9500

• UCREFRP Catalog Number: 9500
• Author: Pitlick, J. and R. Cress
• Title: Downstream Changes in the Channel Geometry of a Large Gravel Bed River
• USFW Year: 2002
• USFW - Doc Type: American Geophysical Union
• Copyright Material: YES

PITLICK AND CRESS: DOWNSTREAM CHANGES IN CHANNEL GEOMETRY 34 - 9
1
100 100
80 1 80
60 60
40 40
f * 0
20 -_L - - ----- 20
365 350 335 320
i ,
100 100
• c
? m
w --, D50 W
8r
° 10 t..._, 10 W
u?
300 280 260 240 220 200
en
70 ti . .. 79
50 50
30 t, • € + U 30 3
•
.
10 _ i 10
180 170 160 150- 144
100 • 100
w?
---- D50 ••
T ••
10._... 10
145 135 125 115 105 95
Distance (km)
Figure 9. Comparison of downstream trends in bank-full
shear stress and median grain size within individual
segments of the Colorado River. Parameters of the
regression equations for individual segments are given in
Table 4. Dotted lines indicate trends in D50, and solid lines
indicate trends in Tb; the third line in Figure 9d indicates the
trend with the two data points at rkm 139 and 136 removed.
channel slope. The relatively large increase in bank-full
depth is thus driven by a mass balance requirement of
carrying coarse sediment across a relatively low slope with
little additional flow. Ferguson and Ashworth [1991]
describe similar trends on the Allt Dubhaig, a small stream
in Scotland that undergoes systematic changes in channel
geometry with almost no change in discharge. The Allt
Dubhaig is characterized by a rapid decrease in slope and
grain size, and a modest increase in channel depth [Ferguson
and Ashworth, 1991]. One difference here is the increasing
abundance of fine sediment and bank vegetation, which help
constrict the channel and promote vertical accretion during
floods, similar to what Allred and Schmidt [1999] have
observed on the Green River in Utah. Limits to this process
are determined by bank stability and the resistance of the
channel to high shear stresses during floods. Fine and coarse
sediment in the Colorado River thus interact to form a
channel that generates sufficient shear stress to maintain
bed load transport without widening.
[26] Our observation that the width-depth ratio of the
Colorado River decreases downstream contrasts with results
from many other studies [Church, 1992, Knighton. 1998].
The differences in this case are most likely the result of a
small change in discharge, coupled with a small change in
total bed load flux. To support this point we compare data
from the Colorado River with a set of hydraulic geometry
relations formulated by G. Parker [personal communica-
tion]; for purposes of discussion we will say that Parker's
data set is representative of "typical" gravel bed rivers.
Using data from 62 gravel bed rivers in Canada, the UK and
the USA, Parker formulated the following dimensionless
hydraulic geometry relations:
B* = 4.9 Q* 0.46 (la)
H* = 0.37 Q*"' (lb)
S = 0.098 Q* -1.34 (Ic)
Tb = 0.049 (ld)
[27] The variables are defined as follows: B* = BID50 is
the dimensionless bank-full width; H* = HID50 is the
dimensionless bank-full depth, Q* = Q1( RgD50D20) is
the dimensionless bank-full discharge; Q is the bank-full
discharge-, and R is the submerged specific gravity of the
sediment, assumed to be .1.65.
[28] Figure 11 shows corresponding hydraulic geometry
relations for the Colorado River. In this case, values of Q
were calculated for individual cross sections using the
Manning equation for velocity and the measured cross
sectional area. Estimates of Manning's n were made on
the basis of flow modeling results, verified by field obser-
vations [Pitlick and Cress, 2000]. The resulting hydraulic
geometry equations are
B* = 39.3 Q* 0.32 (2a)
H* = 0.07 Q* 1.13 (2b)
S = 0.78 Q* _"' (2c)
Tb = 0.049 (2d)
[29] Comparison of the two sets of equations reveals.
some interesting differences and similarities. The exponent
in the width relation for the Colorado River is low in
comparison to the relation for "typical" gravel bed rivers,
and the exponents for depth and slope are correspondingly