WSP07063

• File Number: 8220.101.17
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 2/1/1986
• Author: Rubin/et al.
• Title: Relation of Inversely Graded Deposits to Suspend-Sediment Grain-Size Evolution During the 1996 Flood Experiments in Grand Canyon
• Water Supply Pro - Doc Type: Report/Study

age between the size disuibution of suspended
sediment in the eddy and main I.:hannel.

DISCUSSION AND CONCLUSIONS
In...erse grading of flood deposits has been
described by Osterkamp and Costa (1987) and
studied by lseya (1989). who attributed formation
of inversely graded beds 10 changing sediment
supply during floods in Japan~se rivers. Iseya
found thaI basal muddy deposils formed during
me beginning of a flood when the concentration
of silt and clay was high. These deposits were
overlain by inversely graded sands that also
formed early during a 1l00d (while discharge was
still increasing). (seya proposed that the increase
in grain size during the flood was caused by a
decrease in concentration of fine suspended sedi.
ment. in addition 10 an increase in flow intensity
over the flood plain. As in the Japanese rivers,
deposits of the 1996 tlood in Grand Canyon
e:lthibit upward coarsening and at some siles
exhibit replacement of ripples by dunes; we
hypothesize mat the cause of both changes was
deplerion of tines on the bed in the main channel.
As fines were winnowed from the bed., the bed
sediment coarsened, which caused the mean size
of sediment supplied to the channel, eddies, and
channel-margin deposits to coarsen. In the main
channel, the coupling between the c~ning of
the bed sedimenl and suspended sediment can be
e;ltplained mechanistically in the following
manner. For a sandy bed. the overall concentra-
tion of suspended sediment near the bed scales
approximately with the shear stress in excess of
the critical value for initiation of motion for the
median grain size of the bed material. whereas the
concentration of each size fraction scales approxi-
mately linearly with the fraction of thai size class
in the bed material (e.g.. Smith and McLean
1977; Mclean. 1992; Topping. 1997). Thus. the
ncar-bed overall concenu-ation of suspended sedi-
ment will decrease slightly as the median grain
size increases from 0.3 mm to 0.4 rom, but the
concenlration of the coarser fractions will in-
crease as a resuh of higher represemalion in the
bed material. as shown in Figure 4 (A and B). In
other words, as finer malerials are winnowed
from the bed. the median size in the bed coarsens
slighlly and the total concentration in suspension
drops. but the concentrations of the coarsest frnc-
tions in suspension acrually increase as they
become more common in the bed. Although our
observations can be e)lp/ained solely in tenns of
the winnowing process described abo...e. we can
not rule out the possibility that upward-coarsening
may have been enhanced by other processes-
e.g.. grain-size segregation by differing tranSpoll
rates (Hand, 1997) into the eddies, or by excava-
tion of coarser grains from underlying deposits.

The JocaHy observed change from ripples 10
dunes (a depositional sequence of climbing
ripples overlain by crossbedding) was caused by
the increase in mean grain size of sediment sup-
plied to the eddies. The conditions causing this
change can be envisioned as a line representing a
constant velocity and an increasing grain size on
a plm showing bed phase as a function of grain
size and velocity (Southard, J 971; Rubin and
McCulloch. 1980; Southard and Boguchwal.
J990).]n the rock record, observations of upward
coarsening and change in bed configuration from
ripples to dunes are typically interpreted to indi-
cate stronger flows. but this is not necessarily the
case. In the 1996 1l00d. peak discharge was held
conSlant: coarsening of lhe bed caused [he
change in bed configurntion.
Our observations indicate that limitations in
sediment supply can drive changes in main-
channel concentration and grain size. and thereby
playa dominant role in the fonnalion and stratig-
raphy of eddy deposils. Accordingly. dam-release
scenarios for the purpose of sand-bar mainte-
nance must consider the effect of decreasing
main-channel concentrations on deposition rntes
in eddies and on transport of sediment through
the canyon. The rate of decrease ofmain-channel
concenmuion and the associated grain-size evo-
lution during a given flood will depend on the
timing of previous high Ilows and tributary inpuls
of sediment; this complexity mandales further
investigation and monitoring of tributary inputs
and subsequent main-channel tranSport as a func~
tion of grain size. It may be possible to exploit the
bed-coarsening process and to manipulate dam
releases in such a way as to maximize transport
into eddies, while minimizing sediment transport
out of the canyon. Although the dam enhanced
the degree of supply limitation. grain-size evolu-
tion is a namra! process that also occurred during
predam tloads.

ACKNOWLEDGMENTS
The u.s. Bureau of Reclamation provided suppon
and equipment: the National Park. Service prOVIded
houslOg. J. Bennett C. Crouch. T. Hop~on. 1. Lyons.
R. Shreve. and D. Schoellhamer helped with sediment
sampling. R. Anima. F. Iseya. M. RubIO. J. SchmIdt.
M. Smelser. and R. Stanley helped wilh trenching and
~pling. F. lseya contributed stimulaling discussions
about inverse gradmg in Japanese n\'ers. P. Kinzel III
conducled most of the laooralory analyses of sediment
concenlration and grain-size. The manuscripl was
re\.iewed by P. Half. R Hunter. C. Paola. and 1. Xu.

REFERENCES CITED
Andrews. E. D.. 1990. TIle Colorado River. a perspective
from Lees Ferry. Arizona. in WoJman. M. G.. and
Rigg"i. H. C..eds.. Surface water hydrology: Boul
der. Colorado. Geologi~al Soclely of Amenc;).
Geology of Nonh America. v. 0-1. p. 304-) 10.

'.1 C,

Collier. M. P.. Webb. R. H.. and Andrews. E. D.. 1997.
Experimental flooding in Grand Canyon: Scien-
ufic American. .... 276. p. 66--73.
Edwards. T. K.. and G1l"iSOO. G. D.. 1988. Field_
for measuremenl of Ou\'ial sediment: U.S. Gee--
loeicSurv<yOpen-F>leRepon86-53J.118p.
Guy. H. P.. 1969. Laboralory theory and methods for
sedimenl analysis: U.s. Geological Suno:y Tech.
niquesofWater-Re:soun:es InvestIgations. Book 5.
Chapler C 1.58 p.
Hand. B. M.. 1997. In\lt'rse grading resulcing from
coarse-sedimenl transpon lag: Journal of Sedi-
men[ary Research. v. 67, p. 124-129.
[seya. E. 1989. Mechanism ofinvelY grading of sus-
pended load deposits. in Taira, A.. and Masuda.
F.. eds.. Sedimentary facies in the aClj\'e plate
margin: Tokyo. Terra Scientific Publishing Com-
pany. p. 113-129.
Leopold. L. B.. and Maddock.. T.. Jr.. 1953. The
hydraulic geometry of stream channels and some
physiographic implications: U.S. Geologic Sur.
vey Prof~'onal Paper 252. 57 p.
Mclean. S. R.. 1992. On the calculation of suspended
load for noncohes.ive sediments: Journal of ~o-
physical Rese.m:h, v. 97. p. 5759--5770.
OSlerk.amp. W. R.. and Costa. 1. E.. 1987. Changes
accompanymg an on-aordinal')' flood on a sand-
bed stream. in Mayer. L.. and Nash. D.. eds...
CalaStropruc flooding: Boston. Allen and Unwin.
p. 201-224.
Rubin. D. M.. and McCulloch. D. S.. 1980. Single and
superimposed bedfonns: A synthesis of San
Francisco Bay and flume observations. in Bouma.
A. H., Gorsline. D. S.. Allen. G.. and Monl}'. c.,
eds.. Shallow marine processes and products:
Sedimentary Geology. v. 26. p. 207-231.
Rubin. D. M.. Schmidt.. 1. c.. and Moore. 1. N.. 1990.
Origin. Slructure. and evolution of a reattach-
ment bar, ColQJ'"ado River. Grand Canyon. Ari~
zona: Journal of Sedimentary Pelro[ogy. v. 60.
p.982-991-
Sdunidt.1. C. and GraI.l. B.. 1990. Aggr:ulation and
degradation of alluvial sand deposits. 1965-1986.
Colorado River, Grand Canyon National Park.
Arizona: U.S. Geological Survey Professional
Paper 1493.100 p.
Smith. 1. 0.. and Mclean. S. R.. 1977. Spatially a...er-
aged flow o...er a wavy surface: Journal of Ge0-
phYSical Resean:h. v. 82. p. 1735-1746.
Southard. J. B.. 1971, Represenlation of bed configura-
tions in deplh-veklciry-size diagr.uns: Joumal of
Sed.imentary Petrology. v. 41. p. 903-915.
Southard. 1. 8.. and Boguchwal. L. A.. 1990. Bed con-
figurations in steady unidirectional .....ater flows.
Purt 2. Synthesis of flume dam: Journal of Sedi.
mentary Petrology. v. 60. p. 658-679.
Topping, D. J.. 1997. Physics of flow. sediment tran!.-
port, hydraulic ~om('[ry. ,:md channel geo-
morphic adjustment during flash floods in an
ephemeral river. the Pana River. Utah and
Ari7.0na IPh.D. thesis]: Seattle. University of
WashinglOn. 405 p.

Manuscnpt receiVed June 23.1997
Revised manuscript receh'ed November 7, 1997
MJIluscript accepted November 17. 1997

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