BMP - Utility Tree Risk Assessment - 26

of other trees that in turn could fall into a line (i.e., domino effect). Conversely, the target zone may be reduced in size by adjacent larger trees,
strong lower branches, or other factors that provide protection and reduce
the likelihood of impact or prevent the tree or branch from striking the line.
Trees and branches sometimes fall in unusual and unpredictable ways, striking outside of what would normally be considered the target zone. The
direction of tree failure is often more closely related to wind direction, terrain, and lean and crown weight rather than the location of the defect on the
tree. While the prevailing wind direction provides a baseline reference, storm
wind directions can be unpredictable and vary considerably from prevailing
wind directions.

Assessment Area

The area in which trees are to be assessed in a UTRA project is established
in the scope of work and typically defined in terms of a tree's distance from
overhead conductors, from the edge of the maintained corridor, or from the
centerline of the ROW. In cases involving critical utility infrastructure, such
as substations and high-voltage transmission lines, the assessment area may
include any tree tall enough to strike the line or encroach within the Minimum Vegetation Clearance Distances as defined in NERC FAC-003. However, in many cases on overhead distribution systems, there is no expectation
that all trees tall enough to strike the line should be included in the assessment (Figure 3).

Occupancy Rate

Targets can be categorized by the amount of time that they are within the
target zone-their occupancy rate. Utility infrastructure is generally static,
occupies a fixed location, and is continuously present so the occupancy rate
is constant. In some situations, the dynamic motion of conductors due to
loads, such as wind and ice, or sag due to thermal heating, may warrant additional consideration of occupancy in the context of its position relative to
trees.

Site Factors

Site factors have significant influence on both the likelihood and consequences
of tree or branch failure. Site factors that can be used to evaluate the likelihood of tree failure impacting the target include, but are not limited to, the
history of previous failures, such as branch, trunk, root, and soil failure;
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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
BMP - Utility Tree Risk Assessment - vii
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BMP - Utility Tree Risk Assessment - Cover4
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