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Windbreak Functions�or�How Do Windbreaks Work?

Richard Straight – US Forest Service

USDA National Agroforestry Center

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What to Expect

Definition
Very abridged history
How barriers influence wind flow
Discuss how different windbreak elements influence wind speed and flow
Climate Change and windbreak design
Sources of related information

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Windbreak Definition

A barrier placed on the land surface that obstructs the wind flow and alters flow patterns both up-wind (windward) and down-wind (leeward) of the barrier.
Plantings of single or multiple rows of vegetation (trees, shrubs, grass) that are established for one or more environmental and economic purposes.

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Fabric & Gates

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Brief History

Scottish Parliament urged the planting of tree belts to protect agricultural production in the mid-1400’s
Westward expansion in the U.S. saw homesteaders planting windbreaks
Dust Bowl conditions led to the Prairie States Forestry project
1930’s Conquest, Saskatchewan project included planting 960 miles of shelterbelts using about 7 million seedlings

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Wind does not move uniformly around or through a windbreak?

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How does the change in wind speed affect microclimate?

The change in wind flow affects the exchange rates between the surface of an object and the air above that object.

The net vertical motion of air parcels is brought about by turbulent transfer.

As these parcels move up they carry with them the various “properties”.

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How does the change in wind speed affect microclimate?

Almost any measurable property of interest in the atmosphere is moved from levels of high concentration to levels of low concentration.

Properties such as:

Water vapor - Pollen
Heat - Ozone
Dust - Carbon dioxide

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How does the change in wind speed affect microclimate?

Also:

Lower wind velocity causes particulate matter to be deposited
Windbreak vegetation physically traps particulate matter
Windbreak vegetation may adsorb some of the chemicals attached to particulate matter
Even changes in atmospheric pressure

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Results of Modifying Air Flow with Windbreaks

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How do Windbreaks provide these Benefits?

Windbreak function depends upon six key windbreak elements:

Height
Density
Orientation
Length
Width
Continuity

Criteria for elements vary by purpose

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What is H?

H = Height of the Windbreak

H

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10H to 15H

H = Height of windbreak

H

Why Is Windbreak Height Important?

The height determines the distance of the sheltered zone. For example, select the tallest trees suited to the site for large fields and fewest windbreaks.

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Windbreak Height

The leeward distance of wind protection is directly proportional to the height of the windbreak.

Note: 4H is about the mid-point of maximum wind reduction

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Windbreak Density

Windbreak density is the percentage of the solid portion of the barrier to the total area of the barrier.

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Air Flow Differences

2-D fence

(top view)

VS

3-D barrier

(top view)

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Not to scale

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Proposed Turbulence Model

Around a Tree Shelterbelt:

Time

Spatial

variable

Velocity

Air

density

Drag

coefficient

Total

velocity

Vegetative surface area density

Cubic

porosity

Pressure

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Density - The density level is manipulated by the choice of plant materials (e.g. deciduous trees versus conifers) and the way the plants are arranged in the windbreak. By combining low growing shrubs with medium and tall deciduous trees, dense plant material is provided at three levels (low, middle and upper) of the windbreak during the growing season. However, during the winter, the density would decrease due to the loss of foliage. Consequently, a conifer component would be desirable for year-round protection.

For practical application purposes density is dealt with on a gross scale of low, medium and high density.

Low density is 25-40% dense

Medium density is 40-60% dense

High density is greater than 60% dense

100% density is not possible with vegetation.

Remember that density will likely vary from the bottom to the top of the windbreak.

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Windbreak Density

Dense = �Maximum wind reduction but short wind shadow

Moderately Dense = Less wind reduction but longer wind shadow

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Impact of Density on Windbreak Effectiveness

Downwind multiples of Windbreak Height

Solid

60-80%

40-60%

25-35%

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Windbreak Orientation

Orient windbreaks perpendicular to troublesome winds

Troublesome

Wind(s)??

Plan multiple windbreaks for whole field protection

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Working With Variable Wind Direction

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Wind Roses –

Bismarck, ND

January

6%

3%

9%

12%

15%

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Wind Roses – Mandan, ND

January

February

March

August

April

May

June

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Wind Roses – Mandan, ND

July

August

September

August

November

December

October

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Wind Roses – Billings, MT

January

February

March

April

May

June

July

August

September

October

November

December

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Windbreak Length

Length: At least 10 times the windbreak height

For full protection, the windbreak needs to extend the full length of the field needing protection.

End

Eddy

End

Eddy

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Windbreak Length

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Windbreak Length

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Windbreak Needs to be Continuous

Gaps in the windbreak can result in damage downwind

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Windbreak Needs to be Continuous

Avoid creating gaps in the windbreak

Access Lanes

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Field Windbreaks

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Livestock Windbreaks

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Farmstead Windbreaks

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Snow management

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Use These Criteria to Design Any Windbreak

Windbreak function depends upon six key windbreak elements:

Height
Density
Orientation
Length
Width
Continuity

Criteria for elements vary by purpose

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Changing Weather -- Resilient Landscapes

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Wind speed in 2050; % of models that predict an increase

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Agroforestry: is one tool for producing food, fuel and fiber with a changing climate

….While providing other services

Branching Out: Agroforestry as a Climate Change Mitigation & Adaptation Tool for Agriculture. JSWC (2012)

Sequestering carbon (C)
Reducing GHG emissions
Avoiding C emissions

Reducing threats
Enhancing resilience
Facilitating species migration

Mitigation

Adaptation

www.jswconline.org

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Predicted Changes in Climate That Will Affect Agricultural

Hotter, with more frequent hot temperature extremes, fewer cold temperature extremes. Increased frequency and duration of heat waves.

Increasingly frequent and intense severe weather events

Increasing frequency of drought

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Predicted Changes in Climate That Will Affect Agricultural

Hotter, with more frequent hot temperature extremes, fewer cold temperature extremes. Increased frequency and duration of heat waves.

Increasingly frequent and intense severe weather events

Increasing frequency of drought

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Impacts on Agricultural and Forest Ecosystems and Landscapes

Increased soil erosion from intense wind and rainfall events

Greater plant stress and damage

Increasing water demands

Greater weather volatility, wider extremes

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Agroforestry Systems Also Need Adaptation

Drought Tolerance
Species Selection
Consider Seed Sources
Diversity of Genera Over Species

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Sources of Information

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Related resources

_{T R E E C R O P S}

^{A PERMANENT AGRICULTURE}

^BY

^{J. RUSSELL SMITH}

^{Harcourt Brace and Company Inc,}

^{SOMETIME PROFESSOR OF INDUSTRY WHARTON SCHOOL OF FINANC E AND COMMERCE UNIVERSITY OF PENNSYLVANIA}

^{NOW PROFESSOR OF ECONOMIC GEOGRAPHY COLUMBIA UNIVERSITY}

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H