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