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

Figure 1. Flood deposit al
eddy sRe. Photo was taken
after recession of flood
end shows upper 1.5 m of
a 5-m-thlck deposR. Lower
left shows finer-grained
climbing-ripple structures
(ripple loresets dip loward
the right), whereas top
0.5 m shows coarser
crossbedded strata. Inset
shows study shes.

this typically occurred over the time scale of
week.s or months. but in the postdam era (with
reduced sediment supply) this adjustment may
occur over a few days of high flow.

FIELD OBSERVATIONS
COBr.itning-Upward Deposits
Sediment deposited during the 1996 flood
experiment was examined in trenches on several
dozen eddy bars (Fig. I) and was sampled verti-
cally at five of these siles. The mean grain size
coarsens upward by roughly a Factor of two. From
0.06-0.10 mm at the base toO.IQ-{).19 mm at the
lOp of the deposits <Fig. 2). The increase in mean
grain size occurs not merely by the removal of
fines, but also by an increase in the modal size and
an increase in size of the coarsest fraction. In the
field, the upward coarsening is great enough to be
visible to the eye. Most of the flood deposits are
composed of climbing-ripple deposits. but at
some locations the finer-grained climbing-ripple
stnlctures are confined to lhe basal deposit and are
overlain by coarser-grained crossbedding (Fig. 1).
Predam deposils also coarsen upward (Fig. 2).

Sediment Concentration and Grain Size
To study the connection be[Ween eddy deposits
and main-channel sediment transpon. we moni-
tored bed and suspended sediment during the
flood experiment at two sites. one in the main
channel al the "near Grand Canyon. AZ gage
station number 09402500" and one in an eddy
located 55 kIn downs{ream (Fig. I). At the main

channel site. suspended-sediment samples were
collected From the U.S. Geological Survey cable-
way using both a P-61 poinl sampler and a bag
sampler with a 0-77 head The P-61 sampler was
deployed at two verticals located at one-third and
two-thirds of the channel width Following both a
point-sampling and depth-integrating methodol-
ogy. In the point-sampling methodology. three
samples were collected at six points in each veni-

____ 1996 flood
(Tog) - -0- - Predam floods
1.0
~ /Jj 1/
.~ 9_
0
0- "
" ,
-0 0.'
-0 "
" , , I'
0 " // I: " i
0
'"
c 0.6
:s ,i .' 1/ I
.~ 1 II '
~ " ;,! I
.c 0< r " ;tV
0> 1<;'"
iii :'
.c
" ' :
> 0.2 , ' ,
.'"
~ , ' ,
" I , , : '
0: 0 o , / . J
0.0
(B~5e) 0.04 0.06 00. 01 0.2

Median grain diameter (mm)
Figure 2. Plot of median grain size as function
01 relatlye height within flood deposils (1996
flood deposit at five sites and four predam
flood deposits). Grain size nearly doubled dur-
Ing each flood.

L L

,
so

km

l
~'"
t

caI on tluee days during tile seven-day flood. In the
depth-integrating methooology, [wo to four sam-
ples were coUecrecl at each vertical on each day of
the flood. The 0-77 sampler was used to collect
cross-sectionally averaged suspended-sediment
samples on four days using the equal-discharge-
increment methodology described by Edwards
and Glysson (1988). Bed material was sampled at
as many as five equally spaced locations across the
channel daily using a 8M-54 sampler. At the eddy
site. suspended-sediment samples were collected
using a D-74 depth-inregrating sampler deployed
from a boat at four locations spanning the width of
lhe eddy; each of the points was sampled twice
daily during the flood. Concentrations of sus-
IXnded sediment were determined using slaJ1dard
U.S. Geological Survey techniques (Guy. 1%9).
Grain-size distributions of the suspended sand
were measured at 114 $ intervals using a visual-
accumulation rube. and groin-size distributions of
me bed marerial were measured al In 1ft intervals
using dry sieving.
During the seven-day flood. 1O(al sediment
concentralions determined by averaging depth-
integrated samples from the main channel site
decreased by approximately a factor or two.
beginning with an average concentration of
0.15% by volume measured on March 27.1996,
and decreasing to a value of about 0.068'10 on
April 2, 1996 (Fig. 3). This was a decrease of
almost a factor of five in silt and day concentra-
tion (From 0.035% to 0.008% by volume) and a
decrease of slighlly less than a factor of two in