WSP05649

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 2/1/2000
• Author: Topping et al.
• Title: Colorado River Sediment Transport
• Water Supply Pro - Doc Type: Report/Study

568

TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 2

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Figure 18. Model-predicted changes in suspended-sediment concentration and grain size as a function of
coarsening the grain-size distribution of the fine sediment on the bed. For these calculations, equations (2),
(3), and (4) were solved for Dow over a planar bed; the depth was held constant at 5 m, and the shear velocity
was held constant at 5 cm/s (typical values based on measurements made at the Grand Canyon gage); the
sorting of the fine sediment on the bed was held constant at 0'., = 0.55 (a typical value for the fine sediment
on the bed of the Colorado River); the grain-size distribution of the fme sediment on the bed was divided into
97 1/16-<1> size classes; the settling velocity of each size class was determined by the method of Dietrich [1982]
for sediment with a Power> index of 3.0 and a Corey shape factor of 0.7 (typical values for Colorado River
sand); the water temperature was set at I50C. (a) Depth-averaged suspended-sediment concentration as a
function of the median size of the fine sediment on the bed for two cases: a finer-bed case with an initial bed
D,o = 0.1 mm and a coar>er-bed case with an initial bed Dso = 0.3 mm. Values in Figures I8a and 18b are
nondimensionalized by [he initial values. For either the finer- or coarser-bed case, as the fine sediment on the
bed coarsens, the suspended-sediment concentrations decrease nonlinearly and substantially. (b) Depth-
averaged median size of the suspended sediment as a function of the median size of the fine sediment on the
bed for the same cases as in Figure 18a. For either case, as the fine sediment on the bed coarsens, the median
size of the suspended sediment coarsens.

ically much finer than that on the bed of the Colorado River
(Dso - 0.3-0.7 mm), substantial changes in the grain size of
the bed occur as a sediment wave migrates downstream. As the
fine front of a sediment wave reaches a given location and the
upstream supply of sediment becomes enhanced, the concen-
tration of the finer grain sizes in suspension will be higher than
that which can be supported by the grain-size distribution of
the bed downstream. This produces a mass transfer of the
finest sizes from the suspended sediment to the bed, resulting
in fining of the bed. This was observed at all measurement

locations following the 1983 Little Colorado River flood, at the
Lower Marble Canyon gage following the 1997 Paria River and
ungaged tributary Doods, and on the bed in the upper 80 kIn of
Marble Canyon following the 1998 Paria River floods. As the
front of a sediment wave passes a given location and the up-
stream supply of sediment becomes depleted, the concentra-
tion of the finer grain sizes in suspension will decrease to be
lower than that which can be supported by the now finer
grain-size distribution of the bed. This produces a mass trans-
fer of the finest sizes from the bed back to the suspended