Windthrow Hazard Assessment and Management Terry Rollerson Golder Associates Ltd.
Windthrow HazardAssessment and Management
Terry Rollerson
Golder Associates Ltd.
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Workshop outline
• Terminology / windthrow damage• Mechanics• Factors affecting windthrow• Hazard assessment / classification / risk• Management strategies • Monitoring and operational trials• Case studies• Discussion groups
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Terminology
• Windthrow– rotational falls– hinge falls
• Breakage– stem break– stock break
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Terminology
• Endemic windthrow– occurs regularly on a small scale – individual trees or groups of trees
• Catastrophic windthrow– occurs infrequently when exceptionally
strong winds cause widespread and extensive damage to large areas
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Terminology
• Windthrow hazard– The likelihood or expected severity of
endemic windthrow
• Windfirmness– the ability of a tree to resist overturning
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Silvicultural systems
• Edges dominant:– Clearcutting– Clearcutting with
reserves– Group retention– Strip shelterwood
• Spatial pattern dominant:– Single tree selection– Group selection– Irregular shelterwood– Strip shelterwood– Uniform shelterwood
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Variable Retention Cutting Patterns
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Windthrow Mechanics
• Torque (Fihi)
• Wind force Ai CDi ui2
• Gravitational force mixig
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Terminology
• Sway period– amount of time required for a tree crown to move
though a complete sway
• Sway amplitude– the distance the tip of the crown moves from the
vertical to its outermost sway point
• Damping– dissipation of energy in a tree through movement
and contact of branches, stem and roots
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Basic truths
• The smaller the perimeter/area ratio the less windthrow per unit area logged
• Stumps don’t blow over
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Windthrow hazard classification
• Wind force factors
• Resistance to overturning
• Other indicators
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Windthrow hazard
Appliedforces
Resistance tooverturning
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Wind force factors
• High hazard– topographically
exposed locations– windward boundaries– tall trees– large dense crowns– heavy removal
• Low hazard– topographically
protected– lee boundaries– short trees– small open crowns– light removal
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Resistance to overturning
• High hazard– trees with low taper
and no butt flare– shallow rooting
(<0.4m)– shallow soils– poorly drained soils
(low shear strength)– root rot areas
• Low hazard– trees with high taper
and large butt flare– deep rooting (>0.8m)– deep soils– well drained soils– coarse soils that
favor deep rooting
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Other indicators
• High hazard– moderate to
extensive natural windthrow present
– extensive windthrow on similar logged areas nearby
– pit and mound micro topography
• Low hazard– no natural windthrow– no windthrow in
similar logged areas nearby
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Windthrow hazard classWindforcehazard L M H
Overturninghazard
L L LM M
M LM M H
H M H VH
Otherhazardindicators L M H VH
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Windthrow risk assessment
• Risk = Hazard x Consequence– Hazard =
• likelihood or severity of windthrow
– Consequence =• life / safety• erosion - stream channel damage• landslide activity• fish habitat damage• timber damage
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Windthrow risk rank
Windthrow hazard LL
MM
H VHConsequence
L L L LM M
M L M MH H
H LM MH H VH
VH M H VH VH
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Windthrow hazard assessments
• Office review:– air photos, maps, local knowledge
• Falling boundary traverses - edges
• Traverses within cutting area - spatial
• Collection of relevant field data
• Synthesis of all data
• Development of management options
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Windthrow hazard mitigation
Minimize appliedforces
Maximize Resistance tooverturning
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Management strategies
• Select
• Avoid
• Remove
• Retain
• Modify
• Induce change
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Windthrow management
• Minimize total edge• Minimize edge on
high hazard sites• Feather windward
boundaries• Top/prune high risk
stems• Limit % removal on
high hazard sites
• Edges• Edges
• Edges
• Edges/Spatial
• Edges/spatial
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Windthrow management
• Remove vulnerable stems and stands
• Maintain smooth edges and canopies
• Retain stable windthrown edges
• Plan for salvage• Vary treatments
locally & monitor
• Edges/spatial
• Edges /spatial
• Edges
• Edges/spatial• Edges/spatial
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Windthrow toolbox
Field assessment
Common sense
Managementoptions
LoggingMonitoring
Existing knowledge
Research
Windthrow history maps
(vector maps)
Currenttoolbox
Newtools
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Research approaches
• Process studies
• Treatment and effect experiments
• Retrospective studies
• Operational trials
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Monitoring approaches
• Maps and simple databases• Opportunistic / systematic• Stratification by:
– topographic and/or physiographic zones– soil/terrain types– stand types– treatments
• Field observations over time