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