Windbreak Function(How do they work?)

Joe ZeleznikNDSU Extension Forester

Windbreak Function - Basics

• They slow down the wind

• They re-direct the wind

• Which modifies the microclimate and environment in protected areas

Based on Structure

Based on Structure

• Density - % solid

Based on Structure

• Density - % solid

• Height – changes over time

– height of tallest row

Functional Height = Height of the tallest row

Based on Structure

• Density - % solid

• Height – changes over time

– height of tallest row

• Length –

PROTECTED AREA

Based on Structure

• Density - % solid

• Height – changes over time

– height of tallest row

• Length –

• Orientation – perpendicular to prevailing winds

WIND

Predominant Wind DirectionsLocation

Direction % Time Direction % Time Direction % Time Direction % TimeNNW 17 NNW 17 SSE 17 SSE 18SSE 17 SSE 13 N 14 NNW 10NW 11 NW 10 SE 14 NW 16E 10 SE 10 NW 12 SE 14

NW 18 NW 16 NW 14 NW 17SW 13 SE 12 SE 12 SW 13

Fargo

Bismarck

Minot

Time of yearJanuary April July October

From Northerly or northwesterly Direction 12 out of 24

From Southeasterly or Easterly Direction 10 out of 24

From Southwesterly Direction 2 out of 24

Based on Structure

• Density - % solid

• Height – changes over time

– height of tallest row

• Length –

• Orientation – perpendicular to prevailing winds

• Continuity

Based on Structure

• Density - % solid• Height – changes over time – height of tallest row• Length – • Orientation – perpendicular to prevailing

winds• Continuity• Species/Number of Rows

How does structure affect wind speed and wind flow?

Ground

Windspeed in the open

Fastest

Slowest

Ground

Windspeed in the open

Fastest

Slowest

No turbulence over smooth ground

Turbulent flow over a rough surface

Ground

Windspeed in the open

Fastest

Slowest

No turbulence over smooth ground

Turbulent - Random

Eddy – Circular, against main flow

Turbulent flow over a rough surface

Terms to know:

Upwind

Windward

Downwind

Leeward

H = height of object

Solid barrier

Pressure eddy

1-2H

Solid barrier

Suction eddy

1-3H 3-6H

Turbulent wake zone

6-8H

Resumption of open-field flow

Lower effect of pressure eddy

Porous barrier

Lower effect of suction eddy

Turbulent wake farther downwind

Resumption of open-field flow

Porous barrier

Even lower effect of suction eddy

Turbulent wake even farther downwind

Resumption of open-field flow

Lower branches pruned

Increased snow spreading?

How does structure affect wind speed and wind flow?

• Density – determines degree of wind speed reduction (and length of protected zone)

Density

Upwind Downwind

How does structure affect wind speed and wind flow?

• Density – determines degree of wind speed reduction (and length of protected zone)

• Height – along with density, determines length of protected zone, upwind and downwind

WIND

Protected Area

Protected Area

Protected Area

HEIGHT

2-5 H 10-20 H

How does structure affect wind speed and wind flow?

• Density – determines degree of wind speed reduction

• Height – with density, determines length of protected zone, upwind and downwind

• Length – determines total area of downwind protection

LENGTH

End effect: Windspeeds increased around end of windbreak by 1.1-1.3 times

LENGTH

PROTECTED AREA

PROTECTED AREA

Length:Height - > 10:1

How does structure affect wind speed and wind flow?

• Density – determines degree of wind speed reduction

• Height – with density, determines length of protected zone, upwind and downwind

• Length – determines total area of downwind protection

• Orientation – place perpendicular to prevailing winds

Wind

ORIENTATION

Wind

?

Wind

? PROTECTED

AREA

Wind

?

PROTECTED AREA

PROTECTED AREA

Predominant Wind DirectionsLocation

Direction % Time Direction % Time Direction % Time Direction % TimeNNW 17 NNW 17 SSE 17 SSE 18SSE 17 SSE 13 N 14 NNW 10NW 11 NW 10 SE 14 NW 16E 10 SE 10 NW 12 SE 14

NW 18 NW 16 NW 14 NW 17SW 13 SE 12 SE 12 SW 13

Fargo

Bismarck

Minot

Time of yearJanuary April July October

From Northerly or northwesterly Direction 12 out of 24

From Southeasterly or Easterly Direction 10 out of 24

From Southwesterly Direction 2 out of 24

How does structure affect wind speed and wind flow?

• Density – determines degree of wind speed reduction

• Height – with density, determines length of protected zone, upwind and downwind

• Length – determines total area of downwind protection

• Orientation – place perpendicular to prevailing winds

• Continuity

CONTINUITY

Brandle and Finch

How does structure affect wind speed and wind flow?

• Density – determines degree of wind speed reduction

• Height – with density, determines length of protected zone, upwind and downwind

• Length – determines total area of downwind protection

• Orientation – place perpendicular to prevailing winds

• Continuity• Species/Number of Rows – affect density, rate

of height growth

How does structure affect wind speed and wind flow?

• Density – determines degree of wind speed reduction

• Height – with density, determines length of protected zone, upwind and downwind

• Length – determines total area of downwind protection

• Orientation – place perpendicular to prevailing winds

• Continuity• Species/Number of Rows – affect density, rate

of height growth• Cross section

Cross section – no affect, really

Microclimate modifications

Microclimate modifications

• Windspeed

Density

Upwind Downwind

Microclimate modifications

• Windspeed– One consequence – particle fallout Snowdrift placement

Microclimate modifications

• Windspeed

• Air Temperature

• Soil Temperature

• Humidity

Microclimate modifications

• Windspeed

• Air Temperature

• Soil Temperature

• Humidity

• Heat loss (animals and buildings)

Goals for the windbreak

Goals for the windbreak

• Design considerations (density, placement) are based on goals:

Goals for the windbreak

• Design considerations (density, placement) are based on goals:– Crop protection– Reduced soil erosion– Increased snow capture in field– Farmstead protection– Living snow fence– Sound barrier– Wildlife food/cover

Any Questions?