8267

• UCREFRP Catalog Number: 8267
• Author: Elliott, J. G. and L. A. Hammack.
• Title: Geomorphic and Sedimentologic Characteristics of Alluvial Reaches in the Black Canyon of the Gunnison National Monument, Colorado.
• USFW Year: 1999.
• USFW - Doc Type: Water-Resources Investigations Report 99-4082,
• Copyright Material: NO

Table 4. Sediment-size statistics and critical shear stress for the median particle size, Red Rock Canyon study reach
[Station, sample location on cross section from left endpoint; Percentile, percentile of the size distribution; lc, sampled across the entire ]ow-flow channel;
Critical shear for the median sediment size calculated with Equation 2; Shields pazameter is variable depending on pavement and subsurface properties;
ft, foot; mm, millimeters; ]b/ftZ, pound per square foot; DF, debris flow)
Critical Critical
shear shear
16th 50th 84th Maximum with with
Cross Sample Sample Station Percen- Percen- Percen- particle Shields Shields Remarks
section year code (ft) the the the size param- param-
(mm) (mm) (mm) (mm) eter eter
0.030 0.045
(Ib/ft2) (Ib/ftz)
F2 1994 RBank 85 43 165 500 1,000 1.67 2.51 DF-2 deposit neaz
distal fan mazgin.
Fl 1994 RBank 78 43 165 500 1,000 1.67 2.51 DF-2 deposit near
distal fan margin.
H1 1994 RBank 90 13 50 174 500 0.51 0.76 DF-3 deposit neaz
distal fan mazgin.
Hl 1994 Fan 105 11 33 109 320 0.33 0.50 DF-3 deposit on fan.
H2 1994 RB1dBaz 181 79 320 450 650 3.24 4.87 Reworked boulders
from debris fan.
Gl 1994 XChan lc 11 64 168 600 0.65 0.97 Sampled from
station 52 to 210.
G1 1994 LB1dBaz 45 2 100 250 1,050 1.01 1.52 Sand covers boulders
many places.
G1 1994 LGv1Baz 79 16 36 76 120 0.36 0.55 Well sorted gravels.
G2 1994 XChan lc 51 90 250 900 0.91 1.37 Sampled from
station 38 to 142.
approximated by the relation between boundary shear
stress, flow depth, and energy gradient, given by the
duBoys equation (Chow, 1959, p. 168):
do=yD S (1)
where 1;o is the mean boundary shear stress, in pounds
per square foot;
y is the specific weight of water (62.4 pounds
per cubic foot);
D is the mean flow depth, in feet; and
S is the energy gradient, in foot per foot, for a
specific discharge.
Assumptions for using equation 1 are (1) the channel
cross section has a regular, or trapezoidal, shape and
width at least 10 times greater than its depth, (2)
streamflow is steady (there is a continuity of discharge
from cross section to cross section in the reach), and
(3) streamflow is uniform (velocity is constant in both
magnitude and direction through the reach). Applica-
tion of equation 1 is inappropriate in channel sections
where there is a strong lateral variation in acceleration
or where abrupt, local changes in streambed gradient
occur. Cross sections in this study were not trape-
zoidal, although all had single-thread channels at less
than flood discharges; all had width at least 20 times
mean flow depth, and many were approximately
symmetrical at less than flood discharges. streamflow
in the Warner Point and Red Rock Canyon study
reaches was assumed to be steady; there were no
significant local inflows to either reach, and infiltration
losses were assumed to be insignificant.
Most natural streams do not completely satisfy
the assumptions for equation 1, and the boundary
shear stress associated with any specific discharge is
i6 Geomorphic and Sedimentologic Characteristics of Alluvial Reaches in the Black Canyon of the Gunnison National Monument,
Colorado