BMP - Tree Risk Assessment - 34

Likelihood of Impacting a Target
To estimate the likelihood of the failed tree impacting the target, the arborist
should consider the occupancy rate of any targets within the target zone, the
direction of fall, and protection factors that could affect the impact of the
failed tree or tree part as it falls toward the target (Figure 7).
The likelihood of impacting a target can be categorized using the following guidelines:
High-The failed tree or tree part is likely to impact the target. This is
the case when there is a constant target, with no protection factors, and the
direction of fall is toward the target.
Medium-The failed tree or tree part could impact the target, but is
not expected to do so. This is the case for people in a frequently used
area when the direction of fall may or may not be toward the target. An
example of a medium likelihood of impacting people could be passengers
in a car traveling on an arterial street (frequent occupancy) next to the
assessed tree with a large, dead branch over the street.
Low-There is a slight chance that the failed tree or tree part will impact
the target. This is the case for people in an occasionally used area with no
protection factors and no predictable direction of fall; a frequently used area
that is partially protected; or a constant target that is well protected from
the assessed tree. Examples are vehicles on an occasionally used service
road next to the assessed tree, or a frequently used street that has a large tree
providing protection between vehicles on the street and the assessed tree.
Very low-The chance of the failed tree or tree part impacting the specified target is remote. Likelihood of impact could be very low if the target is
outside the anticipated target zone or if occupancy rates are rare. Another
example of very low likelihood of impact is people in an occasionally used
area with protection from being struck by the tree failure due to the presence
of other trees or structures between the tree being assessed and the targets.
Targets are usually assessed on an individual basis, but they can also
be combined to provide the client with a better perspective of what would
happen in case of a tree failure. For example, consider a situation with a
straight tree, with considerable root decay, in a courtyard surrounded on
three sides by a house, a garage, and a workshop, at various distances from
the tree. The house, which is only partially within the target zone, can be
considered as an individual target with a medium likelihood of impact. Another approach is to consider all three targets in aggregate as " structures. "
In that case, the likelihood of impact would be high, since the structures
surround the tree on three sides and, if the tree were to fail, it would prob34

BMP - Tree Risk Assessment

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