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. Wind erosion:- It is the process of detachment,
transportation and deposition of soil particles
by the action of wind.
The basic causes of wind erosion are few & very
simple. Most commonly, it takes place, when:
1) Soil is loose, finely divided & dry
2) Soil Surface is smooth & bare
3) Wind is strong enough to detach the soil
particles from the soil surface

4. Wind Erosion Problem Area :-
• Coastal sandy area – In India about 1.47 m.ha lands are spread as
coastal sandy areas. Such areas are lashed by high wind velocity.
The formation of sand dunes near the sea coast is major problem.
• Desert Areas - The desert areas are severely affected by wind
erosion. In India about 11.796 m.ha lands are grouped as desert
area. The formation and movement of sand dunes is the major
problem of desert areas. Moving sand dunes covers the roads,
bridges, rail lines & even human settlements.
• Semi-arid Areas – These areas are found at the boundary of the
deserts. In semi-arid areas the desert is in advancing stage, which
covers the valuable lands. The semi-arid areas are also affected by
severe wind erosion and formation of sand dunes.
• Inland River Sands - These are found along the plain of major rivers.
Such areas also have the problem of wind erosion, viz. Ganga and
Chambal rivers.
• It is estimated that about 17.2 m.ha of land is suffering from wind
erosion & extension of deserts.

5. Factors affecting Wind Erosion
1) Climatic factor: precipitation, temperature, humidity, velocity
& density of prevailing wind. When mean annual rainfall is
comparatively less, the moisture content in soil is less, & thus
the soil particles are in loose condition & they detaches easily
due to blowing wind & causes soil erosion. Due to rise in
atmospheric temperature the soil moisture reduces & that soil
is more susceptible for erosion.
2) Soil Condition: Texture, structure, size of soil particles, density
of soil mass, organic matter, moisture content & surface
roughness. A high soil moisture content always reduces the
effect of wind on soil erosion, while dry soil is subject to severe
wind erosion. Roughness of soil surface creates retarding effect
on wind velocity, thereby the wind erosion gets reduced.
3) Vegetation: It creates direct obstruction in flow path of
blowing wind & deflects the wind currents

6. Mechanics of Wind Erosion:
Phases of wind erosion –
1) Initiation of soil movement
2) Transportation of soil particles
3) Deposition of soil particles

7. (1)Initiation of soil movement :
• Movement of soil particles is caused by wind
forces against or parallel to the surface of ground.
The erosive wind is turbulent at all heights,
except at close to surface of ground, it is laminar.
The velocity is lowest near the ground and
increases in proportion to the logarithm of the
height. Wind may lift soil particles from the
surface in the initiation of soil movement.
Three types of particle movements are:
(a) Suspension, (b) Saltation and (c) Surface
creep.

8. (a) Suspension
• Def: It is the movement of very fine soil particles,
generally less than 0.1 mm in diameter.
• A small particles has low settling velocity and once they
lifted up, it remains suspended in air for a long period of
time by the effect of turbulence. Suspension represent
the floating of small sized particles in the air stream.
Movement of these fine particles in suspension is
initiated by the impact of particles in saltation.
• They are often carried many kilometers before being
deposited. Such upward eddies have velocities up to 3-5
kmph which are sufficient to lift very fine sand, silt and
clay.
• Dust particles will come down to the surface only when
the wind subsides. It has been found that 3% to 40% soil
are transported through the suspension mechanism.

9. (b) Saltation
Def:- The soil particles carried by wind is moved in a series of
bounces or jumps (0.05 to 0.5 mm size) is called as saltation.
• Soil particles of size ranging from 0.05 to 0.5 mm usually moves in this
manner.
• The main cause of saltation movement is by the pressure of wind on
the soil particles and collision of particles with other particles.
• The direct hit of the wind against soil particles dislodges them from
their seats.
• They move a short distance along the surface and then suddenly shoot-
up in a jumping movement (saltation).
• Height of the jump varies with the size and density of the soil particles
and the velocity of wind.
• They continue to gain velocity until they strike the ground.
• Then they either rebound and continue their movement in saltation.

10. • As the soil particle leaves the surface, it moves nearly vertical
direction.
• The horizontal distance through which the particle continues to rise
is about1/5th to 1/4th of the distance L.
• As the particle descends to the surface it travels in a straight line
with an angle of descent of about 60 to 120.
• From the mechanics of saltation, the movement of soil by wind is
dependent not so much on the force of the wind acting on the
ground surface.
• The major portion of soil (50% to 75%) is carried by saltation.

11. (c) Surface Creep
Def.:- It is the rolling or sliding of large soil particles (0.5 to 2
mm size) along the ground surface. They are too heavy to
the lifted by wind & are moved primarily by the impact of
particles in saltation rather than by the direct force of wind.
• The proportion of the three types of movements varies
greatly for different soils. Between 50 to 75% of the weight of
soil eroded by wind is carried in saltation, 3 to 40% in
suspension and 5 to 25% in surface creep.
• Coarsely granulated soil erodes mainly in surface creep, while
pulverized soil moves in saltation and suspension.
• If the movement by saltation is controlled, there will be little
soil movement by surface creep and suspension.
• For ordinary wind velocities 93% of the total soil is carried at
less than 30 cm height, and only a small trace of soil is wind
born above 1 m from ground level.

12. (2) Transportation of soil particles:
Transportation is influenced by particle size,
gradation of particles, wind velocity & distance
across eroding area.
3) Deposition of soil particles:
Deposition of particle depends upon particles
weight and wind velocity. The deposition of soil
particles occurs when gravitational force is grater
than the resisting force holding the particle in air.

13. Sand dunes
Def:- Sand dune is a sand wave of approximately triangular
cross section formed by wind.
• Once a bed of sand becomes exposed to wind action, it start
moving in the direction of wind.
• The sand dunes once formed can submerged good fields.
• The typical sand dune has a gentle slope of 5-100 to
windward side and very much steeper slope of about 300 to
leeward side.
• The wind tends to push the individual particles onwards and
upwards until the dune tip reaches a constant height.
• This height depends upon strength of wind, size of sand
grain, moisture and presence or absence of vegetation.