of the CEM 5 reaches within the
DEC monitoring reaches can be used to select a design slope. Figure 6.18 provides a summary of the
sediment concentration for CEM types for 1993, 1994 and 1995; the line through the data is the average
for each CEM type. Figure 6.16 can be used to estimate the energy slope. The data indicates the design
slope for the CEM 5 concentration of 1,000 mg/l would be approximately 0.001, and the CEM 4
concentration of 2,000 mg/l would be 0.0014. Structures could be located using this range of bed slopes,
which would reduce sediment concentration below the existing average sediment transport. A check could
then be made comparing bed slope and energy slope, and adjustments could be made if required.
The proposed optional procedure has the limitation of depending on the present field identification
of CEM 4 and CEM 5 reaches. Just as with the GDM No. 54 slope-area curve, as the
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