Soil Erosion

Course: B.Sc. Agricultural

Subject : Fundamental of Soil and Water conservation Engineering

Unit: 3

Soil Erosion

• Is the process of detachment of soil particlesfrom the top soil and transportation of thedetached soil particles by wind and/orwater.

• The agents causing erosion are wind andwater. The detaching agents are fallingraindrop, channel flow and wind. Thetransporting agents are flowing water, rainsplash and wind

Losses due to erosion:

i) Loss of fertile top soil

ii) Loss of rain water

iii) Loss of nutrients

iv) Silting up of reservoirs

v) Damage to forests

vi) Reduction in soil depth

vii) Floods

viii) Adverse effect on public health

ix) Loss of fertile land

x) Economic losses

Types of erosion:

• There are two major types of soil erosiona) Geological erosion (Natural or normal erosion): is

said to be in equilibrium with soil forming process.It takes place under natural vegetative covercompletely undisturbed by biotic factors. This isvery slow process.

b) Accelerated erosion: is due to disturbance innatural equilibrium by the activities of man andanimals through land mismanagement, destructingof forests over grazing etc.Soil loss through erosion is more than the soilformed due to soil forming process

Based on the agents causing erosion

Erosion is divided into

a. Water erosion

b. Wind erosion

c. Wave erosion

Water erosion

• Loss of soil from land surface bywater including run off frommelted snow and ice is usuallyreferred to as water erosion.

• Major erosive agents in water erosion are impacting/falling raindrops and runoff water flowing over soilsurface.

• Process of water erosion

Detachment of soil particles is by either raindropimpact or flowing water.

Individual raindrops strike the soil surface atvelocities up to 9 m/s creating very intensivehydrodynamic force at the point of impact leading to soilparticle detachment.

Over land flow detaches soil particles when theirerosive hydrodynamic force exceeds the resistance of soilto erosion.

Detached soil particles are transportedby raindrop splash and runoff.

The amount of soil transported by runoffis more than due to raindrop splash. Thusthe falling raindrops break the soilaggregates and detach soil particles fromeach other.

The finer particles (silt and clay) blockthe soil pores and increase the rate ofrunoff and hence loss of water and soil.

Forms of water erosion

• Water erosion occurs in stages identified assheet erosion, rills, gullies, ravines, landslidesand stream bank erosion.

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a) Sheet erosion:• It is the uniform removal of surface soil in thinlayers by rainfall and runoff water.• The breaking action of raindrop combined withsurface flow is the major cause of sheet erosion.• It is the first stage of erosion and is leastconspicuous, but the most extensive.

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• b) Rill erosion:

• When runoff starts, channelization begins anderosion is no longer uniform.

• Raindrop impact does not directly detach anyparticles below flow line in rills but increases thedetachment and transportation capacity of theflow.

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• Rill erosion starts when the runoff exceeds 0.3to 0.7 mm/s.

• Incisions are formed on the ground due torunoff and erosion is more apparent thansheet erosion.

• This is the second stage of erosion.

• Rills are small channels, which can beremoved by timely normal tillage operations.

• c) Gully erosion:

• It is the advanced stage of water erosion. Size of theunchecked rills increase due to runoff.

• Gullies are formed when channelized runoff form vastsloping land is sufficient in volume and velocity to cutdeep and wide channels.

• Gullies are the spectacular symptoms of erosion. Ifunchecked in time no scope for arable crop production

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• d) Ravines:

• They are the manifestation of a prolongedprocess of gully erosion. They are typically foundin deep alluvial soils. They are deep and widegullies indicating advanced stage of gully erosion.

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• e) Landslides:

• Landslides occur in mountain slopes, when theslope exceeds 20% and width is 6m. Generallyland slides cause blockage of traffic in ghatroads.

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• f) Stream bank erosion:

• Small streams, rivulets, torrents (hill streams) aresubjected to stream bank erosion due to obstruction oftheir flow. Vegetation sprouts when streams dry up andobstructs the flow causing cutting of bank or changingof flow course.

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Factors affecting water erosion

• a) Climate:

• Water erosion is directly a function of rainfall and runoff.Amount, duration and distribution of rainfall influencesrunoff and erosion.

• High intensity rains of longer duration causes severeerosion. Greater the intensity, larger the size of theraindrop. Rainfall intensity more than 5 cm/hr isconsidered as severe.

• Total energy of raindrops falling over a hectare land withrainfall intensity of 5 cm /hr is equal to 625 H.P. Thisenergy can lift 89 times the surface 17.5 cm of soil fromone ha to a height of 3 ft.

• Runoff may occur without erosion butthere is no water erosion without runoff.

• The raindrop thus breaks down soilaggregates, detaches soil particles andleads the rainwater with the fineparticles.

• These fine particles seal the pores of thesurface soil and increases runoff causingerosion.

• b) Topography:

• The degree, length and curvature of slope determinethe amount of runoff and extent of erosion. Waterflows slowly over a gentle slope where as at a fasterrate over a steeper one. As water flows down theslope, it accelerates under the forces of gravity.When runoff attains a velocity of about 1 m/s it iscapable of eroding the soil.

• If the percent of slope is increased by 4 times thevelocity of water flowing down is doubled. Doublingthe velocity quadruples the erosive power andincreases the quantity of soil that can betransported by about 32 times and size of theparticles that can be transported by about 64 times.

• c) Vegetation:

• Vegetation intercepts the rainfall and reducesthe impact of raindrops. It also decreases thevelocity of runoff by obstructing the flow ofwater.

• The fibrous roots are also effective in formingstable soil aggregates, which increasesinfiltration and reduces erosion.

• d) Soil Properties:• Soil properties that influence soil erodibility by water

may be grouped into two types.• i. Those properties that influence the infiltration rate

and permeability• ii. Those properties that resist the dispersion,

splashing, abrasion and transporting forces of rainfalland runoff.

• The structure, texture, organic matter and moisturecontent of upper layers determine the extent oferosion. Sandy soils are readily detachable but notreadily transportable. Soils of medium to high claycontent have low infiltration capacities and they arereadily transported by water after they are dispersed,but their detachability is generally low.

• e) Man and beast

• Man and beast accelerates erosion byextensive farming and excessive grazing.

• Faulty practices like cultivation on steepslopes, cultivation up and down theslope, felling and burning of forests etc.,leads to heavy erosion.

• Excessive grazing destroys all vegetationand increases the erosion.

Estimation of soil loss by water erosion

• Based on the mechanism and factors influencing soilerosion, a universal soil loss equation (USLE) developed byWischmeier (1959) is most useful for predicting soil lossdue to water erosion.

• A= R x K x L x S x C x Pwhere, A= predicted soil loss in t/ha/yearR= rainfall erosivity factor or indexK= soil erodibility factorL= length of slope factorS= slope steepness factorC= soil cover and management factor andP= erosion control factor

Wind erosion

• Erosion of soil by the action of wind is knownas wind erosion. It is a serious problem onlands devoid of vegetation. It is more commonin arid and semi arid regions. It is essentially adry weather phenomenon stimulated by thesoil moisture deficiency.

• The process of wind erosion consists of threephases: initiation of movement, transportationand deposition. About 33 m.ha in India isaffected by wind erosion.

• This includes 23.49 m.ha of desert and about 6.5m.ha of coastal sands. The Thar Desert is formedmainly by blow in sand.

Mechanism of wind erosion :

• Lifting and abrasive action of wind results indetachment of tiny soil particles from the granules orclods. The impact of these rapidly moving particlesdislodge other particles from clods and aggregates.

• These dislodged particles are ready for movement.Movement of soil particles in wind erosion isinitiated when the pressure by the wind against thesurface soil grains overcomes the force of gravity onthe grains.

• Minimum wind velocity necessary forinitiating the movement of most erodable soilparticles (about 0.1 mm diameter) is about 16km /hr at a height of 30.5 cm.

• Most practical limit under field conditions,where a mixture of sizes of single grainedmaterial present is about 21 km/hr at a heightof 30.5 cm.

• In general movement of soil particles by windtakes place in three stages:

- A. Saltation

- B. Surface creep and

- C. Suspension.

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• A. Saltation:

• It is the first stage of movement of soilparticles in a short series of bounces or jumpsalong the ground surface. After being rolled bythe wind, soil particles suddenly leap almostvertically to form the initial stage ofmovement in saltation. The size of soilparticles moved by saltation is between 0.1 to0.5 mm in diameter.

This process may account for 50 to 70% of thetotal movement by wind erosion.

• B. Surface creep:

• Rolling and sliding of soil particles along theground surface due to impact of particlesdescending and hitting during saltation is calledsurface creep.

• Movement of particles by surface creep causesan abrasive action of soil surface leading to breakdown of non-erodable soil aggregates.

• Coarse particles longer than 0.5 to 2.0 mmdiameter are moved by surface creep.

• This process may account for 5 to 25% of thetotal movement.

• C. Suspension:

• Movement of fine dust particles smaller than0.1 mm diameter by floating in the air isknown as suspension. Soil particles carried insuspension are deposited when thesedimentation force is greater than the forceholding the particles in suspension.

• This occurs with decrease in wind velocity.Suspension usually may not account for morethan 15% of total movement.

References/Sources1.https://lh3.ggpht.com/W3w2gJLqs7FyjdbIVnJIMU622EVH7ljT9FbycU1p5Ece7VkvMglwjQD4Xt9uWkwdiyHRTQ=s106

2.https://lh4.ggpht.com/pO4Z4bnv4uF0PatmNO_DkVsZYEoMf5a9TSrNH2wNnRD79EWl0oVcapxTDCZVQUCJp8-4=s157

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Text book and web sources

- www.agriinfo.in

- ecourses.iasri.res.in

- Soil erosion and Conservation by R. P. Tripathiand H. P. Singh