section 3: debris flow initiation potential in gullies
TRANSCRIPT
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