WSP03492

• File Number: 8220.101.10.A
• Description: Colorado River-Water Projects-Glen Canyon Dam/Lake Powell-Glen Canyon Adaptive Management-AMWG
• State: AZ
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 3/3/2004
• Author: USGS
• Title: AMWG Meeting Attachments-March 3-4 2004-Variations in Sand Storage Measured at Monumented Cross Sections in the Colorado River Between Canyon Dam and Lava Falls Rapid-Northern Arizona 1992-99
• Water Supply Pro - Doc Type: Report/Study

two measurements (1 period) is calculated to be 22 m2,
the standard error is 0.39 m2, and the range is
22 :to.39 m2 with a 68.3-percent level of confidence
that the true mean change in area lies within this range.
For a 95-percent level of confidence, the standard error
is 0.78 m2, and the range is 22 :to.78 m2 If, for
example, changes in area are accumulated (summed)
between 13 measurements (12 periods) and calculated
to be -519 m2, the standard error is 1.4 m2, and the
range is -5] 9 :t 1.4 m2 with a 68.3-percent level of
confidence. For a 95-percent level of contidence, the
standard error is 2.8 m2, and the range is -519 :t2.8 m2.

Table 5. Standard error estimates applied to changes in area

Number of
measurements

Standard error
(68.3-percent
level of
confidence;
square meters)

Standard error
(95-percentlevel
of confidence;
square meters)

Number of
period!s}

2 0.39 .78
3 2 .55 1.1
4 3 .67 1.4
5 4 .77 1.6
6 5 .86 t.8
7 6 .95 1.9
8 7 1.1 2.2
9 8 I.t 2.2
10 9 J.2 2,4
II 10 1.2 2.6
12 tI 1.3 2.6
13 t2 t.4 2,8

Bed-elevation corrections from the water-surface
reference points enable comparisons between
measurements for a given cross section. but do not
enable relative comparisons between cross sections
because differences in channel g~ometry are not taken
into account. For example, an absolute change in area
of 5 m2 is more significant in a narrower cross section
with limited capacity for sediment storage than in a
wider cross section with great capacity for sediment
storage. To compare relative changes between cross
sections without regard to differences in channel
geometry, the changes in area were nonnalized
(normalized change in area) for cross sections in the
primary data set (table 4).

Changes in area from measurement to
measurement were nonnalized by dividing the change
in area by the cross-sectional area. The cross-sectional
area used for normalization was calculated by
estimating the elevation of the water surface at the
wates-surface reference points at a discharge of
850 m3!s. A discharge of 850 m3!s was chosen to
maximize the cross-sectional area and because many of
the cross sections were measured at discharges near
this value. Using the date and time of the measurement
and data from the nearest upstream streamflow-gaging
station, a discharge value was estimated for each water-
surface elevation measurement made at the water-
surface reference points. Discharge reference-point
elevation curves were plotted for each water-surface
reference point and regression equations were filled to
each curve. Using the regression equations, a water-
surface elevation was calculated for a discharge of
850 m3!s for each water-surface reference point.
Selected discharge-elevation curves are shown in
figure 10. Figure 10A,C illustrate discharge-elevation
relations having high R2 values. Figure lOB shows a
discharge-elevation relation having a low R2 value.
Possible reasons for the outliers in the discharge-
elevation curves include actual changes in the channel
that affected the water-surface elevation or inaccurate
water-surface measurements. To detennine the cross-
sectional area for a discharge of 850 m3!s, the area for
each incremental distance then was computed as the
difference between the water-surface elevation at a
discharge of 850 m3/s and the lowest measured bed
elevation at that distance multiplied by 0.25 m. The
nonnalized area for the cross section was computed by
summing the incremental areas.

Presentation of Data

Changes in sediment storage in the Colorado River
downstream from the Pari a and Lillie Colorado Rivers
are documented from June ]992 to February ]994
(Graf and others, 1995) and April 1994 to August 1995
(Graf and others, 1997). Konieczki and others (1997)
documented changes in sediment storage before and
after the 1996 controlled flood. All cross-section data
collected from June 1992 through September 1999 are
avai]able from the USGS as three types of tab-
delimited ASCII electronic files. The first file type
contains the interpolated data for each measurement.
These tiles contain the transect number; the distance
from the left-bank end point, in meters; and the bed

0226~

Melhods 21

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