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Timber Harvest and Road Impacts on Landslides and Sediment Delivery

Timber Harvest and Road Impacts on Landslides and Sediment Delivery. David O. Wallin Department of Environmental Sciences Huxley College of the Environment Western Washington University Bellingham, WA david.wallin@wwu.edu 360-650-7526. The forests of the Lake Whatcom Watershed then.

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Timber Harvest and Road Impacts on Landslides and Sediment Delivery

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  1. Timber Harvest and Road Impacts on Landslides and Sediment Delivery David O. Wallin Department of Environmental Sciences Huxley College of the Environment Western Washington University Bellingham, WA david.wallin@wwu.edu 360-650-7526

  2. The forests of the Lake Whatcom Watershed then..

  3. …and now

  4. Timber Harvest Impacts • Tree removal • Roads

  5. Forest Hydrology: the basics

  6. Forest Hydrology: the basics • In a forested environment, with no disturbance, surface runoff is minimal.

  7. Forest Hydrology: the basics • In a forested environment, with no disturbance, surface runoff does not occur • Evapotranspiration (water loss to the atmosphere) accounts for about 50% of total annual precipitation

  8. Forest Hydrology: the basics • In a forested environment, with no disturbance, surface runoff does not occur • Evapotranspiration (water loss to the atmosphere) accounts for about 50% of total annual precipitation • Tree removal significantly reduces water loss to the atmosphere

  9. Forest Hydrology: Effects of Timber Harvesting • Tree removal reduces evapotranspiration resulting in much higher soil water content: • Increased mass of soil profile • Higher pore water pressure increases lubrication • Following tree removal, residual roots decay resulting in reduced support for soil profile • Net result is an increased risk of landslides

  10. “Hydrologic Maturity” • These recently harvested stands are referred to as being “hydrologically immature.” • Based on the DNR, the effects described above persist for about 40 years following a timber harvest. • DNR manages their timber land on a 60 year rotation. • This means that, on lands that are actively managed for timber production, about 2/3rds (40/60) of the landscape will have an increased risk of landslides at any given time.

  11. Hydrologic Maturity: How much area is “at risk.” • The area proposed for reconveyance includes about 8400 acres. Of this, about half is currently “off-limits” to timber harvesting under the Lake Whatcom Landscape Plan. Hence about 4200 acres are actively managed for timber production. • This means that, under the landscape plan, about 2700 acres (2/3rds of 4200) within the reconveyance area would always be maintained under conditions that have an elevated risk for landslides.

  12. Roads and Flow Routing • A: Subsurface flow interception • B: Surface flow on roads • C: Flow routing along ditches to stream • D: Flow along roads to stream • E: Flow from relief culverts can deliver water to streams

  13. Roads and Slope Failure • A: Cutslope slides • B: Fillslope slides • C: Debris flow that pass roads • D: Fillslope or culvert failures that become debris flows

  14. By how much is the risk increased? • Over 30 years of research has shown that timber harvesting and roads increase the risk of landslides by a factor of 2X to well over 100X. • Forest practices are constantly evolving and many changes have been implemented to reduce the risks. • Even the most recent publications (2007) document elevated risk even with improved timber management practices

  15. How will reconveyance help? • No new timber harvesting. Gradual elimination of hydrologically immature stands. • Approximately 20 miles of new roads planned by DNR under the current landscape plan will not be constructed. • Approximately 9 miles of existing roads will be decommissioned.

  16. What about Recreational Impacts Associated with Reconveyance? • There is no scientific literature to support the contention that recreational impacts will in any way come close to the known and well documented effects of timber harvesting. • There will be some minimal impacts associated with traffic and trail use but the current park plan takes steps to mitigate these impacts.

  17. Slides triggered on slopes above buffered stream. Note debris flow of sediment has passed through trees above and flowed down the stream channel.

  18. Area of previous photograph. Path of debris flow further down the slope. Small slides can cause huge sediment loading.

  19. This slide destroyed two homes

  20. Slides along logging road Older slide

  21. Slides along logging roads triggered a debris flow Note multiple failures in young stand, but none in older mature stands.

  22. End

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