BMP - Utility Tree Risk Assessment - 35
8. Tree Risk Categorization
The scope of work for UTRA assignments should define whether findings
should include a rating of risk. In some cases, determination of the risk rating may be done by the utility vegetation manager based on data provided by
the utility tree risk assessor. In a qualitative tree risk assessment, matrices are
used to categorize risk. The risk category can then be compared to the level
of risk that is acceptable to the utility vegetation manager. If the risk category
exceeds the level of acceptable risk, mitigation should be considered by the
utility vegetation manager.
The likelihood of a tree or tree part failure having an effect on a target is
determined by considering two factors:
1. The likelihood of the failed tree or branch impacting the specified target (overhead power line).
2. The likelihood of a tree failure occurring within a specified
time frame under normal weather conditions.
UTRA begins with considering the likelihood of a strike, then the likelihood
of tree failure. This is the reverse order used in general tree risk assessment
performed by arborists working for other clients. This is because UTRA
focuses exclusively on utility infrastructure as the target. If a tree strike of
utility lines is unlikely, there is no expectation that the likelihood of tree failure should be evaluated. 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 Impacting a Utility Target
The first consideration in UTRA is whether the tree can potentially strike
the utility target of interest. If it can't, there is no need to continue the
assessment process. To estimate the likelihood that a tree or branch failure
could result in a strike to an overhead line, the utility tree risk assessor
should consider the height of the line, height of the tree, distance from the
conductor(s), likely direction of its fall, and any target protective factors.
35
BMP - Utility Tree Risk Assessment
Table of Contents for the Digital Edition of BMP - Utility Tree Risk Assessment
Table of Contents
BMP - Utility Tree Risk Assessment - Cover1
BMP - Utility Tree Risk Assessment - Cover2
BMP - Utility Tree Risk Assessment - i
BMP - Utility Tree Risk Assessment - ii
BMP - Utility Tree Risk Assessment - Table of Contents
BMP - Utility Tree Risk Assessment - iv
BMP - Utility Tree Risk Assessment - v
BMP - Utility Tree Risk Assessment - vi
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BMP - Utility Tree Risk Assessment - Cover4
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