WSP05376

• File Number: 8220.101.17
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 2/1/2001
• Title: Monitoring of Coarse Sediment Inputs to the Colorado River in Grand Canyon
• Water Supply Pro - Doc Type: Report/Study

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River (tig. 21, SI3ndard aerial phOlographs
must be digilized, georeferenced. and rec-
tified before use, requiring the establish-
ment of control pomts in the field. When
pre-even! topography IS not available. fan
volume can he calculaled by multiplying
the fan area measured from aerial photo-
graphs by an average thickness.
Panicle-size dislributions are hesl
measured by comhining JX>int CQunls in
the field with standard sieve analysis in
the laboratory to capture the full range of
panicles !"ound in debris flow deposits.
Larger particles may be measured on site.
Both unreworked deposits on fan surfaces
and reworked deJX>sils along diSlal fan
edges are evaluated (fig. 3). Particle-size
measurements should document sulur-

ing. or the interlocking of panicles. Sutur-
ing is caused by the rearrangement and
wearing together of panicles, and its
OCL'UITence makes debris fans difficuh to
rework.
Olher measuremenls include survey-
ing the waler-surface fall through the
rapid. which can be used to calculale
stream power for a given discharge (fig.
4). Surface velocity through the rapid can
be measured by timing the passage of
Hoats through the rapid. Other on-site
measurements include documentation of
changes in hydraulic features such as the
shifting of waves and holes, and the
movemenl. appearance, or disappearance
of rocks,

Particle Size, In mm

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- 80
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Particle

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Debris-now deposit
- Tributary reworked deposit
- P&rtilI.lly river reworked deposit
- - - - - Fully river relNorked deposit
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Size, in q. units

Figure 3. Particle-size distributions reflecting reworking on distal margins of a recently
aggraded debris fan at mile 127.6 in Grand Canyon. The curve tor partially river
reworked deposit is the result of releases from Glen Canyon Dam. The curve of fully
river reworked deposit is for reworking under pre-dam conditions.

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100

200
Thalweg

3/25/96
4/9/96

300 400
Distance (m)

600

500

Figure 4. Water-surface profiles through Lava Falls Rapid showing the effects ot
reworking by the March 1996 flood,

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Percent conSlriClion of the river chan-
nel, a ratio of the average channel widlh
lhrough [he rapid to (he average channel
width ahove and below lhe rapid (Webb
and Of hers. 1999a), is a useful measure of
lhe impact of a debris flow on river chan-
nel morphology (fig. 5). For ease of mea-
surement and consistency. measures of
channel width are best obLained from geo-
referenced remole-sensing data. particu-
larly aerial photographs, Aerial
photography may also reveal qualitative
changes in the hydraulics and navigability
of rapids. re"ecling underlying changes in
the positions of boulders, and can be com-
bined with field observations as a qualita-
live measure of channel change.
Because dehris bars typically are
unsLaole, much of the monitoring of these
bars is best performed using remote sens-
ing. This monitoring is most accurate
when digital aerial photographs georefer-
enced hy geographical posilioning sys-
lems (GPS) are used because stable. long-
lerm conlrol poinlS may be difficull to
loc3le. Altemalively, L1DAR data of sub-
aerial topography and multi-beam bathy-
metric data can be combined to give an
importanl three-dimensional portrail of
bar evolulion over time. Particle-size dis-
tributions, and particularly lithologic
counls, may be useful for assessing the
smbilily and fonnation of debris bars.

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Long-Term Monitoring

Repeal moniloring of older fans could
be performed althe same time as monitor-
ing at recently 3ggraded debris fans.
lhough additional moniloring should be
done following the occurrence of atypi-
cally large reworking floods at other times
of the year. Monitoring could start with an
examination of new aerial photography
and recalculation of debris-fan area, width
of the reworked zone, and channel con-
Slriclion. Because regulated noods do not
overtop most fans but instead are eroded
13lerally. lack of change in any of these
parameters indicates a stable fan where
armoring is likely. Field work on these
fans could be Iimiled to annual point
counts along the fan edge to evaluate the
degree of armoring. Muhi-year stabilily in
particle-~ize distJibulion as well as fan
area and channel constriction indicates a
stahle fan and monitoring could be sus-
pended. However. any river discharge