Section 3: Debris flow initiation potential in gullies

Debris flow initiation in gullies

• A slope failure (landslide) starts on the headwall, sidewall, or outside of the channel

• The failure mass enters the gully channel, channel sediment starts to move - a channelized debris flow

• About 98% of Coastal B.C. debris flows result from slope failures

Table C. Gully wall failure potential (GWFP)Surficial materialGully wall

slope angle R C M, F W, L NF>70 L H H H H

61 – 70 L M H H H50 – 60 L L M H H

<50 L L L M HEnter results in Table E

Table D. Gully geometry potential for debris flow initiationChannel gradient (%)

Gully wallslope

distance (m)

0 - 30 31 - 40 41 – 50 51 - 60 61 - 70

>20 L H H H H>15 – 20 L M H H H>10 – 15 L L M M H>5 – 10 L L L M M

0 – 5 L L L L L___ m Enter results in Table E

Table E. Debris flow initiation potentialGully Geometry Potential (Table D)GWFP (Table C)

Low Moderate HighHigh L M H

Moderate L M MLow L L L

Debris flow initiation study: objectives

• To better define the factors that affect debris flow initiation in gullies

• To develop more accurate methods of identifying gully reaches prone to debris flow initiation

Study areas

• Vancouver Island, north of Nitinat Lake

• Vancouver Island, south of Nitinat Lake

• Mainland Coast near Squamish

• Queen Charlotte Islands

Data collection - site selection

• Within an area, we chose gullies that:

1) Were logged 5 - 15 years ago

2) Had at least one slope failure

3) Had reasonable access

• In each gully, inventoried slope failures >25m2

Data collection - predictor variables

• Headwall or sidewall location

• Gully wall slope angle

• Gully wall slope distance

• Channel gradient

• Terrain type

• Soil drainage

Data collection - predictor variables con’t

• Surficial material depth and soil depth

• Initial slope failure dimensions

• Volume of debris delivered to channel

• Original slope gradient

• Failure plane slope

• Angle of entry

Response

• ChDF - the initial slope failure resulted in a channelized debris flow

• NochDF - the initial slope failure did not result in a channelized debris flow

Analytical methods

• Univariate analysis

• Logistic regression - uses continuous, ordinal and nominal variables combined

• Logistic regression ideal for a binomial response (either a debris flow initiated, or it did not)

Results

• Number of gullies assessed: 144

• ChDF: 75

• NoChDF: 211

Headwalls vs. Sidewalls

• Headwalls: 66% ChDF (39 of 59 failures)

• Sidewalls: 16% ChDF (37 of 227 failures)

Median angle of entry

ChDF NochDF

Headwall 0 22

Sidewall 67 75

Volume of debris into channelChannel gradient set at 25 degrees

0.0

0.2

0.4

0.6

0.8

1.0

10 100 1000

Debris volume into channel (m 3)

Prob

abili

ty o

f a C

hDF

N. Nitinat

3 Areas

Minimum failure sizes for ChDF

• Headwalls: 11 m3 or 33 m2

• Sidewalls: 25 m3 or 50 m2

Headwalls: Volume into channel vs. Initial volume

1

10

100

1000

10 100 1000

Initial volume (m 3)

Vo

lum

e in

to c

han

nel

(m

3) NochDF

ChDF

Sidewalls: Volume into channel vs. Initial volume

1

10

100

1000

10 100 1000

Initial volume (m 3)

Vo

lum

e in

to c

han

nel

(m

3) NochDF

ChDF

What about the GAP criteria?

• Gully wall slope angle and surficial material

• Gully wall slope distance and channel gradient

GWSA and surficial material

1: C and/or C/R. 2: M 3: C & M

Headwalls: Terrain type vs. Gully wall slope angle

1

2

3

40 60 80 100 120 140

Gully wall slope angle (percent)

Terr

ain

typ

e

NochDF

Chdf

GWSA and surficial material

Sidewalls: Terrain type vs. Gully wall slope angle

1

2

3

50 70 90 110 130 150

Gully wall slope angle (percent)

Ter

rain

typ

e

NochDF

ChDF

1: C and/or C/R. 2: M 3: C & M

Gully wall failure potential

• Headwalls1) Till slopes: failures >50%2) Colluvial slopes a few failures >60%

• Sidewalls1) Till slopes: failures >60%2) Colluvial slopes: failures >70%

Channel gradient (SW only)Debris volume into channel set at 50 m3

0.0

0.2

0.4

0.6

0.8

1.0

10 15 20 25 30 35 40 45

Channel gradient (degrees)

Prob

abili

ty o

f a C

hDF

N. Nitinat

3 Areas

Headwalls: GWSD vs. Channel gradient

0

20

40

60

80

100

20 40 60 80 100 120

Channel gradient (percent)

GW

SD (m

)

NochDF

ChDF

Sidewalls: Channel gradient vs. GWSD

0

10

20

30

40

50

60

0 20 40 60 80 100 120

Channel gradient (percent)

GW

SD (m

)

NochDF

ChDF

Does the 1995 DFIP method work?

• Gully wall slope angle and surficial material - good, needs tweaking

• Gully geometry potential for debris flow initiation - fairly good, needs tweaking

• No recognition of differences in headwalls vs. sidewalls

GAP 2001: GWFP

Table A1. Headwall failure potential (HWFP)Headwall slope angle Headwall surficial material

______ (%) R C M, F W, L FS>70 L H H H H

>60–70 L M H H H>50–60 L L M H H

<50 L L L M HEnter the results in Table C

Table A2. Sidewall failure potential (SWFP)Sidewall slope angle Sidewall surficial material

______ (%) R C M, F W, L FS>70 L H H H H

>60–70 L L M H H>50–60 L L L H H

<50 L L L M HEnter the results in Table C

GAP 2001: GGPDFI

Table B. Gully geometry potential for debris flow initiationSidewall slope Channel gradient ______(%)

distance ______ (m) <30 >30–<40 >40>15 L M H

7–<15 L L M0–<7 L L L

All headwalls M H HEnter the results in Table C