Windthrow Hazard Assessment and Management

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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 dont blow over

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Windthrow hazard classification

• Wind force factors
• Resistance to overturning
• Other indicators

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Windthrow hazard

Applied forces
Resistance to overturning
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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 (lt0.4m)
• shallow soils
• poorly drained soils (low shear strength)
• root rot areas

• Low hazard
• trees with high taper and large butt flare
• deep rooting (gt0.8m)
• 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 class
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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
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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 applied forces

Maximize Resistance to overturning
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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
Existing knowledge
Research
Current toolbox
Common sense
New tools
Management options
Windthrow history maps
(vector maps)
Logging
Monitoring
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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