BMP - Tree Risk Assessment - 42
Steps in Developing a Tree Risk Rating
1. Identify possible targets, including those identified by the client.
2. Identify tree part(s) and/or failure mode(s)-that is, the location or
manner in which failure could occur.
3. Assess and categorize likelihood of failure for each part of concern
within a specified time frame.
(Imminent / Probable / Possible / Improbable)
4. Assess and categorize likelihood of tree/part impacting target.
(High / Medium / Low / Very low)
5. For each failure mode/target combination, categorize the likelihood of
failure and impact (Matrix 1).
(Very likely / Likely / Somewhat likely / Unlikely)
6. For each failure mode/target, assess consequences.
(Severe / Significant / Minor / Negligible)
7. For each failure mode/target, categorize the risk (Matrix 2).
(Extreme / High / Moderate / Low)
Risk aggregation is the consideration of multiple risks in combination;
it is difficult to do even with complex mathematical analyses. Therefore,
the tree risk assessor cannot simply add or multiply the risk ratings for the
individual failure modes to reach a whole-tree risk rating.
What the tree risk assessor can do is identify-among all the failure mode/
target combinations assessed-the failure mode/target combination having
the greatest risk, and report that as the tree risk rating. Assigning a tree risk
rating for a tree may be useful, especially when prioritizing mitigation of risk
for a population of trees. For example, in a given situation, whole-tree failure
may be unlikely, but it could have significant consequences if it occurs; using
Matrix 2, the risk rating is low. At the same time, failure of a dead branch
may be very likely, but with minor consequences; the risk rating is moderate.
The risk rating would be reported as moderate, the higher of the two ratings.
This rating often is presented as the single risk level for the tree, especially
when dealing with populations of trees and limited visual assessments. It is
important to note, however, that if measures are taken to mitigate the highest
risk, there still is residual risk associated with that tree, including the remaining
risk factors. The risk rating for that tree may or may not change based upon
the remaining risk factors.
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BMP - Tree Risk Assessment
Table of Contents for the Digital Edition of BMP - Tree Risk Assessment
Table of Contents
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