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time, for instance the number of years, for which the assessor is deciding
whether or not a specific failure is likely to occur. Without a stated time
frame, the rating for likelihood of failure is meaningless. The longer the time
frame, the less reliable the rating, because conditions that affect failure are
prone to change over time. Time frames of one to three years are common.
Time frames greater than five years are often not appropriate because the
uncertainty over that period can be excessive. Long time frames can unnecessarily increase the rating for likelihood of failure, which could lead
to unnecessary mitigation.
Assessments may also be conducted using several time frames. For example, an assessment can be done for the next 12 months and for the next
five years. In some cases, the results will be different. Sometimes, having
assessments consider more than one time frame can be helpful for the tree
owner/manager in making mitigation choices. Whatever time frame is established, the time frame cannot be considered a " guarantee period " for the
risk assessment.
The likelihood of failure can be categorized using the following guidelines:
Imminent-Failure has started or is most likely to occur in the near future,
even if there is no significant wind or increased load. This is a rare occurrence
for a risk assessor to encounter, and may require immediate action to protect
people from harm. The imminent category overrides the stated time frame.
Probable-Failure may be expected under normal weather conditions
within the specified time frame.
Possible-Failure may be expected in extreme weather conditions, but it
is unlikely during normal weather conditions within the specified time frame.
Improbable-The tree or tree part is not likely to fail during normal
weather conditions and may not fail in extreme weather conditions within
the specified time frame.
Tree defects and conditions are typically considered individually when
assessing single trees, but they can be considered in aggregate as long as
the risk being assessed is clearly defined that way. For example, the likelihood of failure of a specific dead branch might be rated as possible, while
the likelihood of failure of any one of several dead branches in a tree might
be rated as probable. Similarly, two or more modes of failure (codominant
stems, dead branch, etc.) might be rated in aggregate, although this is more
complex to consider. Once again, it is essential to define the risk that is being assessed.

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BMP - Tree Risk Assessment

Table of Contents for the Digital Edition of BMP - Tree Risk Assessment

Table of Contents
BMP - Tree Risk Assessment - Cover1
BMP - Tree Risk Assessment - Cover2
BMP - Tree Risk Assessment - i
BMP - Tree Risk Assessment - ii
BMP - Tree Risk Assessment - Table of Contents
BMP - Tree Risk Assessment - iv
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