How Debris Flow Channels Refill in the Western Cascades

Arianna LambieAdvisor: Frederick Swanson

Ecosystem Informatics Summer Institute

August 2007

Why we careDebris flows change landscapes and

influence ecosystems

What influences flow impacts?

Frequency

Volume

My objectives

1) Quantify material volume

• Wood

• Sediment

2) Determine accumulation rates

3) Identify factors influencing accumulation

Methods

Study Site: HJ Andrews Forest

Data Collected

• Wood– diameter, length, orientation

• Sediment– width, length, depth, composition, obstruction

• Channel in general– Slope, width, other observations

Surveying the Channels

Surveying the Channels

Surveying the Channels

Results

Wood and sediment volumes correlate and increase over time

Relationship Between Wood and Sediment Volume

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0.00 0.20 0.40 0.60 0.80

Volume of w ood per channel length (m3/m)

Vol

ume

of s

edim

ent p

er c

hann

el

leng

th (

m3/

m) 1996

1964

>1964

Approximating Accumulation Rates

Wood Accumulation Over Time

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 100 200 300

Time since last debris f low (years)

Vol

ume

of w

ood

in c

hann

el

(m3/

m) HJ Andrew s

Coast Range

Approximating Accumulation Rates

Sediment Accumulation Over Time

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

0 100 200 300

Time since last debris f low (years)

Vol

ume

of s

edim

ent i

n ch

anne

l (m

3/m

)

HJ Andrew s

Coast Range

Large wood is a primary sediment storage mechanism

Sediment Obstructions

45%

20%

22%

9%

1%2%

2%

.5%

Large Wood

Large and Small Wood

Large Wood and Boulders(and sometimes small wood)

Small Wood

Boulders (>.64m)

Small Wood and Boulders

Hillslump or Bank alone

Shallow Slope alone

The Sediment Story

Sediment

Wood

Hillslump

Remnant debris flow deposits

Material in channel

Root throw, Soil creep, and subsequent transport

Slump earthflow

Previous debris flow

Fluvial transport

Bedrock channel slope

Forest management, Road building

Size, age, health of trees

Debris flow impacts

Debris flow frequency

Solid line = transport

Dashed = influence

Extending the Study

• Fill and rework the influence diagram– Compile data from previous studies– Continue channel data collection, plantations

• Apply math– Fit equations to accumulation rates– Quantify influence pathways

• Investigate biologic impacts– In channel– Downstream

Summary

Frequency

Volume

Important Debris Flow Characteristics

HJ Andrews Forest

• Sediment and wood accumulate faster than in Coast Range

• Wood blocks 87% of sediment

Implications for forest management that changes wood input

Piecing together the sediment budget story