BMP - Utility Tree Risk Assessment - 82

2. Variability in direction and speed (acceleration) of wind
3. Mass of wind (air density, precipitation, and even debris)
4. Resistance/drag of tree to wind
The combination and significance of each of these factors define the characteristics and destructive potential of a windstorm. As a result, the impact of
windstorms on overhead power systems vary by storm and tree attributes.
Trees fail when wind loading exceeds a tree component's capacity to withstand the mechanical load. Factors such as exposure, tree form, wood
strength, structural defects, topography, soils, and precipitation can all interact to affect the structural stability of individual trees during wind events.
Each factor can vary across a utility's service territory. The same variability
can be said of the power system's exposure to a storm event-being localized or widespread.
It is important to recognize changes in the profile of a tree as wind speed
and loading increases. Leaves and branches are deflected, reducing and streamlining the canopy. Flexing of branches and stems decreases peak loads. As a
result, the " sail area " presented to the wind varies by species and time of year,
and may be reduced by more than 50 percent as wind speeds increase.
While recognizing the variability in vulnerability of individual trees to the force
of wind, it is possible to characterize the pressure of wind exerted at the
Wind Pressure on Trees
14

Pressure (lbs/ft2)

12
10
8
6
4
2
0
0

10

20

30

40

Wind Speed (MPH)

Figure 10. Wind pressure on trees.
82

50

60

70

80



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
BMP - Utility Tree Risk Assessment - viii
BMP - Utility Tree Risk Assessment - ix
BMP - Utility Tree Risk Assessment - x
BMP - Utility Tree Risk Assessment - xi
BMP - Utility Tree Risk Assessment - xii
BMP - Utility Tree Risk Assessment - 1
BMP - Utility Tree Risk Assessment - 2
BMP - Utility Tree Risk Assessment - 3
BMP - Utility Tree Risk Assessment - 4
BMP - Utility Tree Risk Assessment - 5
BMP - Utility Tree Risk Assessment - 6
BMP - Utility Tree Risk Assessment - 7
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BMP - Utility Tree Risk Assessment - 12
BMP - Utility Tree Risk Assessment - 13
BMP - Utility Tree Risk Assessment - 14
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BMP - Utility Tree Risk Assessment - Cover4
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