development of a geomorphic model to predict erosion of pre-dam colorado river terraces containing...
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Development of a Geomorphic Model to Predict Erosion of Pre-Dam
Colorado River Terraces Containing Archaeological Resources
Kate Thompson and Andre Potochnik
Principal Investigators
SWCA, Inc. Environmental Consultants
and
Gary O’Brien, Ron Ryel, and Lynn Neal
Problem
Apparent rapid gully erosion of pre-dam terraces during the past two
decades has caused loss of numerous cultural sites in Grand Canyon river
corridor
Objectives of Study
• Test hypotheses:– Has erosion increased in the post dam period?– If so, is erosion climate driven or dam-related?
• Develop a model that predicts relative vulnerability of sites to erosion
Gully Erosion
Arroyo Development
Processes Driving and Resisting Erosion
Illustrated by Gary O’Brien
Process Restoring Erosion
Cut and Fill (Cataract Canyon)
Study Locations
Comparison of Furnace Flats to Cataract Canyon
Characteristic Cataract Canyon Furnace Flats
River gradient 2.42 1.95
Reach length 11 13
Elevation 1158 807
Mean annual precipitation 220 233
Mean monsoon season precipitation 86 97
Percent of months with > 50mm precip 9.1 9.0
1957 flood stage 101,000 120,000
Cross Section of Terraces in Cataract Canyon
Part 1
Test Hypotheses
Arroyo at Paria River
Arroyo Aggradation (Paria River)
Null Hypothesis: Degree of gully erosion has remained unchanged
from the pre-dam to post-dam period
• Test 1 - Use air photos to determine the degree of channel lengthening since 1965
• Test 2 - Compare amount of gully erosion in Cataract Canyon (control section) to Furnace Flats section in Grand Canyon
Channel Lengthening Over Timen = 23
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
6/15/61 6/15/69 10/20/76 10/20/80 10/10/88 4/5/92
Calendar Year
Rel
ativ
e E
xten
t o
f D
egra
dat
ion
arroyos fully formed (%)
arroyos partly formed (%)
arroyos absent (%)
Gully Density/Depth of 3 Paired Areas
Sitescompared
Length ofArea (m)
Number ofcatchments
Gullydensity
Mean gullydepth
#1Palisades CkCross Cyn
4242
32
0.070.05
0.680.83
#2Upper UnkarRapid 12
112108
84
0.070.04
0.740.73
#3Tanner CynRapid 4
11283
43
0.040.04
2.850.20
Climatic Variation Hypothesis: High precipitation anomalies in the
post-dam period increases severity of gully erosion
• Test 1 - Evaluate previous research on variation of 20th century precipitation
• Test 2 - Investigate variation in monsoon season rainfall at equivalent time periods before and after closure of the dam
Decadal Variation in 20th Century Precipitation
Webb et al. in prep.
Monsoon Precipitation for 13 Weather Stations,
Colorado River Corridor (> 50 mm / month)
R2 = 0.89
0%
10%
20%
30%
40%
50%
60%
0% 10% 20% 30% 40% 50% 60%
PERCENT OF MONTHLY PRECIPITATION EXCEEDING 50 MM, PRE-DAM (1932-1963)
PE
RC
EN
T O
F M
ON
TH
LY P
RE
CIP
ITA
TIO
N E
XC
EE
DIN
G 5
0 M
M,
PO
ST
-DA
M (
1964
-199
5)
Lake Powell - Canyonlands
K-M Highlands
Western Grand Canyon
Lower Colorado River
Line of equal events
Monsoon Precipitation for 13 Weather Stations, Colorado River Corridor (>25 mm/day)
R2 = 0.30
0
5
10
15
20
25
30
35
40
45
0 10 20 30 40
NUMBER OF DAYS EXCEEDING 25 MM, PRE-DAM (1932-1963)
NU
MB
ER
OF
DA
YS
EX
CE
ED
ING
25
MM
, P
OS
T-D
AM
(1
96
4-1
99
5)
Line of equal events
Base-Level Hypothesis: Reduction of sand supply and large
floods in the post-dam period increases degree of gully erosion
• Test 1 - Report on rebuilding of high-elevation sand bars in both Grand Canyon and Cataract Canyon
• Test 2 - Assess catchment and river processes at each study site in Grand Canyon
222 mile - 1923 (E.C. LaRue photo)
Granite Park1963
1000 cfs(Belnap
collection)
Granite Park1996
8000 cfs(Lisa Leap
photo)
Old Unkar Camp1963
(Belnap collections)
Old Unkar Camp1998
Cross CanyonMarch 1999
Cross CanyonAugust 1999
Rapid 12March 1999
Rapid 12August 1999
Percent of sites containing 1983 and pda deposits
0
10
20
30
40
50
60
70
80
1996 1983 1996 & 1983 pda
Sites Supporting Base-Level Hypothesis
47%
31%15%
7%
not supported
weakly supported
supported
strongly supported
n = 119
Geomorphic ProcessNo Evidence Of:
Pre-dam arroyo-cuttingCutbank retreat by riverSide-canyon erosion
Score
111
Sandy Deposit Present:
Pda and 1983Active eolian sand
Maximum ScorePossible
11
51
< 3 not supported= 3 weakly supported
= 4 supported= 5 strongly supported
Part 2
Geomorphic Model for the Small-Catchment System
Steps to Building Geomorphic Model
• Classify catchments by geomorphic setting• Construct process-based conceptual model
• Construct predictive mathematical model
• Use model to predict vulnerability of individual sites
Geomorphic Settings
Illustrated by Gary O’Brien
Mathematical Model - Step 1
• Quantify driving and resisting parameters
• Q = C*I*A (Am. Soc. Civil Engineers) Total runoff (m3 ) upper catchment
• Axt = Wt * Dt Cross-sectional area of terrace segment
Add Geomorphic Factors to Model
Mathematical Model - Step 2
• Vr = ln(Q)/ln[Axt * (1+TF)]
Vr is raw vulnerability
• FVCi = (Vr * FVCi-1)/100
FVCi is cumulative vulnerability
thus: vulnerability rating of archaeological terrace = Vr of highest terrace
and: vulnerability rating/catchment = mean FVCi
Vulnerability Plot
0
0.2
0.4
0.6
0.8
1
1.2
0 20 40 60 80 100
Vulnerability of top terrace
Gu
lly
dep
th r
atio
Grand CanyonCataract Canyon
n = 128
Threshold line
Mean Vulnerability by Geomorphic Setting
0.00
10.00
20.00
30.00
40.00
50.00
60.00
alluvial fan tributaryplain
talus slope debris lobe dune field
Geomorphic Setting
Rel
ativ
e V
alue
s of
Cat
chm
ent A
rea
and
Vul
nera
bilit
y
mean area (1000 m2)
% mean vulnerability
Conclusions(hypotheses testing)
• Gully erosion in terraces is more severe from 1978-1999 than 1942-1977
• Gully erosion is more extensive in Grand Canyon today than in Cataract Canyon control site.
• Gully erosion is increased due to both a high precipitation anomaly and a decrease in sediment renewal.
• 78% of channels draining archaeological sites show most elements of restorative base-level process
• Eolian redistribution of fresh flood sand is on-going at about 50% of catchments
Conclusions(the predictive model)
• Process-based model works best for this small-catchment geomorphic system– it simplifies enormous variety and complexity of
small catchments
• Statistically based model does not work well– poor correlation of gully depth/width to most
measured parameters
• Highest vulnerabilities are function of large catchment area and narrow terrace width
Recommendations
• Site mitigation achieved by slowing erosion:– decrease stream power
– increase terrace diffusivity
• Data recovery suggested at sites where:– outliers occur on plot
– gully-depth ratios are close to 1.0
– there are few base-level controls
• Use vulnerability plot to:– identify high risk sites
– use as a base to track how points shift in future
Future Work
• Integrate mathematical model results with mainstem studies (Wiele, 2000).
• Refine model through application and observation.
• Quantify drainage density of uppermost terrace: could be more important than gully depth and width.
• Eolian studies: quantify redistribution of sand.• Repeat historic photography.