8157

• UCREFRP Catalog Number: 8157
• Author: Pitlick, J., M. V. Steeter, B. Barkett, R. Cress and M. Franseen.
• Title: Geomorphology and Hydrology of the Colorado and Gunnison Rivers and Implications for Habitats Used by Endangered Fishes.
• USFW Year: 1999.
• USFW - Doc Type: \
• Copyright Material: NO

Table 2. General characteristics of flow modeling sites
average bankfull conditions surface (subsurface) sediment
River Mile width, depth, slope, D84, D50, D16,
(RM) m m m/m mm mm mm
------------------
184.2 ------------------
85 ------------------
2.89 -------------------
0.0024 ------------------
140 --------------
75 -----------
32
(120) (26) (1.4)
177.3 75 3.23 0.0020 100 48 24
(64) (30) (0.7)
166.0 126 3.27 0.0020 86 57 34
(64) (30) (1.4)
162.4 148 3.36 0.0015 105 55 27
(72) (16) (0.5)
159.0 148 2.86 0.0017 90 46 25
(n/a) (n/a) (n/a)
139.5 106 4.19 0.0012 90 50 25
(80) (25) (2.4)
134.0 137 3.22 0.0014 80 50 35

------------------
------------------
------------------
------------------- (80)
------------------ (48)
-------------- (16)
-----------
The key problem in estimating discharge thresholds for sediment transport and channel change is to
develop appropriate measures of the boundary shear stress, r, and the critical shear stress,'r, The
average boundary shear stress is given by
i = p g R Sf (1)
where p is the density of water, g is the gravitational acceleration, R is the hydraulic radius (which
in wide channels is very nearly equal to the flow depth, h), and Sf is the friction slope or energy
gradient. We used a series of observations over a range of flows to calibrate a one-dimensional
hydraulic model for each study site (we adapted the step-backwater modeling procedure outlined
by Henderson, 1966, to a spreadsheet program). The step-backwater model finds Sf at individual
stream channel cross sections using an iterative solution to the energy equation:
S f - dx - dx (u2 + h + z) (2)
g
where dH/dx is the gradient in total energy, u is the mean velocity, z is the average bed elevation,
and x is the downstream direction. The model results allow us to evaluate the boundary shear
stress and the roughness (Manning's n) for a range of discharges.
In the absence of direct observations of particle entrainment from tracer gravels or bed load
samples, the only practical means for estimating r, is to use the Shields' criterion:
i* - iC (3)
c - (R-P)gD
where z, is the critical dimensionless shear (Shields) stress, p, is the density of sediment, and D is
the particle diameter.
13