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

Transcript of Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing...

Page 1: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 2: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 3: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 4: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Gully Erosion

Page 5: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Arroyo Development

Page 6: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Processes Driving and Resisting Erosion

Illustrated by Gary O’Brien

Page 7: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Process Restoring Erosion

Page 8: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Cut and Fill (Cataract Canyon)

Page 9: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Study Locations

Page 10: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 11: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Cross Section of Terraces in Cataract Canyon

Page 12: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Part 1

Test Hypotheses

Page 13: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Arroyo at Paria River

Page 14: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Arroyo Aggradation (Paria River)

Page 15: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 16: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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 (%)

Page 17: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 18: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 19: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Decadal Variation in 20th Century Precipitation

Webb et al. in prep.

Page 20: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 21: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 22: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

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222 mile - 1923 (E.C. LaRue photo)

Page 24: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Granite Park1963

1000 cfs(Belnap

collection)

Granite Park1996

8000 cfs(Lisa Leap

photo)

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Old Unkar Camp1963

(Belnap collections)

Old Unkar Camp1998

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Cross CanyonMarch 1999

Cross CanyonAugust 1999

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Rapid 12March 1999

Rapid 12August 1999

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Percent of sites containing 1983 and pda deposits

0

10

20

30

40

50

60

70

80

1996 1983 1996 & 1983 pda

Page 29: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 30: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Part 2

Geomorphic Model for the Small-Catchment System

Page 31: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 32: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Geomorphic Settings

Illustrated by Gary O’Brien

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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

Page 34: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

Add Geomorphic Factors to Model

Page 35: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 36: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 37: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 38: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 39: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 40: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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

Page 41: Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik.

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.