9670

• UCREFRP Catalog Number: 9670
• Author: Pitlick, J.
• Title: Channel Monitoring To Evaluate Geomorphic Changes On The Main Stem Of The Colorado River.
• USFW Year: n.d.
• USFW - Doc Type: Boulder, CO.
• Copyright Material: NO

To examine the importance of spatial variations in grain size, the modeled estimates of r*
were re-calculated using a "local" grain size for each cross section. The local grain size was
determined by taking the average of several values, centered around the section. Calculations for
higher flows were based on samples from the thalweg as well as exposed bar surfaces, since
these areas would both be under water. Calculations for lower flows were based only on samples
from the thalweg. The effect of using spatially variable grain sizes in the model is to reduce the
estimates of r* slightly, as shown in Figure 27. The inclusion of coarser sediment in certain
areas of the channel has the most noticeable effect on flow conditions in the riffle, and then
mostly only in the intermediate range of flows, 125-224 m3/s. At those flows, the shear stress
through the riffle is quite high because the energy slope is high; however, when the shear stress
produced by those flows is balanced against the coarser bed grain sizes, the modeled values of r*
through the riffle decrease substantially (Fig. 27). The net effect of using spatially variable (and
generally coarser) grain sizes is to reduce the potential for bed load transport at flows much less
than about 300 m3/s, thus that value is retained as the threshold discharge for initial motion.

0.04
N
0.03

m
O
z
y 0.02

C
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0.01
E
0

0.00
0 100 200 300 400 500
Discharge (m3/s)
Figure 27. Relation between dimensionless shear stress and discharge, RM 176,
after adjusting for spatial variations in grain size.
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• R2 = 0.63
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