BMP - Tree Risk Assessment - 37

that could be caused by the impact of the failure. The amount of damage
depends on the part size, fall characteristics, fall distance, and any factors
that may protect the target from harm.
It is often said that without a target, a tree poses no risk, but that may not
mean that the failure has no consequences because trees have value. If a tree
fails, the value and benefits associated with that tree are lost or diminished.
Therefore, there are always some consequences associated with failure.
When evaluating consequences, consider the size of the tree or tree part
that could fail, and how it could impact a target. Generally, a small branch
has less potential to cause damage than a large branch. Small branch failures
occur more frequently, and even a small branch could cause personal injury, a
power outage, or a traffic accident (Figure 8). Branches less than 1 inch (2.5
cm) in diameter usually are not considered in most tree risk assessments. The
minimum size branch that should be considered by the tree risk assessor may
be specified in the scope of work. In some situations, the minimum branch
diameter of concern may be as large as 4 inches (10 cm) in diameter. For
example, if a 4-inch (10 cm) diameter branch falls on a house from a height
of 10 feet (3 m) above the roof, the degree of damage would be low, and no
injury to people inside would be expected. If the same size branch were to
fall from near the top of a large tree with no branches in between to slow it
down, more extensive damage could occur.
In estimating how much damage could occur from a tree failure, consider
the relative amount of force with which it is likely to strike the target. A falling tree or branch will gain speed as it accelerates toward the ground. So,
in general, the higher the distance from which a branch falls, or the greater
the distance from the tree to the target, the greater the force that the tree or
branch will have at the point of impact. If the distance from a tree trunk to a
well-built, multi-story house is short, a tree that fails may simply lean against
the house, causing minor damage. On the other hand, if the distance is such
that the tree can accelerate significantly before the trunk strikes the house,
damage may be much greater. If there are lower branches in the tree that are
likely to slow or stop the fall of the trunk, damage may be lessened. In this
example, the lower branches serve to protect the target. Large-diameter, widegrowing branches that are low on the trunk also may affect the fall pattern of
a tree. If the branches contact the ground well before the trunk, the fall may
be slowed or stopped, or the tree may roll.
Protection may be provided by structures that surround the people in the
target zone. If the protection factor is not strong enough to stop the impact,
then the assessor should judge whether it will reduce the consequences.
37

BMP - Tree Risk Assessment

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