8274

• UCREFRP Catalog Number: 8274
• Author: Pitlick, J. and R. Cress.
• Title: Longitudinal Trends in Channel Characteristics of the Colorado River and Implications for Food-Web Dynamics.
• USFW Year: 2000.
• USFW - Doc Type: Recovery Implementation Program Project 48-C,
• Copyright Material: NO

Thresholds for Coarse Sediment Transport
Our estimates of discharges required to move framework gravel particles are arrived at through a
two-step process. The first step involves calibrating the hydraulic flow model in short reaches
where we have measurements of the channel geometry and corresponding water surface elevations
for a series of known discharges. These calculations provide verified values of the roughness
coefficient (Manning's n), and site-specific relations between discharge and dimensionless shear
stress. Counting the three sites discussed below, and the seven sites studied by Pitlick et al. (1999),
we have now developed discharge-shear stress relations for ten sites. The second step of the
process involves extrapolation of the results from individual sites to longer reaches. This is done
by using eqn. (7) along with field-verified values of n and measured values of width and depth at
each of the 149 main-channel cross sections.
To illustrate the basic elements and results of the modeling procedure, Figure 22 shows data from a
series of measurements in a 450-m reach near RKM 345. The lower line connecting the average
bed elevations shows that the reach is predominantly a run. The closely spaced lines at the top of
the figure show calculated water surface profiles for discharges of 300 and 282 m3/s; an additional
set of points corresponding to the latter discharge shows observed water surface elevations. The
agreement between observed and calculated water surface elevations for this flow is excellent, with a
maximum difference of only 6 cm. This fit was achieved using a Manning's n of 0.035. The same
value was used in modeling the water surface profile for 300 m3/s (about half of the bankfull
discharge), with the downstream water surface elevation estimated from a uniform-flow equation
(see Bailey and Ray, 1966), and upstream elevations obtained through the trial and error procedure
of the step-backwater method. The same steps were repeated for other discharges to give a series of
modeled water surface profiles. In the absence of field measurements, the one constraint that can be
used to check these results is that, at high flows, the modeled water surface slope should be roughly
the same as the reach average slope, which we estimate is 0.0021 on the basis of the GPS
measurements.
98.5
RKM 345
98.0
97.5-

.0 97.0
is
m
U1 96.5
96.0-
95.5
calc ws, 300 m3/ s
calc ws, 242 m3/ s
0 obs ws, 242 m3/ s
0 50 100 150 200 250 300 350 400
Distance (m)
Figure 22. Bed and water surface elevations in a reach near RKM 345.
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