short course on gullies and streams tom millard, vancouver forest region
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Short Course on Gullies and Streams
Tom Millard, Vancouver Forest Region
Objectives
• Report on recent studies
• Provide an update on GAP changes
Debris definitions
• Woody debris - “slash”
• Landslide debris - includes woody debris, rocks, soil, mud
• Debris flows and debris torrents - types of landslides
Water transport of woody debris
• Woody debris floats downstream during floods
• Small WD easier to float than large slash• Flood may also transport small or moderate
amounts of sediment• Channel bed and banks remain relatively
unchanged• Occurs in any gradient channel
Debris flow (or debris torrent)
• Debris flows almost always starts as a landslide into a gully channel
• Needs a steep channel to start in (>30%)
• Almost all wood and sediment is eroded - channel often scoured to bedrock
• Often no channel bed or banks remain
• Woody debris does not start a debris torrent
History of GAP
• Started with a district request to identify gullies that would respond badly to logging
• Became a guidebook in 1995, first formal procedure for identifying different gully hazards and risks to downstream resources
• Recognized in 1995 that the procedures were preliminary and needed to be tested
• Studies undertaken from 1996 - 2000 have resulted in changes to the GAP
Section 2: Water Transport Potential
Water transport potential assessment
• Objective: to identify channels that will be disturbed if logged, or if woody debris is left in the channel.
• Disturbance: Erosion of channel banks and bed, woody debris jams, avulsions (breakouts), transport of woody debris into fish reaches
No slash movement
No slash movement
Little slash movement
Little - moderate slash movement
Moderate slash movement
Moderate slash movement
Extensive slash movement
Severe slash movement
Severe slash movement
Severe slash movement
Old assessment method
ChannelGradient (%)
Channel Cross-section Area (m2)Channel width ____ (m) x Channel depth ____ (m) = XS Area ____ (m2)
____ CG < 0.5 0.5 - < 2 2 - < 5 5 - < 8 8 - <121
40 L M H H H20 - < 40 L L M H H8 - < 20 L L M M H
< 8 L L L M M
Van. Island & Boston Bar Slash in Streams Studies
• Objective - to identify channels that become disturbed if slash is left in the channel after yarding is completed
• Channel disturbance = high water transport potential
Vancouver Island study location
• Near Nitinat Lake, Southwest Vancouver Island
• Winter floods from rain storms
• Mostly rocky terrain, coarse grained soils
• “Nitinat”
Boston Bar study location
• All streams located with Anderson River basin, east of Boston Bar
• Almost Interior conditions
• Snowmelt dominated floods
• Deep, fine-grained till soils
• “Anderson”
Slash in Streams Study Methods
• Select streams logged 5 - 15 years ago, with the slash left in the stream
• Inventory amount and types of disturbance
• Collect data on width, depth, gradient, and other variables that may be used to predict disturbance
• Streams <5 m wide
Disturbance
• Large sizes of woody debris transported
• Large amounts of woody debris transported
• Frequent and large woody debris jams
• Channel and bank erosion
Severe disturbance
Predictor variables
• Channel width and depth
• Gradient
• Channel location
• Channel and bank types
• Size of sediment moving
Results
• Nitinat (Vancouver Island) study - 99 stream reaches
• Anderson River (Boston Bar) study - 42 stream reaches
0
10
20
30
40
50
60
canyon fan gully openslope
valleyflat
Broadlyincised
Stream Location
Nu
mb
er
of
cases
Anderson
Nitinat
0
5
10
15
20
25
30
0 - <10 10 -<20
20 -<30
30 -<40
40 -<50
50 -<60
>60
Gradient (percent)
Nu
mb
er
of
cases
Anderson
Nitinat
05
10
1520253035
404550
0 - <1 1 - <2 2 - <3 3 - <4 4 - <5
Width (m)
Nu
mb
er o
f ca
ses
Anderson
Nitinat
0
5
10
15
20
25
30
35
0 -<0.1
0.1 -<0.2
0.2 -<0.3
0.3 -<0.4
0.4 -<0.5
0.5 -<0.6
0.6 -<0.7
0.7 -<1.0
Depth (m)
Nu
mb
er o
f ca
ses
Anderson
Nitinat
0
10
20
30
40
50
60
70
0 >0 - 0.001 0.001 -0.01
0.01 - 0.03 >0.03
Bank erosion (m3/m)
Nu
mb
er
of
cases
Anderson
Nitinat
0
10
20
30
40
50
60
70
80
0 >0 - 0.001 0.001 -0.01
0.01 - 0.03 >0.03
Bed erosion (m3/m)
Nu
mb
er
of
cases
Anderson
Nitinat
0
10
20
30
40
50
60
70
SWD LWD Logs
Size of woody debris moving
Nu
mb
er o
f ca
ses
Anderson
Nitinat
0
5
10
15
20
25
30
35
0 0 -0.1 0.1 - 1 1 - 10 >10
Average Jam Volume (m3)
Nu
mb
er o
f ca
ses
Anderson
Nitinat
0
5
10
15
20
25
30
35
40
45
0 0 -0.1 0.1 - 1 1 - 10 >10
Average Sediment Wedge Volume (m3)
Nu
mb
er
of
cases
Anderson
Nitinat
Significant predictors of channel disturbance
• Channel width, depth, and area - width is best
• Size of sediment transported in the stream
Channel width and size of woody debris moved
Channel width (m)Anderson Nitinat
Woodydebris size
moved Min. Median Max Min. Median MaxSWD 1.1 1.6 2.8 .8 2 3.3LWD 2.2 2.9 5.7 1.9 3 4.6Logs 4.9 3.1 4.1 5.8
Jam volume and width: Nitinat
0
10
20
30
40
50
60
70
80
0 1 2 3 4 5
Channel width (m)
Avera
ge j
am
vo
lum
e (
m3)
0
1
2
3
4
5
6
7
8
0 1 2 3 4 5
Average Channel Width (m)
Ave
rag
e ja
m v
olu
me
(m3 )
Jam volume and width: Anderson
Sediment size and woody debris size moving
Sediment size moved (mm)Woody debris sizemoved Anderson Nitinat
Min. Median Max Min. Median MaxSWD 1 100 120 2 90 280LWD 110 200 310 45 150 360Logs 340 140 210 360
Jam volume and sediment size: Nitinat
0
10
20
30
40
50
60
70
80
0 50 100 150 200 250 300 350 400
Maximum sediment size moved (mm)
Avera
ge j
am
vo
lum
e (
m3 )
Jam volume and sediment size:Anderson
0
1
2
3
4
5
6
7
8
0 50 100 150 200 250 300 350 400
Maximum sediment size moved (mm)
Avera
ge j
am
vo
lum
e (
m3 )
What about gradient?
• Gradient has no real effect
• Steeper streams have more channel roughness
• Lower gradient streams are more able to float woody debris
Useful predictors of water transport potential
• Channel width (could also use channel area, but no better)
• Size of sediment moving in channel
• In these two studies, size of woody debris moving in channel was used as an response variable, but - can also be used as a predictor prior to harvesting
Water transport assessment and cleaning strategies
Water transport potential Low Moderate HighBankfull channel width (m) < 2 > 2 - < 3.5 > 3.5Size of water transported woody debris SWD LWD LogsLargest sediment transported (mm) < 100 > 100 - < 200 > 200
Water transport potential Cleaning strategyLow Do not cleanModerate Clean all introduced SWD and most LWDHigh Clean all logging debris except larger logs
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