Modeling Debris-Flow Sed

Transport in Grand Can

Robert H. Webb, U.S. Geological Surve

For reprints: rhwebb@usgs.gov
History of Research Pro

GCES I GCES II 

1980 1985 1990 1995 

Webb’s 1st river trip

First funded 

debris-flow project

First debris-flow report 

(Webb et al., 1989)

Inventoried debris-flow activity in 

40 tributaries (Melis et al., 1994) Debris-fa

reworkinduring flo

Prospect Canyon 

debris flow

Co

COLLABORATORS:Thure Cerling (U of Utah)James Pizzuto (U of Delaware)John Elliott (USGS)

M.S. STUDENTS:

Peter Griffiths (1990-96)Dan Hartley (1998-00)Tillie Klearman (2000-2002)Andy Persio (2002-present)

P

TC

Monitor and study the contribution of coarse-grained sedim

within Grand Canyon from all tributaries and evaluate 

Purpose of Research Pr
What is a Debris Flow

• Debris flows are slurries of poorly s

and water

• Particles range from clay to boulder

b-axis diameter

• Typical moisture contents reconstitu

Grand Canyon debris flows range fr(n=49)

• Three types of flood hydrographs w

flow component occur in Grand Can

Ref: Webb et al., 1989, USGS Prof. Paper 1492
SIGNIFICANCE OF DEBRI

• Debris flows threaten humans 

and structures

• Debris flows create rapids and 

eddies that are efficient sand traps and create beaches

• Debris flows add boulders and 

cobbles to the river that form substrate for aquatic plants

• The occurrence of debris flows 

influences the types of native fishes present in the river 

90 92 

Ref: Webb et al., 1989, USGS Prof. Paper 149
Significance of Debris F

Ref: Webb et al., 1989, USGS Prof. Paper 1492
Debris Flows and Bedrock 

• The weak

contribute matrix

• Hermit Sh

are imporcolluvium failures

• Boulders 

form rapi

Kaibab 

Limestone

Redwall 

Limestone

Hermit 

Shale

Supai 

Group
Initiation of Debris Flo

• Single-layer clays (e.g., kaolinite) produ

flows than multilayer clays (e.g., smectit

• Four initiation mechanisms• Rainfall intensities >1 in./hr • Intense storms, not wet seasons

0 10 20 30 40 

Bedrock Failure

Firehose Effect

Colluvial Wedge Failures

Combination of Failures

Percent of Debr

Ref: Webb et al., 2003
Distribution of Historical Debris Flows (

Ref: Webb et al., 2000, WRIR 00-4055

Figure 11 from Webb et al., 200
How do you study rare events when f

watching or measuring?

1. Stratigraphy (14C, 

3He, other datin

2. Historical diaries (1869-1964)3. Direct observation (1984-2002) 4. LIDAR remote sensing (2000)5. Repeat photography (1871-2002)

Frequency of Debris Fl
Stratigraphic Analyses of Deb

• It is very difficult to recognize individual debr• Stratigraphy is self-censoring and does not rec

Ref: Hereford et al., 1996, GSA Bull.; Hereford et al., 1

Res.; Webb et al., 1999, USGS Prof Paper 159

• Oldest known flow, 

(

3He

c), in Whitmore 

189). This debris flothe river.

• Largest known debr

river is at Lava Falls 3,000±500 years (

3H

• Many debris flows d

300 – 5,400 years u
Historical Observations: The 

• Human observations are unreliable when they 

recognize complex phenomena like debris flow

• Some people saw important events but in gene

recognized changes in debris fans

• Other changes, such as increases in riparian ve

erosion, were accurately identified

• Examined 67 diaries (1872-1964)• Sponsored “Old Timers” float trip 

in 1994

Bottom Line:
Historical Observations: 

• The earliest riv

Colorado salmand humpback 

• Dynamiting fo

common in GlFerry, and nea

• At first, dynam

pikeminnow. Lyielded only ca

• Trout, first sto

the 1920s, werbefore Glen Cacompleted.
Historical Observations: The 

• Many sandbars used on pre-dam river trips 

present (e.g., downstream from Tapeats Cre

• The largest change they saw in Grand Cany

of sand bars downstream from Nankoweap 
Observed Debris Flows, 19

• From 1984-2002, a total of 95 de

were observed in Grand Canyon 

• 9 increased the severity of existi• 7 changed existing riffles into ra• 3 created new riffles
Observed Debris Flows, 19
1923 Birdseye Expedition 2000 LIDA

Geomorphic Change Detection in Gra

Comparison of 2000 LIDAR and 1923 

Ref: Magirl et al., in preparation

Grand Canyon water-surface profile has been mea

1. By Colonel Claude Birdseye on the 1923 US2. Through analyzing data from the 2000 LIDA
Direct Comparison of Water-S

Profiles Over 77 Years

• Quantify change in water-surface elevation from 19• Detect the presence of previously unknown debris f• Calculate new set of geomorphic statistics for Gran

Ref: Magirl et al., 2002, GSA Abstracts

700710720730740

90 92 

2000 Lidar1923 Survey

Water-surface Elevation (m)

Granit

900910920930940

8 10 12 14 16 18 20

2000 Lidar1923 Survey

Water-surface Elevation (m)

River Mile

House Rock Rapid

Soap Creek Rapid

Badger Rapid

Upper Marble Canyon 
Largest Rise at Head of a 

House Rock Rapid, mile 16.8

1923 

Net Rise

910912914916918920

16 17 18 19 20

1923 Survey 2000 Lidar

Water-surface Elevation (m)

River Mile

Redneck Rapid (1.20 m)

House Rock Rapid (1.83m)

18-Mile Wash (1.08 m)
Detection of Previously Unknown D

• 1890: Stanton reports a 8-10 foot dro• 1923: Birdseye measures a 1 foot dro• 1940: Doris Nevills swims an enlarge• 2000: LIDAR measures a 5 foot drop

The riddle of Doris Rapid (mile 1

580582584586588590

136 

12

Water-surface Elevation (m)

1. Debris flow occurs between 

1884-1890

2. The 220,000 ft

3/s flood in 

1921 reworks the first deposit

3. A second debris flow occurs 

between 1923-1940

Possible Explanation:
Geomorphology of the R

Luna Leopold (1969) states ...50% of total decrea

place in only 9% of the total river distance...[based 

New estimate, based on 2000 LIDAR profile: 66% of drop in 9% of distance

0

20406080

100

0 20 40 60 80 

PERCENTAGE OF TOTAL FALL IN ELEVATION

PERCENTAGE OF TOTAL DISTANCE

Leopold Estimate, 196

LIDAR (2000) Estim

66%

50%

9% of total river distance

Ref: Magirl et al., in preparation
Repeat Photography and Deb

• Matched 1,365 photos showing debris-flow • Earliest photo—1871; Most useful group—• 113 debris flows at 160 tributaries (1890-19• Extrapolating...4.6 debris flows per year (1

Ref: Webb 1996; Webb et al., 1999, USGS Prof 

Matched set from cover of 

Century of Change

Matched 

Centu
Crystal Rapid

1966 Debris Flow Effec

1890 

Ref: Webb 1996
Logistic Regressio

• Repeat photography captures 160 trib

mouths, 1890-1990

• We record 94 tributaries that had at le

debris flow (~60% of tributaries)

• We analyze debris-flow occurrence a

categorical data with 22 geologic and variables

• We calculate debris-flow probabilitie

significant variables

Debris Flows and Native 

• Areas where humpback chub are mos

are areas with highest debris-flow fre

• In 1993, radio-tagged humpback chub 

attracted to the newly aggraded debriTanner Rapid.

• Debris flows occur in Little Colorado 

canyon and could block upstream migthe chub if a large event occurs in fut
Debri

Little 

• 74 tribut

Blue Sprflow pro

• LCR trib

those aloshould gof coarse 

• LCR cha

that of thsusceptibflows.

• Debris fl

in reach Confluen
Based on substantial data on debri

developed predictive models to pre

changes in Grand Canyon rapids (ha

1. Debris-flow frequency model (logisti2. Sediment-yield model for Grand Can3. Particle-size distributions for debris f4. Debris-fan reworking

MODELING DEBRIS-F

SEDIMENT YIELD
10 2

10 310 410 5

0.1 1 10 100 1000

Debris-Flow VolumesV

max

 = 11808 . A0.294, R2 = 0.89

V

avg

 = 5728 . A0.278, R2 = 0.47

Debris Flow Volume (m

3

)

Drainage Area (km 2)

Ref: Webb et al., 2000, USGS WRIR 00-4015

Debris-Flow Sediment-Yield M

Grand Canyon Tributa

Qsdf = 0.

whereQsdf = se

F[π(x)] = tha, b = em0.17 is a 
Modeling Coarse Sediment 

• Debris-flow sediments, on average, a

boulders, 65% gravel and cobbles, an

• Model predicts sediment inputs into t

based on long-term averages.

• This model could be used to predict (

reworking component) where gravel waccumulate in Grand Canyon

Ref: Melis, 1997, Ph.D; Webb et al., 2000, USGS WRIR 
Sediment Yield of 

Grand Canyon Tributa

Seven geomorphic reaches

Reach B: 25% of Paria inputs
River Re

• Glen Canyon D

1963

• Pre-dam flood

removed all paaxis diameter)

• Post-dam flood

move smaller pft in diameter

• Particles now e

instead of the s

Ref: Webb et al., 2000, USG
Reworking of Aggraded Debris Fans (t

Lava Falls Rapid. A. March 25, 1996. B. April 6, 1996. The rapid widened by about 60 ft by reworking of 1995 debris-flow deposits.

Ref: Webb et al., 1999; Pizzuto et al., 1999
Reworking of Debris Fan at Gr

9,0009,2009,4009,6009,800

10,00010,20010,40010,600

Entire fanNon-sand area

Date

Photogrammetric analysis using ER
Future Work: 1-D Hydraulic M

900000 

27002750280028502900

Entire Co

M

Elevation (ft)

31103120313031403150316031703180319032003210

0 100 200 300 400 500

• Based on 1923 and som• Limited to 30,000 ft

3/s 

• Most cross sections we

trapezoids

Randle and Pemberton (1987) STAR

Converted into HEC-RAS working m

• Entire river length modeled• Uses STARS cross sections

(still based on 1923/1984 data)

• Ultimate goal is modeling of 

gravel and coarse particle transport for conceptual modeling
Improved Hydraulic Model using GC

Mark Gonzales, GCMRC Survey

Mike Breedlove, GCM

2002 Bathymetry 2002 LIDA

State-of-the-Art 1-D Hydrauli

HEC-R

Inundation to PDebris fan rewCombine w/ DF 

GSTARS

Gravel transportFish spawningSand storage
Integrated Science

-4.0 Bar 

• 1-D and 2-D modeling in Glen and Marble Camanagement of salmonids and suitability of fish • Use results to drive conceptual model by EcomKorman).
Conclusions

• As Alan Howard and Robert Dolan predicted, 

profile through Grand Canyon is becoming an drop profile as a result of operations of Glen Chas “good” aspects (traps more sand for beachchallenging rapids) and “bad” aspects (more hnative fish).

• Modeling of sediment transport by episodic ev

flows is possible with sustained research effor

• Reworking of aggraded debris fans is possible 

powerplant releases from Glen Canyon Dam (flood). More frequent floods would move both boulders.

• We don’t know why, but native fish are related 

frequency. More research by fisheries scientist