WSP04447

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 1/1/1999
• Author: Schmidt/John C.
• Title: Summary & Synthesis of Geomorphic Studies Conducted During the 1996 Controlled Flood in Grand Canyon
• Water Supply Pro - Doc Type: Report/Study

the concentration of suspended sediment of the main flow
(J.M. Nelson and R.R. McDonald, U.S. Geological Survey,
written commun., 1995), especially on the side of the flow
near the eddy. These findings are consiSlent also with
measurements at 2 eddies in lower Marble Canyon
upstream from the Liule Colorado River, where the area-
weighted deposition was low throughout the flood (Figure
2a).
Reworking of recently-aggraded debris fans also
occurred rapidly and early during the 1996 controlled flood.
Pizzuto er aL [this yoIume] showed that particle
entrainment on debris fans occurs by Iwo mechanisms: (I)
slab failure by lateral erosion at the edge of the flow and (2)
entrainment of indiyidual particles from inundated pans of
the fan. They showed that entrainment of individual
particles occurred at 2 study sites, where dimensionless
critical Shields stresses as large as 0.24 and 0.3 were
measured. Lateral erosion is an important mechanism in
debris-fan reworking, because slab failures impart initial
movement to large particles that might not otherwise be
entrained. Webb el al. [this volume] showed that reworked
debris fans were coarser than they were before the flood,
suggesting that bed coarsening results from winnowing of
fine particles by seIectiye entrainment. Additionally, large
particles falling into the flow from bank failure eventually
limit fan reworking. This process of fan annoring occurs
rapidly and early and most reworking probably takes place
within the firsl day of a flood. Pizzuto el al. [this volume]
reported that reworking of the Prospect Canyon debris fan,
which forms Laya Falls Rapid, ended approximately 4 hrs
after the rise of the flood. On the basis of these observa-
tions, Webb el al. [this volume] stated, "Very short-duration
and high-magnitude controlled floods would be highly
effective in reworking aggraded debris fans."

6.3. Large-Scale Erosion during Ihe Flood

The rapid erosion of sand bars was first documented by
Cluerel al. [1994J and Cluer [1995] who photographed and
observed large portions of reauachment bars erode into the
main channel. This erosion occurred when lhe upper
surface of the bar was exposed and discharge was less than
powerplant capacity. Cluer [I 995J auributed rapid erosion
to destabilization of the base of reattachment bars by
change in direction and magnitude of downstream flow as it
exits a channel constriction. Wiele el 01. [1996] showed that
reorientation of the core of downstream flow within a
channel expansion could occur when deposition in channel
pools and in adjacent eddies was high. The processes
observed by Ciuer et 01. [1994] had not occurred. however.

SCHMIDT 335

TABLE l. Large erosion events in 5 eddies during the 1996
controlled flood in Grand Canyon

Eddy (day range)
Eminence (0. I) 1.4
Saddle (0- 1)2
Crash (1_2)3.4
SaIl Mine (0.1)1
Sail Mine (3_4)1.4
Salt Mine (4_5)3.4
Salt Mine (5-6))
Carbon (5.6)3.4
Carbon (6-7)1.4

Average
thickness of
erosion (m)
1.44

1.82
LI2

L3I
1.53

3.15
1.23

1.63

1.49

I Upstream end of reaH3chment bar.
2 Upstream half of reattachment bar.
3 Reauachment-bar platform.
4 Probable mass failure.

Proponion of
eddy (hat
eroded

0.53

0.68

0.66

0.39

0.39

0.58

0.60

0.59

0.51

during high-transport conditions such as Lhose during the
first days of the 1996 controlled flood.
Rapid erosion of large proportions of reattachment bars
occurred during the 1996 controlled flood. Andrews el 01.
[this yolume] measured erosion of more than 50,000 mJ of
sand from the upstream part of a reattachment bar at Salt
Mine eddy between days 4 and 5 of the flood, when as
much as 7 m of sediment were eroded rrom the bar.
KOllieczki et al. [1997] measured an ayerage of 4 m of scour
during a 2-hr erosional event that occurred during the last
day of the flood at (he same site. A similar event occurred at
a reattachment bar immediately downstream from 122 Mile
Creek (D.M. Rubin, U. S. Geological Survey, pers.
commun., 1998).
These erosion events did not occur on any specific day;
they occurred on dirrerent days at difrerent sites (Figure
2b). Large areas of erosion occurred between days 0 and I
in eddies at Eminence, Saddle, and SaIL Mine, between days
I and 2 at the eddy at Crash Canyon, between days 3 and 5
in the eddy at Salt Mine, between days 5 and 6 in the eddies
at Salt Mine and Carbon Creek, and between days 6 and 7
in the eddy at Carbon Creek (Table I).
Alldrews et 01. [this volume] suggested that mass failures
of more than 10,000 mJ occurred at each of the 5 eddies that
they measured and each measured reattachment bar
probably had at least one erosion event during the flood.
Howe....er. every erosion event was not necessarily a mass
failure (Table I). The onshore side of some areas of large
erosion had a scalloped shape, characteristic of head walls