WIND EROSIONWIND EROSION

MechanismsMechanisms

SaltationSaltationserious if v > 21 kph @ 0.3 m serious if v > 21 kph @ 0.3 m 0.1 to 0.5 mm0.1 to 0.5 mm

SuspensionSuspensiondust storms can transport dust across dust storms can transport dust across continents (as happened at Bangor in continents (as happened at Bangor in 1988)1988)size usually < 0.1 mmsize usually < 0.1 mm

Contributing factorsContributing factors

ClimateClimate

Becomes more important in lower rainfall Becomes more important in lower rainfall areas.areas.

Wind erosivityWind erosivity

Record wind speed & total time for each Record wind speed & total time for each velocity increment in each segment, 1 to 16, of velocity increment in each segment, 1 to 16, of the compass. the compass. Minimalist approach - plot number of days for Minimalist approach - plot number of days for which wind blows in each direction to guide. which wind blows in each direction to guide. Make length of the radius equal to number of Make length of the radius equal to number of days. Result is known as a "wind rose".days. Result is known as a "wind rose".

The histogram will look something like the The histogram will look something like the following for each section of the compass (1 to following for each section of the compass (1 to 16) 16)

winds below 19 mswinds below 19 ms-1-1 would not would notcontributecontribute

Analysis of wind speed for one directionAnalysis of wind speed for one direction

Wind erosivity index for direction j is calculated from:Wind erosivity index for direction j is calculated from:

ijijj fvI 3

where i designates velocity (m swhere i designates velocity (m s-1-1) band and j the ) band and j the directiondirection

vvijij is the mid-point of the velocity band less 19 m s is the mid-point of the velocity band less 19 m s-1-1, ,

the threshold velocity)the threshold velocity)

ffijij is the total amount of time wind in that direction that is the total amount of time wind in that direction that

is within a particular velocity band, is within a particular velocity band, i.e.i.e. for each for each

compass point, jcompass point, j

In the example, IIn the example, Ijj would be would be

tt19.519.5x 0.5x 0.533 + t + t20.520.5 x 1.5 x 1.533+ t+ t21.521.52.52.53 3 etcetcThis is repeated for each section of the compass.This is repeated for each section of the compass.

16

1

j

jjII

This is an empirical equation; wind kinetic This is an empirical equation; wind kinetic energy is proportional to venergy is proportional to v22 but v but v33 seems to seems to work better.work better.

TextureTexture

More erosion if soil high in particles between 40 More erosion if soil high in particles between 40 µ - 300 µ µ - 300 µ

......

silt sizesilt size

OMOMWorse if high unless un-decomposedWorse if high unless un-decomposed

LimeLimepresence of calcrete (petrocalcic horizon) on presence of calcrete (petrocalcic horizon) on surface leads to reduced erosionsurface leads to reduced erosionbut if rock or calcic soil becomes powdery, but if rock or calcic soil becomes powdery, increased erosion resultsincreased erosion results

Aggregate stabilityAggregate stability

Wind erodibility of soilWind erodibility of soilOne equation is given as follows but does not One equation is given as follows but does not include factor for aggregate stability, organic include factor for aggregate stability, organic matter content or lime:matter content or lime:

KKww = 100 - (34.7 + 0.9 X = 100 - (34.7 + 0.9 X11 - 0.3 X - 0.3 X22 - 0.4X - 0.4X33))

wherewhere XX11 = < 1µ size range = < 1µ size range XX22 = 50 - 250 µ = 50 - 250 µ XX33 = > 250 µ = > 250 µ

See web site for WEPS - wind erosion See web site for WEPS - wind erosion prediction system linked from the Wind prediction system linked from the Wind Erosion Research UnitErosion Research Unit

PreventionPrevention

Stubble mulchStubble mulch • increased drag effect on wind traps particles increased drag effect on wind traps particles

travelling through saltationtravelling through saltation

Effect of vegetation on wind velocity near the Effect of vegetation on wind velocity near the surfacesurface

Plane of zero wind velocity occurs at height of Plane of zero wind velocity occurs at height of about 70 per cent of the height of vegetation about 70 per cent of the height of vegetation above the ground surfaceabove the ground surface

(a) zero wind velocity occurs at a height z0 which lies above height of mean surface;

(b) crop cover raises height of the effective surface by a distance D and also increases the value of z0

(c) velocity profiles plotted onlinear scale

(d) velocity profiles plotted onlogarithmicscale

Minimum tillageMinimum tillage

Reduces wind erosion as well as water erosion Reduces wind erosion as well as water erosion though little hard research data of measured though little hard research data of measured erosion rateserosion rates

Wind strip croppingWind strip cropping

• Plant @ right angles to average wind Plant @ right angles to average wind direction direction

• Alternate tall and short crops Alternate tall and short crops

Wind breaksWind breaks

• Protect for 10 to 20 x height of trees leeward Protect for 10 to 20 x height of trees leeward and 5 x windwardand 5 x windward

• Design considerations:Design considerations:• make as long as possiblemake as long as possible• do not plant too densely or eddies will do not plant too densely or eddies will

form and increase erosionform and increase erosion• plant several rows with smaller bushes / plant several rows with smaller bushes /

trees on either side of treestrees on either side of trees• align roads at angle to windalign roads at angle to wind

• Wind breaks:Wind breaks:• reduce evaporation reduce evaporation • increase yields (10 to 20% in average increase yields (10 to 20% in average

year, 50 to 60% in dry year in parts of year, 50 to 60% in dry year in parts of Kenya)Kenya)

• stone walls and hedges also reduce erosion stone walls and hedges also reduce erosion and evaporationand evaporation

Windbreaks which are too wide will reduce the Windbreaks which are too wide will reduce the efficiency -----efficiency -----

It is important that wind breaks consist of species at different levels - not just one (usually tall) species

Crop yield normally declines in the vicinity of a Crop yield normally declines in the vicinity of a wind break but many researchers have found wind break but many researchers have found that away (about 1 to 1.5 tree heights) away that away (about 1 to 1.5 tree heights) away from the wind break, yields increase, mostly from the wind break, yields increase, mostly because of reduction of evapotranspiration because of reduction of evapotranspiration losses.losses.

Dune stabilisationDune stabilisation

• fences, reeds, etc. for micro wind breaksfences, reeds, etc. for micro wind breaks

• chemical mulchingchemical mulching

• re-vegetationre-vegetation