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Trees for Wind Shelter
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Trees for Wind Shelter

Shelterbelts or windbreaks are narrow belts of trees designed to block damaging winds. As a result of wind tunnel research we now know a great deal about how the design and management of a belt of trees might influence wind speed on both sides of the belt. Field based studies undertaken across Australia as part of a national windbreaks research program has provided practical experience to support this theory.

Basic Function Of A Shelter Belt And Surface Roughness
A universal function that describes wind flow for a long, thin wind barrier standing on a large, flat surface with the wind blowing normal (perpendicular) to the barrier:

U = mean horizontal wind speed
Uh = mean approach horizontal wind speed at barrier top height in the open
h = shelterbelt height
x = perpendicular distance from the windbreak
z = height above the surface
zo = surface roughness parameter
L = Monin-Obukhov stability length (a measure of atmospheric stability)
P = shelter belt porosity (ratio of perforated area to total area)
Reference: McNaughton, K.G. (1988), 'Effects of Windbreaks on Turbulent Transport and Microclimate', Agriculture Ecosystems and Environment, Vol. 22/23, pp 17-40

Surface roughness is only one of many factors that will influence the efficiency of a shelter belt However the greater the surface roughness preceding a shelter belt, the greater the turbulence of the wind hitting the windbreak and therefore the lower the efficiency of the windbreak. Likewise, atmospheric stability will also influence shelterbelt efficiency. Farmers may be able to influence the orientation, height andporosity of a shelterbelt. They might also be able to manage their crops or stock effectively so as to take advantage of the shelter provided.

The extent of the shelter effect is directly proportional to the height of the belt and this is why it is common to express the extent of the shelter effect as a multiple of tree (or belt) height, e.g. 20H is 20 times the tree (or belt) height. Shelter effects have been routinely measured to 20H - 25H, with the effect of the shelterbelt being marginal after 30H.

Windspeed at Distances from Windbreak

Reference: Cleugh, H. (1997), 'Trees for Shade and Shelter' in Design Principles for Farm Forestry: A guide to assist farmers to decide where to plant trees and farm plantations on farms eds. Abel, N. et al, RIRDC, Canberra, pp.39-52

Shelterbelt Examples
Orientation of Shelterbelts
Porosity of the Shelterbelt
Gaps and End Effects
Effect of Shelterbelts on Microclimate
Effect of Multiple Shelterbelts

The effect of scattered trees and plantations on wind speeds is quite different to that of shelterbelts. In the case of scattered trees, the strongest winds tend to flow evenly over the top of the canopies, leaving the wind speeds at ground level much lower over the whole area. Measurements taken amongst widely spaced trees spread across grazing land indicate that reductions in wind speed of 40% over the whole paddock are possible with just 17 large remnant eucalypt trees per hectare, or about 200 young pruned timber trees per hectare. The larger the trees or the greater the stocking rate, the slower the wind speed will be. Such areas may be valuable as stock havens for ‘off-shears’ sheep or developed as special lambing or calving areas.

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