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Hazard Assessment

Windthrow Hazard Classification
Windthrow hazard classification system.

What different approaches could a forest manager take towards assessing windthrow hazard?

  • There are three basic approaches to windthrow risk assessment: observational, mechanical, and empirical.
  • Observational approaches use a checklist of indicators.
  • Mechanical approaches predict the critical windspeed for overturning from winching and wind tunnel studies, and the probability of critical wind speed from wind mapping/modelling work.
  • Empirical approaches use regression techniques to predict the probability of damage as a function of environmental and management variables.
  • The diagnostic method described below is 'observational' but includes some elements of the empirical approach.

The British system is an advanced example of the mechanical approach. It was developed for Sitka spruce plantations in upland Britain and based on winching tests, wind tunnel work with whole tree crowns, wind mapping using tatter flags. Their work allows prediction of stand height at which endemic winds will produce critical turning moments and trees will fail. Note orientation of trees – blown down in rows.

 

 

 

Critical turning moment. Show Larger Image

Critical turning moment is based on winching tests of trees of different species and sizes on different soil types.

Crown drag. Show Larger Image

Crown drag is measured in wind tunnels; wind speed over terrain is measured in the field or modelling in wind tunnels or with numerical models.

 
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