Wind Resource - Wind Energy: Renewable Energy and the Environment - page 67

C

B A

B

A

C

FIGURE 4.15 Estimation of wind speed by tree deformation.

An index for broad leave trees is the deformation ratio, D, which represents the amount of crown
asymmetry and trunk deflection of trees caused by the wind (Figure 4.15):

D A/B C/45°

The relationship is used for both coniferous and hemispherical crowned trees. For coniferous trees,
A is the angle formed by the crown edge and the trunk on the leeward side, B is the angle formed
by the crown edge and the trunk, and C is the average angle of trunk deflection. For hemispherical
crowned trees, A is the distance between the trunk and the crown perimeter on the leeward side, B
is the distance between the trunk and the crown perimeter on the windward side, and C is the angle
between the crown perimeter and the trunk on the leeward side. The ratio A/B assumes that 1 ≤
A/B ≤ 5. As a result, the minimum value of D is 1, which corresponds to no crown asymmetry, or
trunk deflection C. Since the maximum deflection is 90° for a tree growing along the ground, then
the maximum deformation ratio is D 7.
The relation of deformation ratio to the mean annual wind speed, u, was estimated for Douglas
fir or Ponderosa pine trees [31]. From regression analysis of the data,

u 0.95 D 2.3

Photographs can be used to determine the deformation ratio in lieu of direct examination. The
deformation ratio and Griggs-Putnam Index give similar ranges of wind speeds.
The use of trees as an indicator of wind speed is subject to a number of practical limitations. Of
greatest concern is the tree's exposure to the wind. The deformation should be viewed perpendicu-
larly to the prevailing wind direction so that the full effects of flagging and throwing are taken into
consideration. Hence, trees selected as indicators must be well exposed to the prevailing winds.
Seldom do trees in a forest extend far enough above the canopy to be in an airstream undisturbed by