Lecture 3. Wind-Erosion.pdf

Wind erosion and factors affecting wind erosion

1. Course No: ENGG-121
Course teacher,
Prof. A. C. Fartade
Dept. of Agricultural Engineering
Lecture 2.
Wind erosion : Principle, mechanics, types of
soil movement
Prof. A. C. Fartade, Department of Agril. Engg.

3. Winderosion:-Itistheprocessofdetachment,
transportationanddepositionofsoilparticles
bytheactionofwind.
Thebasiccausesofwinderosionarefew&very
simple.Mostcommonly,ittakesplace,when:
1)Soilisloose,finelydivided&dry
2)SoilSurfaceissmooth&bare
3)Windisstrongenoughtodetachthesoil
particlesfromthesoilsurface

4. WindErosionProblemArea:-
•Coastalsandyarea–InIndiaabout1.47m.halandsarespreadas
coastalsandyareas.Suchareasarelashedbyhighwindvelocity.
Theformationofsanddunesneartheseacoastismajorproblem.
•DesertAreas-Thedesertareasareseverelyaffectedbywind
erosion.InIndiaabout11.796m.halandsaregroupedasdesert
area.Theformationandmovementofsanddunesisthemajor
problemofdesertareas.Movingsanddunescoverstheroads,
bridges,raillines&evenhumansettlements.
•Semi-aridAreas–Theseareasarefoundattheboundaryofthe
deserts.Insemi-aridareasthedesertisinadvancingstage,which
coversthevaluablelands.Thesemi-aridareasarealsoaffectedby
severewinderosionandformationofsanddunes.
•InlandRiverSands-Thesearefoundalongtheplainofmajorrivers.
Suchareasalsohavetheproblemofwinderosion,viz.Gangaand
Chambalrivers.
•Itisestimatedthatabout17.2m.haoflandissufferingfromwind
erosion&extensionofdeserts.

5. Factors affecting Wind Erosion
1)Climaticfactor:precipitation,temperature,humidity,velocity
&densityofprevailingwind.Whenmeanannualrainfallis
comparativelyless,themoisturecontentinsoilisless,&thus
thesoilparticlesareinloosecondition&theydetacheseasily
duetoblowingwind&causessoilerosion.Duetorisein
atmospherictemperaturethesoilmoisturereduces&thatsoil
ismoresusceptibleforerosion.
2)SoilCondition:Texture,structure,sizeofsoilparticles,density
ofsoilmass,organicmatter,moisturecontent&surface
roughness.Ahighsoilmoisturecontentalwaysreducesthe
effectofwindonsoilerosion,whiledrysoilissubjecttosevere
winderosion.Roughnessofsoilsurfacecreatesretardingeffect
onwindvelocity,therebythewinderosiongetsreduced.
3)Vegetation:Itcreatesdirectobstructioninflowpathof
blowingwind&deflectsthewindcurrents

6. MechanicsofWindErosion:
Phasesofwinderosion–
1)Initiationofsoilmovement
2)Transportationofsoilparticles
3)Depositionofsoilparticles

7. (1)Initiationofsoilmovement:
•Movementofsoilparticlesiscausedbywind
forcesagainstorparalleltothesurfaceofground.
Theerosivewindisturbulentatallheights,
exceptatclosetosurfaceofground,itislaminar.
Thevelocityislowestnearthegroundand
increasesinproportiontothelogarithmofthe
height.Windmayliftsoilparticlesfromthe
surfaceintheinitiationofsoilmovement.
Threetypesofparticlemovementsare:
(a)Suspension,(b)Saltationand(c)Surface
creep.

8. (a)Suspension
•Def:Itisthemovementofveryfinesoilparticles,
generallylessthan0.1mmindiameter.
•Asmallparticleshaslowsettlingvelocityandoncethey
liftedup,itremainssuspendedinairforalongperiodof
timebytheeffectofturbulence.Suspensionrepresent
thefloatingofsmallsizedparticlesintheairstream.
Movementofthesefineparticlesinsuspensionis
initiatedbytheimpactofparticlesinsaltation.
•Theyareoftencarriedmanykilometersbeforebeing
deposited.Suchupwardeddieshavevelocitiesupto3-5
kmphwhicharesufficienttoliftveryfinesand,siltand
clay.
•Dustparticleswillcomedowntothesurfaceonlywhen
thewindsubsides.Ithasbeenfoundthat3%to40%soil
aretransportedthroughthesuspensionmechanism.

9. (b)Saltation
Def:-Thesoilparticlescarriedbywindismovedinaseriesof
bouncesorjumps(0.05to0.5mmsize)iscalledassaltation.
•Soilparticlesofsizerangingfrom0.05to0.5mmusuallymovesinthis
manner.
•Themaincauseofsaltationmovementisbythepressureofwindon
thesoilparticlesandcollisionofparticleswithotherparticles.
•Thedirecthitofthewindagainstsoilparticlesdislodgesthemfrom
theirseats.
•Theymoveashortdistancealongthesurfaceandthensuddenlyshoot-
upinajumpingmovement(saltation).
•Heightofthejumpvarieswiththesizeanddensityofthesoilparticles
andthevelocityofwind.
•Theycontinuetogainvelocityuntiltheystriketheground.
•Thentheyeitherreboundandcontinuetheirmovementinsaltation.

10. •Asthesoilparticleleavesthesurface,itmovesnearlyvertical
direction.
•Thehorizontaldistancethroughwhichtheparticlecontinuestorise
isabout1/5
th
to1/4
th
ofthedistanceL.
•Astheparticledescendstothesurfaceittravelsinastraightline
withanangleofdescentofabout6
0
to12
0
.
•Fromthemechanicsofsaltation,themovementofsoilbywindis
dependentnotsomuchontheforceofthewindactingonthe
groundsurface.
•Themajorportionofsoil(50%to75%)iscarriedbysaltation.

11. (c)Surface Creep
Def.:-Itistherollingorslidingoflargesoilparticles(0.5to2
mmsize)alongthegroundsurface.Theyaretooheavyto
theliftedbywind&aremovedprimarilybytheimpactof
particlesinsaltationratherthanbythedirectforceofwind.
•Theproportionofthethreetypesofmovementsvaries
greatlyfordifferentsoils.Between50to75%oftheweightof
soilerodedbywindiscarriedinsaltation,3to40%in
suspensionand5to25%insurfacecreep.
•Coarselygranulatedsoilerodesmainlyinsurfacecreep,while
pulverizedsoilmovesinsaltationandsuspension.
•Ifthemovementbysaltationiscontrolled,therewillbelittle
soilmovementbysurfacecreepandsuspension.
•Forordinarywindvelocities93%ofthetotalsoiliscarriedat
lessthan30cmheight,andonlyasmalltraceofsoiliswind
bornabove1mfromgroundlevel.

12. (2)Transportationofsoilparticles:
Transportationisinfluencedbyparticlesize,
gradationofparticles,windvelocity&distance
acrosserodingarea.
3)Depositionofsoilparticles:
Depositionofparticledependsuponparticles
weightandwindvelocity.Thedepositionofsoil
particlesoccurswhengravitationalforceisgrater
thantheresistingforceholdingtheparticleinair.

13. Sand dunes
Def:-Sandduneisasandwaveofapproximatelytriangular
crosssectionformedbywind.
•Onceabedofsandbecomesexposedtowindaction,itstart
movinginthedirectionofwind.
•Thesanddunesonceformedcansubmergedgoodfields.
•Thetypicalsanddunehasagentleslopeof5-10
0
to
windwardsideandverymuchsteeperslopeofabout30
0
to
leewardside.
•Thewindtendstopushtheindividualparticlesonwardsand
upwardsuntilthedunetipreachesaconstantheight.
•Thisheightdependsuponstrengthofwind,sizeofsand
grain,moistureandpresenceorabsenceofvegetation.