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The Mass Wasting Prescription-Scale Effectiveness ... · PDF file The Mass Wasting Prescription-Scale Effectiveness Monitoring Project Gregory Stewart, Ph.D. Julie Dieu, Ph.D., LEG

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  • The Mass Wasting Prescription-Scale Effectiveness Monitoring Project

    Gregory Stewart, Ph.D. Julie Dieu, Ph.D., LEG. Jeff Phillips, MS. Matt O’Connor, Ph.D., LEG. Curt Veldhuisen, LEG.

    An examination of the landslide response to the December 2007 storm in Southwestern Washington

    CMER Publication 08-802 May 2013

  • Developed by the Upslope Processes Scientific Advisory Group (UPSAG)

    - Scoped in 2005

    - Study Design in 2006

    - CMER and ISPR approval completed in 2007

    Mass Wasting Effectiveness Project

  • Objectives

    1. Evaluate the effectiveness of Forest Practices Rules at reducing sediment delivery to public resources.

    2. Identify prescription-scale management- related factors that might be used to improve unstable slope identification and mitigation efforts.

  • Compare landslide rates under different management scenarios. - Did not evaluate administrative

    components: FPA classification, geotech reports, SEPA, etc.

    Requirement: A population of

    landslides in an area subject to Forest Practices Rules

    Study Outline

  • December 2007 Chehalis storm

    Elaine Thompson The Associated Press Source: http://blog.oregonlive.com/oregonianextra/2008 /11/remembering_the_big_storm.html

  • Aerial reconnaissance

    Mass Wasting Effectiveness study area

    * WDNR aerial reconnaissance

    Study required at least one landslide per sq. mile.

  • Range of rainfall intensities

    Snow followed by 4 – 11 inches of rain in 24 hours.

  • Factors affecting slope stability

    • Slope

    • Precipitation

    • Lithology/Soil

    • Management / Vegetation

    • Others

    Why we used a block design

    Source: http://www.geology.enr.state.nc.us/Landslide_Info/Landslides_background.htm

  • Comparisons within randomly selected blocks

    Random block

  • Field survey of all landslides in the sample areas that delivered to streams, and all road- related landslides.

    Other landslide initiation points were counted when encountered.

    Landslide detection

  • • 91 sq. miles of managed forest and 555 miles of road.

    • Most of the landslides (96%) were debris slides or debris flows.

    - No glacial deep- seated landslides.

    Landslide Inventory

    0

    200

    400

    600

    800

    1000

    1200

    Road Non-road

    No delivery

    Delivery

  • Harvest Treatments

    Full Buffer (more stable?)

    Submature (more stable?)

    Mature (more stable?)

    Partial Buffer (less stable?)

    No Buffer (least stable?)

    Age 40+ Age 0-20 Age 20-40

    1987 1967

  • Convergent headwall Bedrock hollows

    Inner gorge

    Slope > 70%

    Drawing: Jack Powell, DNR, 2003

    0-20 treatment based on buffering of RIL

  • Harvest units in which trees on RIL (if present) were not harvested

    Full Buffer (FB 0-20)

  • Harvest units where some harvest

    Partial Buffer (PB 0-20)

    and some buffering of RIL occurred

  • All RIL, if present, were clearcut

    No Buffer (NB 0-20)

  • Forest stands between 21 and 40 years old

    Submature 21-40 (SM)

  • Forest stands greater than 40 years old

    Mature 41+ (M)

  • 0

    2

    4

    6

    8

    10

    12

    14

    FB 0-20 PB 0-20 NB 0-20 SM 21-40 M 41+

    Harvest treatment results

    Letters indicate statistically significant differences at α=0.1 . Error bars are 90% CI.

    �̅�𝑥 = 5.7 �̅�𝑥 = 9.6 �̅�𝑥 = 7.7 �̅�𝑥 = 6.2 �̅�𝑥 = 5.0

    B,C A A,B B,C C

    𝑎𝑎𝑎𝑎𝑎𝑎 = 3.3 𝑎𝑎𝑎𝑎𝑎𝑎 = 5.8 𝑎𝑎𝑎𝑎𝑎𝑎 = 12.7 𝑎𝑎𝑎𝑎𝑎𝑎 = 36.8 𝑎𝑎𝑎𝑎𝑎𝑎 = 75

  • Did we get as many landslides in Full Buffer (FB) as we would have expected?

    No

    Conclusion

    “Results support the hypothesis that the avoidance of clearcut harvest on unstable terrain reduces the density and volume of landslides.”

  • Road results were largely inconclusive.

    Roads

  • Road abandonment did appear to be effective at reducing landslide volume.

    Formal abandonment

  • • A sizable proportion of delivering landslides originated from terrain that did not fit the definition of any named RIL.

    Other notable findings

  • • Field crews identified contributing factors at only a few landslide initiation sites.

    • Landslides originating in buffers delivered

    significantly more LWD than landslides outside of buffers.

    Other notable findings

  • Study blocks were largely commercial forest.

    Note: Public Resource vs Public Safety

    • Low potential for public safety issues.

    • Study focused

    on initiation, not run-out.

  • There are better data for evaluating the effect of landslides on public safety.

    Public Safety

    Source: www.dnr.wa.gov/Publications/ger_ofr2009-1_landslide_field_trip.pdf

    Slide Number 1 Mass Wasting Effectiveness Project Objectives Study Outline December�2007 Chehalis storm Aerial reconnaissance Slide Number 7 Why we used a block design Comparisons within randomly selected blocks Landslide detection Landslide Inventory Harvest Treatments Slide Number 13 Full Buffer (FB 0-20) Partial Buffer (PB 0-20) No Buffer (NB 0-20) Submature 21-40 (SM) Mature 41+ (M) Harvest treatment results Slide Number 20 Roads Formal abandonment Other notable findings Other notable findings Note: Public Resource vs Public Safety Public Safety

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