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THE CURRENT STATE OF ENGINEERING GEOLOGY, SLOPE STABILITY AND HARVEST UNIT PLANS INTERNATIONAL MOUNTAIN LOGGING AND 11TH PACIFIC NORTHWEST SKYLINE SYMPOSIUM Jeffrey R. Laird, C.E.G. Shannon & Wilson, Inc. Seattle, Washington

EXISTING PUBLIC REGULATIONS RELATING TO HARVEST UNITS AND SLOPE STABILITY CALIFORNIA OREGON WASHINGTON

CALIFORNIA Z’berg-Nejedly Forest Practice Act adopted in 1973. Act requires a THP to be prepared by a RPF for approval by California Dept. of Forestry and Fire Protection. The THP must include measures to avoid accelerated erosion near streams and/or special provisions to protect unique areas. An engineering geologic report may be required as part of the THP. This report can only be prepared by a geologist or engineer who is registered in California.

OREGON Nation’s first Forest Practice Act adopted in 1972. Standards were set for reforestation, road construction and streamside buffers. Enforced by Oregon Dept. of Forestry. Comprehensive revision of rules for harvest and roads was completed in 1983 following severe winter landsliding. Concept of ‘high risk sites’ and ‘written harvest plans’ for streams and wildlife was introduced in these revisions. A rule was adopted in July 1999 to provide for public safety below steep, unstable, forested slopes.  Engineering geology report may be required for these high risk sites. Practice of geology and engineering requires state registration.

WASHINGTON Forest Practices Act enacted in 1974. Timber Fish Wildlife (TFW) established in 1987. Forest Practice Application is required for most forest operations and is reviewed by Dept. of Natural Resources. Watershed analysis program adopted in 1992 to improve FPA process and address ‘cumulative effects’. Forest and Fish permanent rules adopted July 2001 Units with “potentially unstable slopes or landforms” can be designated as Class IV-Special.  FPA must include information prepared by a ‘qualified expert’ regarding the unstable slopes and proposed mitigating measures. Qualified expert approved by DNR. Practice of geology and engineering requires state registration.

APPLICATION OF ENGINEERING GEOLOGY TO HARVEST UNIT PLANNING Field reconnaissance-level, engineering geologic evaluation of existing natural and human-influenced conditions, with consideration of impacts from past and potential future forest practices.

FACTORS TO CONSIDER IN ENGINEERING GEOLOGIC EVALUATION Topography- steep, convergent and/or unusual topography, drainages Geology- rock type, strength, structure, stratigraphy Soils- engineering properties, depths Forest Slope Hydrology-climate, bedrock, topography, drainage network, slope seepage, ET, interception Past and Current Forest Harvest Practices - most state/commercial forests have been previously logged with much less sensitive methods Past Slope Instabilities- where and why Deliverable Hazards - public resources impacted, risk to humans Required Leave Areas - streams, wildlife and green-up trees, HCPs

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SLOPE STABILITY MODELING Generally use limit equilibrium methods (ratio of driving to resisting forces) to calculate Factor of Safety (FS). May be deterministic (one FS value) or probabilistic (range of FS values). Assume groundwater is steady-state and controlled by topography. Most models utilize infinite slope stability model. May be site-specific or distributed (GIS-based).

INFINITE SLOPE STABILITY MODEL

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FIELD REVIEW   Most important element of evaluation - aerial photographs and maps are too large scale, not current   Field visit should include: - identified areas of past instability - high to moderate risk areas identified in slope stability studies - steep, converging slopes; inner gorges - stream channels - proposed road alignments and landings - slopes underlain by weak bedrock, faults and contacts - recently harvested areas in vicinity with similar topography   Include forester or designate unstable sites in field

FIELD REVIEW  Soil and rock characteristics  Seepage, phreatophyte vegetation  Swooped, jack-strawed and pistol-butted trees  Rapid creep: loose soil, deep soil deposits on uphill side of trees  Deep-seated movement - cracks, bulges, hummocks,    benches, sag ponds, deranged drainage, back-tilted benches, scarps  Incised channels, eroding stream banks  Measure range of slope gradients  Characterize both stable and unstable ground

MITIGATIVE MEASURES  Unlike mitigative alternatives for civil structures, there are not a lot of options for harvest units - the trees are either cut or they are not.  How much impact will harvesting have on slope?  Required stream channel buffers of varying widths commonly include marginally-stable ground.  Designate leave areas on and adjacent to marginally-stable ground.  Partial or selective harvests.  Reduce yarding impacts - full suspension, seasonal restrictions.

CONCLUSIONS Frequency of harvest-related landslides has been significantly reduced.  - Regulations  - New forest practices  - Engineering geologic reviews  Landslides are a natural phenomena.  - Always a risk of slope instability.  - Determining the cause - natural or result of harvest - is problematic.  - Timber removal will likely be blamed.  Limited mitigation measures  - Beneficial effects of current measures need further study.

CONCLUSIONS  GIS-slope stability models  - A useful tool, but simplifies complex conditions.  - Too time-consuming for harvest unit scale, use at watershed scale.  - Must compare/calibrate model with actual landslide occurrence.  - Models only consider shallow landsliding.  - Should not be considered a substitute for engineering geologic field evaluation.  - Required leave areas should not be designated solely on model output.

CONCLUSIONS  Historic aerial photographs are very useful and irreplaceable - treat them better than gold!  Most (70-80%) forest-practice-related landslides are due to roads - comparatively more effort should be invested in proper location, construction and maintenance.  Role of root strength - under current forest practices, may be overestimated.  Role of forest slope hydrology - may be underestimated.

CONCLUSIONS  Practical (read dollars) limitation to extent of harvest unit evaluation.  Engineering geologist provides an opinion based on training, experience, personality and company policy.  Landowner/manager and regulatory agency must decide on level of acceptable risk.

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