BMP - Tree Risk Assessment - 32
5.Tree Risk Categorization
Tree risk assessment reports should include a rating of risk. In a qualitative
tree risk assessment, assessors can use a matrix to help categorize risk. The
risk category is then compared to the level of risk that is acceptable to the
client, the controlling authority, or societal standards. If the risk category
defined for the tree risk exceeds the level of acceptable risk, mitigation options should be presented.
When categorizing tree risk, the factors to be considered are the likelihood
of a tree failure impacting a target and the consequences of the failure. The
likelihood of a tree failure impacting a target is determined by considering
two factors:
1. The likelihood of a tree failure occurring within a specified time
frame. The likelihood of tree failure is determined by examining
structural conditions, defects, response growth, and anticipated
loads.
2. The likelihood of the failed tree or tree part impacting the specified
target. Impact may be the tree directly striking the target, or it may
be a disruption of activities due to the failure.
These two factors are evaluated and categorized using a matrix to estimate
the likelihood of the combined event: a tree failure occurring and the tree
impacting the specified target. The likelihood of that combined event is then
compared with the expected consequences of a failure impacting the target
to determine a level of risk.
Likelihood of Failure
Judgment about the significance of defects, conditions, and response growth
can be guided by the information related to likelihood of failure described
in Appendices 3 and 4, species failure profiles, site conditions, and tree risk
assessor experience. While likelihood of failure guidelines are presented
in this BMP for individual defects and conditions, the assessor should
consider known compounding factors as well as any response growth in
the tree, which may have compensated for the condition. Guidelines should
be considered a starting point and should be modified as needed so that
they are appropriate for the tree and site. Significant deviation from these
guidelines or other standards that are used should be noted and in a report.
Before assessing the likelihood of failure, a time frame must be specified to put the likelihood rating in context. The time frame is the length of
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BMP - Tree Risk Assessment
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Table of Contents
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