DISTANCE, IN METERS

DISTANCE, IN METERS

DISTANCE, IN METERS

EXPLANATION

—— 815——— MEASURED BATHYMETRIC ELEVATION CONTOUR —
In meters

——> CALCULATED FLOW FIELD — Dash length is proportional
to magnitude of velocity. Arrows indicate direction of flow

Figure 8. Bathymetry of the modeled reach for the controlled flood, March-April, 1996. A, After 1 day. B,
After 3 days.

10 Modeling of Flood-Deposited Sand Distributions, Colorado River below the Little Colorado River, Grand Canyon, Arizona


Modeling of Flood-Deposited Sand Distributions
in a Reach of the Colorado River Below the Little
Colorado River, Grand Canyon, Arizona

By Stephen Mark Wiele

Abstract

A release from Glen Canyon Dam during March-April 1996 was designed to test the
effectiveness with which the riparian environment could be renewed with discharges greatly in
excess of the normal powerplant-restricted maximum. Of primary concern was the rebuilding of
sand deposits along the channel sides that are important to the flora and fauna along the river
corridor and that provide the only camp sites for riverside visitors to the Grand Canyon National
Park. Analysis of the depositional processes with a model of flow, sand transport, and bed
evolution shows that the sand deposits formed along the channel sides early during the high flow
were affected only slightly by the decline in suspended-sand concentrations over the course of
the controlled flood. Modeling results suggest that the removal of a large sand deposit over
several hours was not a response to declining suspended-sand concentrations. Comparisons of
the controlled-flood deposits with deposits formed during a flood in January 1993 on the Little
Colorado River that contributed sufficient sand to raise the suspended-sand concentrations to
predam levels in the main stem show that the depositional pattern as well as the magnitude is
strongly influenced by the suspended-sand concentrations.

INTRODUCTION

This report describes modeling of sand
deposition and erosion in a reach of the main stem
Colorado River downstream from the Little

1
A controlled flood' was released from Glen Colorado River (fig. 1). The deposition modeled

Canyon Dam during March-April 1996 to
determine the effectiveness with which the riparian
environment along the Colorado River could be
renewed with discharges greatly in excess of the
normal powerplant-restricted maximum. Of
primary concern was the rebuilding of sand
deposits along the channel sides that are important
to the flora and fauna along the river corridor and
that provide the only camp sites for riverside
visitors to the Grand Canyon National Park.

'The Grand Canyon Monitoring and Research Center refers to
this event as a “beach/habitat-building flow” (David Garrett, Chief,
Grand Canyon Monitoring and Research Center, written commun.,
1997).

for the flow of March-April 1996 is compared to
deposition modeled for a flood on the Little
Colorado River in 1993, which delivered sand to
the main stem sufficient to elevate suspended-
sand concentrations in the main stem to predam
levels. Although the suspended-sand concen-
trations at the streamflow-gaging station on the
main stem upstream from the Little Colorado
River during the controlled flood were about 10
times higher than suspended-sand concentrations
predicted by the sand-rating curve developed for
that site, the suspended-sand concentrations were
still lower than would have been typical of
predam spring floods. In addition, the effect of the
declining suspended-sand concentrations on

Abstract 1


channel, but in this case, the flow is in quasi-
equilibrium with the bed shape. Random
fluctuations in the location of the reattachment
point in flume studies have been observed by
Schmidt and others (1993) and Rubin and others
(1990). These fluctuations in a real river would
occur on the time scale of minutes to hours; a
sudden and sustained shift after several days of
depositional development is unlikely. There is a
random component to flow patterns, such as from
turbulence or the fluctuations in the location of
the reattachment point, but for a given bed con-
figuration and upstream boundary conditions, the
flow pattern averaged over a short time period is
fixed by the governing physics of the flow. These
random fluctuations would instead tend to spread
the deposit near the reattachment point and
modify its shape but not cause a sudden direct
scour of the deposit. The modeling shows that the
reduction in  suspended-sand concentration
affected the deposits formed early during the
controlled flood; however, the erosion was minor.
Reduction in suspended-sand concentration alone
could not have resulted in the removal of so much
sand so quickly.

These deposits were probably removed as a
result of a mass failure. This process is consistent
with the removal of the sand over a short time and
does not require a radical change in flow pattern.
Small fluctuations in flow, such as fluctuations in
the location of the reattachment point, could play
a role in triggering such a failure. Cluer (1997)
has proposed that local erosion caused by
increases in flow velocity near the base of bars
could trigger slumping.

Rapid removals of sand such as this one may
have occurred during the flood on the Little
Colorado River; however, there are no
measurements to document the removal. Cluer
(1995) observed similar sudden losses of sand
from deposits during normal dam operations
using daily photographs and found that the
deposits were replenished within weeks to
months. Sudden losses of sand during the flood on
the Little Colorado River probably would be
replaced even more rapidly as a result of the high
concentration of sand in suspension, which would
leave a final deposit that showed no evidence of
such losses. In the absence of surveys made
during an event, the only evidence of such losses

would be found if the conclusion of the event
coincided with a recent loss that allowed no
opportunity for redeposition.

CONCLUSIONS

Changes in suspended-sand concentration
over the course of a controlled release, such as
during the controlled flood, potentially can affect
deposits formed early during the release when
suspended-sand concentrations are higher.
Modeling results for this reach indicate that the
reduction in suspended-sand concentration did
cause a reduction in the size of deposits formed
under conditions more favorable for beach
restoration. The effect, however, was small and
occurred at a much slower rate than the rate at
which the deposits formed.

The modeling results presented here support
the hypothesis that sand residing on the bottom of
the channel can be redistributed effectively to the
channel sides with dam releases greater than
powerplant capacity. The depositional processes,
however, do not appear to duplicate the processes
as they occurred before the closure of Glen
Canyon Dam. High suspended-sand concen-
trations during the flood on the Little Colorado
River led to rapid and massive deposition in the
main channel and a slower continuous buildup of
bars along the channel sides over about 3 days.
During the controlled flood, however, the lower
suspended-sand concentrations led to erosion in
the main channel and deposition near the
reattachment point. Sand carried from the inlet
along streamlines toward the reattachment point
was deposited because of the high magnitude of
the negative divergence of the shear stress along
these streamlines.

REFERENCES CITED

Andrews, E.D., Johnston, C.E., and Schmidt, J.C., 1996,
Topographic evolution of sand bars in lateral
separation eddies in Grand Canyon during an
experimental flood: American Geophysical Union
Transactions, v. 77, no. 46, p. 258.

Cluer, B.L., 1995, Cyclic fluvial processes and bias in
environmental monitoring, Colorado River in Grand

14 Modeling of Flood-Deposited Sand Distributions, Colorado River below the Little Colorado River, Grand Canyon, Arizona


MODELING OF SAND DEPOSITION
AND EROSION

Description of Model and Modeled
Reach

The model was developed to track sand
volumes and to investigate the mechanics of
deposition and erosion of sand deposits in
kilometer-scale reaches of the Colorado River in
the Grand Canyon. The model calculates the
vertically averaged two-dimensional flow field,
the three-dimensional suspended-sand field, the
local sand discharge, and the local sand erosion
and deposition that determine the evolution of the
alluvial bed over time. Channel shape, initial sand
thicknesses, sand size and density, water dis-
charge, sand discharge, water-surface elevation,
average water-surface slope through the reach,
and local roughness must be specified from direct
measurement or estimated from auxiliary
calculations. Details of the model can be found in
Wiele and others (1996). One modification to the
model as it was presented in Wiele and others
(1996) is in the calculation of the skin friction.
Skin friction is extracted from the total shear
stress as a function of the deviation of the
measured bed topography from the model grid
using a calculation similar to a calculation of
Wiberg and Smith (1991).

For the controlled flood, the sand discharge
at the reach inlet was determined by using sand at
the outlet of the first reach below the confluence
with the Little Colorado River (fig. 1) as the
boundary condition for the next reach and
repeating that procedure to the upstream boundary
of the modeled reach presented here. The
upstream boundary condition for the sand at the
first reach below the Little Colorado River
confluence was taken from suspended-sand
measurements made at the streamflow-gaging
station above the confluence (Konieczki and
others, 1997). For the flood on the Little Colorado
River, the four reaches were modeled separately
and used the sand influx from the Little Colorado
River as the upstream boundary condition. The
high suspended-sand concentrations along with
the close proximity to the confluence make this a

reasonable approximation for the flood on the
Little Colorado River.

The modeled reach is about 2.4 km below
the confluence with the Little Colorado River
(fig. 1). The reach is about 350 m long and is
bounded upstream and downstream by rapids that
are formed by debris flows that partially
constricted the channel laterally and deposited
boulders in the main channel. Just below the
entrance to the reach, the channel expands sharply
along the left bank, and the flow forms a large
recirculation zone that occupies about one-half
the length of the channel. A smaller recirculation
zone is on the right side. The channel also
expands vertically just below the inlet, and had a
depression that was about 22 m deep during the
controlled flood. This depression shallows about
midway through the reach to a maximum
cross-stream depth of about 5 m. This
morphology is characteristic of the Colorado
River between the confluence of the Little
Colorado River and Furnace Flats, which are
about 11 km apart. This morphology is especially
effective at trapping sand in recirculation zones,
along margin deposits, and, if the suspended-sand
concentration is high enough, in the main channel.

The model was used to calculate the time
evolution of the sand deposits in four reaches
during the Little Colorado River flood (Wiele and
others, 1996). The model was checked against
cross sections measured before and after the
event and was found to match the cross sections
well. For the comparison between the results of
the flood on the Little Colorado River and the
results of the controlled flood, the model was used
for the one reach that was among the four
included in the study of the effects of the flood on
the Little Colorado River (fig. 1) and that also
was included in daily bathymetry measured
during the controlled flood by Andrews and
others (1996).

Modeling Results

The increase in water and sand discharge in
the main stem during the flood of the Little
Colorado River led to large sand deposits below
the confluence (Kaplinski and others, 1995; Wiele
and others, 1996). Large deposits accumulated in

6 Modeling of Flood-Deposited Sand Distributions, Colorado River below the Little Colorado River, Grand Canyon, Arizona


deposits formed early in the high flow when
suspended-sand concentrations were higher is
examined for evidence of significant loss in initial
sand-deposit volume.

A model of pool-scale flow, sand transport,
and bed evolution (Wiele and others, 1996) was
used to calculate changes in bed topography
between measurements and to examine the effect
of differences in suspended-sand concentrations.
One of the advantages of modeling complicated
geomorphic processes that may have several
competing or complementary effects on the end
result is that the geomorphic processes can be
isolated with the model to test their significance
in forming the final deposit. In this report, for
example, the model is used to test the hypothesis
that the massive erosion of a sand bar was the
result of declining suspended-sand concentration
during the controlled flood. This work was
performed in cooperation with the Bureau of
Reclamation.

FLOOD ON THE LITTLE COLORADO
RIVER, JANUARY 1993

In January 1993, a large flood on the Little
Colorado River contributed 4.2 Tg of sand (G.G.
Fisk, hydrologic technician, U.S. Geological
Survey, written commun., 1993) and increased the
water discharge in the main stem (fig. 2). This
flood led to large sand deposits at stages greater
than the stages associated with normal power-
plant capacity as well as a peak discharge that was
greater than powerplant capacity. A volume of
sand equivalent to that delivered by the Little
Colorado River flood was deposited within about
20 km of the confluence (Wiele and others, 1996),
but fresh sand deposits along the channel sides
were observed for another 260 km downstream.
These deposits farther downstream must have been
formed from sand eroded from the channel bottom
by the high water discharge. After the flood on the
Little Colorado River receded, deposits in the main
channel were eroded as the suspended-sand
concentrations dropped to levels typical of
postdam flows; however, deposits within
environments that are relatively secluded
compared to the main stem, such as recirculation

zones and deposits above the lower stages in the
main stem that followed the Little Colorado River
flood, eroded at much slower rates.

CONTROLLED FLOOD, MARCH—
APRIL 1996

The controlled flood consisted of a steady
low flow of 226 m/s for 96 hours, an increase to
1,270 m/s over a 10-hour period where it was
held steady for 167 hours, then a gradual decrease
to 226 m/s over a 46-hour period (fig. 3).
Modeling of the anticipated release before it
occurred (Wiele, 1996) and modeling associated
with the revision of the one-dimensional model to
include higher discharges using data from the
controlled flood (Wiele and Griffin, 1997) have
shown that the rising limb of the wave steepened
as it traveled downstream to about 250 km below
the dam, where it reached an equilibrium profile.
In the reach below the confluence with the Little
Colorado River, the discharge increased from 350
to 1,100 m?/s in about 3.2 hours.

Suspended-Sand Concentrations

Much of the sand from the flood of 1993 on
the Little Colorado River was still present at the
start of the controlled flood more than 3 years
later. The high discharges during the controlled
flood caused high sand loads throughout its
duration and caused high rates of deposition. The
suspended-sand concentrations decreased over the
duration of the controlled flood (fig. 4). The sand
discharge in the main stem was higher during the
Little Colorado River flood (fig. 5) although the
peak water discharge was lower than the peak
water discharge during the controlled flood. This
combination of lower water discharge and higher
sand discharge led to peak suspended-sand
concentrations during the flood on the Little
Colorado River that were more than twice the
suspended-sand concentrations during the con-
trolled flood and were comparable to predam
suspended-sand concentrations (David Topping,
hydrologist, U.S. Geological Survey, oral
commun., 1996).

Flood on the Little Colorado River, January 1993 3


git UTAH 112° 111°

[ARIZONA

__ NATIONAL
TM\ PARK

36°

EXPLANATION
A STREAMFLOW-

\ 4
sf

GRAND
i> CANYON

GAGING STATION
oO 25 KILOMETERS:
oO 25 MILES
Flagstaff
35° | _
Base from U.S. Geological or
State base map 1:1,000,000
TM~N
Ss Model Reach 2
\ (Wiele and
, Sea - others, 1996)
J ~ “oy
Loos ~,
~ .
ARIZONA 4
Phoenixg 7” &
s
¢
Par
Sections 4
B2-B4

MODEL REACH 4
Modeled reach for this report
(Wiele and others, 1996)

Sections

Area above
low-water
stage

Model Reach 1
(Wiele and others, 1996)

ee

Area above 0 500 METERS

0 2,000 FEET

Figure 1. Study area, modeled reaches, and the cross sections used to test the model, Colorado River, Grand

Canyon, Arizona.

2 Modeling of Flood-Deposited Sand Distributions, Colorado River below the Little Colorado River, Grand Canyon, Arizona


Canyon: Journal of Geology, v. 95, no. 103,
p. 411-421.

1997, Eddy bar responses to the sediment dynamics
of pool-riffle environments: Flagstaff, Arizona,
Bureau of Reclamation, Glen Canyon
Environmental Studies report, 135 p.

Kaplinski, Matt, Hazel, J.E., Jr., and Beus, S.S., 1995,
Monitoring the effects of interim flows from Glen
Canyon Dam on sand bars in the Colorado River
corridor, Grand Canyon National Park, Arizona:
Flagstaff, Arizona, Bureau of Reclamation, Glen
Canyon Environmental Studies report, 62 p.

Konieczki, A.D., Graf, J.B., and Carpenter, M.C., 1997,
Streamflow, and sediment data collected to
determine the effects of a controlled flood in March
and April 1996 on the Colorado River between Lees
Ferry and Diamond Creek, Arizona: U.S.
Geological Survey Open-File Report 97-224, 55 p.

Rubin, D.M., Schmidt, J.C., and Moore, J.N., 1990,
Origin, structure, and evolution of a reattachment
bar, Colorado River, Grand Canyon, Arizona:
Journal of Sedimentary Petrology, v. 60, no. 6,
p. 982-991.

Schmidt, J.C., Rubin, D.M., and Ikeda, Hiroshi, 1993,
Flume simulation of recirculating flow and
sedimentation: American Geophysical Union, Water
Resources ‘Research, v. 29, no. 8, p. 2925-2939.

Wiberg, P.L. and Smith, J.D., 1991, Velocity distribution
and bed roughness in high-gradient streams:
American Geophysical Union, Water Resources
Research, v. 27, no. 5, p. 825-838.

Wiele, S.M., 1996, Calculated hydrographs for the
Colorado River downstream from Glen Canyon
Dam during the experimental release, March
22-April 8, 1996: U.S. Geological Survey Fact
Sheet FS—083-96, 1 sheet.

Wiele, S.M., Graf, J.B., and Smith, J.D., 1996, Sand
deposition in the Colorado River in the Grand
Canyon from flooding of the Little Colorado River:
American Geophysical Union, Water Resources
Research, v. 32, no. 12, p. 3579-3596.

Wiele, S.M., and Griffin, E.R., 1997, Modifications to a
one-dimensional model of unsteady flow in the
Colorado River through the Grand Canyon,
Arizona: U.S. Geological Survey Water-Resources
Investigations Report 97-4046, 17 p.

References Cited 15


0.010

0.009 }-

0.008 -—

0.007 --

SAND VOLUME CONCENTRATION (DIMENSIONLESS)
°
T

0.004 |— 4
0.003 |— N 4
7 Controlled flood \
oot / . 4
/ ‘
0.001 }- 7 4
otf | L i | |
0 1 2 3 4 5 6

TIME, IN DAYS.

Figure 4. Suspended-sand concentrations in the Colorado River near the confluence with the Little Colorado
River during the controlled flood, March-April 1996, and during the flood of 1993 on the Little Colorado River.

Controlled flood

SAND DISCHARGE, IN CUBIC METERS PER SECOND
s
T

TIME, IN DAYS.

Figure 5. Sand discharge in the Colorado River near the confluence with the Little Colorado River during the
controlled flood, and during the flood of 1993 on the Little Colorado River.

Flood on the Little Colorado River, January 1993 5


U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY

Modeling of Flood-Deposited Sand
Distributions in a Reach of the Colorado
River Below the Little Colorado River,
Grand Canyon, Arizona

By STEPHEN MARK WIELE

Water-Resources Investigations Report 97— 4168

Prepared in cooperation with the
Bureau of Reclamation

Tucson, Arizona
1998


150

|

DISTANCE, IN METERS

200

150

100

DISTANCE, IN METERS

505

DISTANCE, IN METERS.

EXPLANATION

—— 815———-_ __ MEASURED BATHYMETRIC ELEVATION CONTOUR —
In meters

——> _ CALCULATED FLOW FIELD — Dash length is proportional
to magnitude of velocity. Arrows indicate direction of flow

Figure 6. Bathymetry of the modeled reach. A, Before the flood of 1993 on the Little Colorado River. B, Before
the controlled flood, March—April 1996.

8 Modeling of Flood-Deposited Sand Distributions, Colorado River below the Little Colorado River, Grand Canyon, Arizona


the main channel as well as along the channel
sides. During the controlled flood in March-April
1996, the main channel generally scoured as fresh
sand deposits accumulated along the channel
sides.

The reach contained more sand before the
start of the controlled flood than before the Little
Colorado River flood (fig. 6). More sand was
stored in the main channel and along the left bank
before the controlled flood; however, the overall
channel shape and flow patterns were similar.

After 1 day during the flood on the Little
Colorado River, the model results indicate that the
main-channel depression had filled, and large bars
had formed along a transition zone from high-
velocity downstream flow in the main channel to
the slower flow near the right bank and along the
boundary between the downstream flow and the
recirculating flow along the left side (fig. 7A).
After 3 days, the left-side bar had grown nearly to
the left bank and extended farther downstream,
and the main channel had filled (fig. 7B). The
total volume of sand in the reach remained nearly
constant after 3 days; however, the shape of the
deposits varied. The bar on the right side migrated
downstream, and the bar on the left side extended
farther downstream.

During the controlled flood, the model
results indicate that the main-channel depression
had been eroded, and sand had accumulated near
the reattachment point along the left bank after 1
day (fig. 84). The model results differ from the
bathymetry of Andrews and others (1996) in that
their measurements show additional accumulation
occurred downstream from the reattachment point
farther from the left bank and extended out
toward the main flow. After 3 days, the main
channel had eroded even more, and the bar on the
left side near the reattachment point had increased
slightly (fig. 8B).

In an alluvial river, deposition and erosion
generally follow the divergence of the shear
stress. A lag between shear stress and deposition
is introduced where the sediment is in suspension,
but in general, erosion occurs where the shear
stress is increasing, and deposition occurs where
shear stress is decreasing. In the study reach,
however, the suspended-sand concentrations
introduced at the upstream end are highly
variable, and unless a major influx of sand from a

tributary occurs, the river carries a load that is less
than its capacity. As a result, whether sand is
scoured or deposited in the main channel depends
on the concentration of sand in suspension as well
as on the channel shape. Following the stream-
lines from the inlet, the shear stress decreases
where the flow expands vertically as it moves into
the river segment that has the deep depression in
the main channel. With sufficiently high sand
loads, this decrease in shear stress would lead
directly to deposition in the depression. During
the flood on the Little Colorado River, which
raised suspended-sand concentrations to predam
levels, model results predict that the majority of
the deposition early in the event was within the
depression. In contrast, if suspended-sand
concentrations are sufficiently low, sand residing
in the depression, such as would result from
deposition during previous low discharges, would
be entrained and scoured from the depression
despite the decrease in shear stress along stream-
lines. During the controlled flood, suspended-sand
concentrations were low enough to cause the sand
in the depression to scour.

During the flood on the Little Colorado
River and during the controlled flood, significant
deposition occurred along the side of the
main-channel flow. In addition to the filling of the
depression in the main channel, the model
indicates that the high suspended-sand concen-
trations during the flood of the Little Colorado
River led to deposition of a bar in the lateral
expansion near the inlet along the left side of the
main-channel_ flow. This bar expanded
downstream and into the recirculation zone. The
lower suspended-sand concentrations during the
controlled flood, however, produced little
deposition near the inlet. Instead, sand in
suspension was carried toward the stagnation
point where the separated flow reattaches to the
left bank.

Response of the channel to the differences in
water discharge and suspended-sand concen-
trations can be compared by summing the change
in the cross-sectional area of the sand deposit
along the channel. After 1 day, the change in sand
cross-sectional area during the flood on the Little
Colorado River shows an increase in the
cross-sectional area of the sand deposit along the
channel and no net erosion (fig. 9). The largest

Modeling of Sand Deposition and Erosion 7


1,000 T T

Discharge to the main stem
(Little Colorado River)

i\
3 ih
8 L Calculated main-stem discharge 4
a hs } ‘i — (Colorado River below Little Colorado River) ran
ir b I Wo\ ra H tn 4
2 I 1 ivf owes
Q t \ A tN vou
© L oo larat " H rT 4
i i \ hon 1

my 1

2 soolry | Voy i\i wa
9° TMy i iow vy \ Lee
a I I 7
2 L 1 s 4
oO
Zz
ul
fo}
c
<
x=
1o}
2
a

0 15 20 25
DAYS IN JANUARY 1993.

Figure 2. Discharge in the main stem of the Colorado River below the Little Colorado River, and the discharge
in the Little Colorado River during the flood of January 1993.

1,500 T T | T

fal L

2

fo)

oO b

if

a

2 L

fry

& 4,000 }—

n

a L

uw

in L

=

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a

5

3 L

Zz

=~ 500 -—-

ui

fo) L

c

=<

= L

3

DB

g
0 | | | | | | | | | ! | | | !
0 1 2 3 4 5 6 7 8 9 0 1 12 18 14 15

DAYS STARTING MARCH 23, 1996

Figure 3. Discharge released from Glen Canyon Dam during the controlled flood of March-April 1996.

4 Modeling of Flood-Deposited Sand Distributions, Colorado River below the Little Colorado River, Grand Canyon, Arizona


FIGURES—CONTINUED
11-12. Graphs showing:

11. Changes in sand cross-section area after 5 days from model results during the
flood in 1993 on the Little Colorado River, during the controlled flood in
March-April 1996, and from measured bathymetry during the controlled

flood as a function of distance from the reach inlet....

12. Measured and modeled cross section 200 meters downstream from the reach inlet:

A. After 4 days.
B. After 5 days

CONVERSION FACTORS

Page

13
13

Multiply By To obtain
meter (m) 3.281 foot
kilometer (km) 0.6214 mile
cubic meter per second (m?/s) 35.31 cubic foot per second
terragram (Tg) 6.852x107 slug

IV Contents


increase in sand cross-sectional area is about
110 m below the inlet where the deep hole in the
main channel, which filled with sand, is located,
and along the eddy fence on the left side of the
inlet flow. The increase in sand cross-sectional
area decreases downstream and corresponds to the
smaller reattachment deposits and margin deposits
along the left and right banks.

After 1 day during the controlled flood, the
modeled and measured bathymetry show a
smaller amount of deposition near the deep hole
(fig. 9). Erosion rather than deposition occurred
during the controlled flood in the main channel.
The deposition near the inlet was along the eddy
fence on the left side. The negative change in
cross-sectional area of the sand at about 200m
corresponds to the loss of sand in the main
channel. Downstream, the change in sand
cross-sectional area increased as a result of
deposition near the reattachment point. The model
shows a pattern similar to that of the measured

bathymetry; however, the pattern is shifted
downstream about 10 percent of the length of the
reach, and the model underestimated the amount
of deposition farther downstream. The discrep-
ancy between the model and the measurements is
due to the additional sand in the main-channel
region as shown in the measured bathymetry.

After 3 days, the deposition from the flood
on the Little Colorado River had increased
throughout the reach. Deposition concentrated
around 150 m (fig. 10) where the bar on the left
side continued to increase. In contrast, the main
channel during the controlled flood continued to
erode as shown by the increased magnitude in the
negative change in cross-sectional area about
200m downstream from the inlet. About 300 m
downstream from the inlet where deposition was
along the left bank near the reattachment point,
the modeled and measured bathymetry show a
small decline in the sand cross-sectional area.

CHANGE IN SAND CROSS-SECTIONAL AREA,
IN SQUARE METERS.

T T T T

500 |- Nes 4
a Model prediction for controlled flood 4
-1,000 -—- 4
-1,500 -—- mn
~2,000 1 | n l 1 ! L l L
0 100 200 300 400 500

DISTANCE DOWNSTREAM FROM INLET, IN METERS

Figure 9. Changes in sand cross-sectional area after 1 day from model results during the flood in 1993 on the
Little Colorado River, during the controlled flood in March—April 1996, and from measured bathymetry during the
controlled flood as a function of distance from the reach inlet.

Modeling of Sand Deposition and Erosion 11


L | 1

Model prediction for flood on the Little Colorado River

Model prediction for controlled flood

Measured during controlled flood 4

! l | Ll

2,000
1500 |
w
wi L
<
z 1,000 |-
3 L
=n
oh 500
ou L
g =
oe ofS
55 L
38
~500 LK
Sz
g L
w ~~ =
ww -1,000
= L
x
GS -1,500
2,000
0

100

200

300 400 500

DISTANCE DOWNSTREAM FROM INLET, IN METERS:

Figure 11. Changes in sand cross-sectional area after 5 days from model results during the flood in 1993 on the
Little Colorado River, during the controlled flood in March—April 1996, and from measured bathymetry during the
controlled flood as a function of distance from the reach inlet.

830

820

810

TITTY Trt yt rrt

Pepsi tiiviils

800

830

ELEVATION, IN METERS

820

810

Lo

Measured

| 14

riosttiiiilis

°

20

40

60

80 100 120
DISTANCE FROM LEFT BANK, IN METERS

8
8

140 160 180 200

Figure 12. Measured and modeled cross sections 200 meters downstream from the reach inlet. A, After 4 days.
B, After 5 days.

Modeling of Sand Deposition and Erosion 13


CONTENTS

Abstract...
Introduction..
Flood on the Little Colorado River, January 199:
Controlled Flood, March-April 1996

Suspended-sand concentrations.
Modeling of sand deposition and erosion .

Description of model and modeled reach

Modeling results......
Conclusions...
References cite

Page

BRAQDQAwWHWW-

FIGURES

255.

9-10.

Map showing study area, modeled reaches, and the cross sections used to test the model,
Colorado River, Grand Canyon, Arizon
Graphs showing:

2. Discharge in the main stem of the Colorado River below the Little Colorado
River and the discharge in the Little Colorado River during the flood of
January 1993

3. Discharge released from Glen Canyon Dam during the controlled flood of
March-April 1996 sesseee

4. Suspended-sand concentrations in the Colorado River near the confluence with the
Little Colorado River during the controlled flood in March—April 1996, and
during the flood of 1993 on the Little Colorado River

5. Sand discharge in the Colorado River near the confluence with the Little Colorado
River during the controlled flood in March-April 1996, and during the flood of
1993 on the Little Colorado River ......

Contour maps showing:

6. Bathymetry of the modeled reach:

A. Before the flood of 1993 on the Little Colorado River
B. Before the controlled flood, March—April 1996......

7. Bathymetry of the modeled reach during the flood of 1993 on the Little Colorado Riv
A. After 1 day
B. After 3 days ..

8. Bathymetry of the modeled reach for the controlled flood, March—April 1996
A. After 1 day .. 10
B. After 3 days .. 10

Graphs showing:

9. Changes in sand cross-section area after | day from model results during the
flood in 1993 on the Little Colorado River, during the controlled flood in
March~April 1996, and from measured bathymetry during the controlled
flood as a function of distance from the reach inlet...

10. Changes in sand cross-section area after 3 days from model results during the
flood in 1993 on the Little Colorado River, during the controlled flood in
March-April 1996, and from measured bathymetry during the controlled
flood as a function of distance from the reach inlet...

12

Contents Ill


U.S. DEPARTMENT OF THE INTERIOR
BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY
Thomas J. Casedevall, Acting Director

The use of firm, trade, and brand names in this report is for identification purposes
only and does not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased
from:

Regional Research Hydrologist, CR U.S. Geological Survey

U.S. Geological Survey Information Services

Water Resources Division Box 25286

Box 25046, MS418 Federal Center

Denver, CO 80225-0286 Denver, CO 80225-0286


200

150

100

DISTANCE, IN METERS

200

150

100 jx

DISTANCE, IN METERS

DISTANCE, IN METERS

EXPLANATION

—— 815—— __ MEASURED BATHYMETRIC ELEVATION CONTOUR —
In meters

> CALCULATED FLOW FIELD — Dash length is proportional
to magnitude of velocity. Arrows indicate direction of flow

Figure 7. Bathymetry of the modeled reach during the flood of 1993 on the Little Colorado River. A, After 1
day. B, After 3 days.

Modeling of Sand Deposition and Erosion 9


After 5 days, little increase occurred in the
cross-sectional areas during the flood on the Little
Colorado River (fig. 11). This pool reached its
capacity to store sand after about 3 days (Wiele
and others, 1996), and as a result, a volume of
sand equivalent to the volume introduced at the
inlet was discharged to the pools downstream.

After 5 days, the results for the controlled
flood show a marked discrepancy between the
modeled and measured bathymetry (fig. 11) as a
result of a process that is not represented in the
model. A large decrease in the cross-sectional
area occurred around 200 m in the measured
bathymetry between 3 and 5 days, although the
model showed little change in the cross-sectional
area. After 4 days, the modeled and measured
bathymetry agreed well (fig. 124); the model
overpredicted the amount of deposition near the
left bank, and underpredicted the amount of
deposition slightly closer to the thalweg. After 5
days, a large difference occurs between the

modeled and measured bathymetry at the same
location (fig. 12B). The model shows the deposit
from the previous day to be stable; whereas, the
measured bathymetry shows a large decrease in
the bed elevation along the left side. Farther
downstream, where the deposit was mainly near
the reattachment point, the model shows a
continuing, although small decrease in the
cross-sectional area; whereas, the measured
bathymetry shows an increase.

The details of the mechanism that caused the
large removal of sand along the left bank are
unknown. The removal of that sand apparently
occurred in only 2 hours (Konieczki and others,
1997). Two possibilities are a radical change in
flow pattern or a response to the reduced sand
concentration over time. E.D. Andrews (hydrolo-
gist, U.S. Geological Survey, oral commun.,
1996) has proposed that the loss is due to mass
failure. Flow patterns in a reach change over time
as deposition occurs, changing the shape of the

2,000 T 1 |

CHANGE IN SAND CROSS-SECTIONAL AREA,
IN SQUARE METERS

-1,500 -—

1 | 1 |

nl | \ | L

~2,000

100 200 300 400
DISTANCE DOWNSTREAM FROM INLET, IN METERS

500

Figure 10. Changes in sand cross-sectional area after 3 days from model results during the flood in
1993 on the Little Colorado River, during the controlled flood in March—April 1996, and from measured

bathymetry during the controlled flood as a function of distance from the reach inlet.

12. Modeling of Flood-Deposited Sand Distributions, Colorado River below the Little Colorado River, Grand Canyon, Arizona