Description and explaination of the various processes of aeolian erosion,transportation and deposition

Wind erosion is largely controlled and determined by- wind velocity,nature and amount of sands dusts and pebbles,composition of rocks,nature of vegetation,humidity,rainfall amount  and temperature.Wind erosion is believed to be defective only upto 182 cm. above the earth's surface.Maximum wind erosion occurs at a short distance above the ground,say at a height of 20-25cm. because here the wind velocity and sediment movement are moderate whereas at the ground sediment transport is high but velocity of transportation is low.At a height the velocity is high but quantity of sand available for abrasion is small.Due to their variable direction they erode rocks from all directions.

There are erosional processes:-

a)Deflation:-(derived from the Latin word 'deflatus' meaning blowing away)

It includes removing,lifting and blowing away dry and loose particles of sands and dusts i.e., incoherent sediments,by wind.It is controlled by 3 factors:-

1)Surface soil's moisture content-soil moisture forms cohesion between the particle's meniscuses which becomes maximum when meniscuses are tightly curved out and extend over maximum grain surface.Thus,high moisture content indicates low deflation and vice versa.

2)Surface roughness-If the ratio of silhouette area of pebbles to the surface area is >0.1 then erosion is restricted and when pebbles are for apart mobility of finer sediments increase as wind scours round the stones.

3)Grain size-Particles of different sizes only begin to move at a certain wind velocity called threshold shear velocity.The most widely used empirical formula is:

                           E=f(I,K,C,L,V)   where,E=potential erosion loss in tons/acre/yr.;I=soil erodibility index;K=ridge roughness factor;C=local climatic index;L=factor relating to field shape in prevailing wind direction;& V=vegetation cover index.

b)Abrasion:-

In this process consolidated,cohesive materials are worn down when the grains are hurled on them,which on hitting the target,roughly spherical grains create circular "Hertzian fractures" roughly 10% - 30% of the grain,depending on impact velocity.The mass of the target removed per impact 'A' can be expressed as:-

As a new facet is abraded,it alters it's angle which may accelerate or decelarate abrasion.Clemens et al concluded that maximum abrasion occurs between about 0.07m - 0.5m.Angular grains chips away and create a steady loss compared to their circular counterparts.

c)Attrition:-

It involves mechanical wear and tear causing reduction of the particle size moving with the wind.

                                                           Wind's transportational works

It varies from other erosional agents  because of wind's highly variable direction.It involves entrainment of loosened grains of sand and dust in the air.The process of transportation are:-

a)Suspension:-

In the suspension grains flow turbulent motion.the vertical velocity in turbulence zone near the ground is approximately equal to the drag velocity(U*),so that Uf

Uf is defined by Stoke's law: it's a function of the balance between the weight of the particle and the drag of the air upon it.Observations over the  level surfaces showed sharp transition to suspension when the grain size is below 0.1mm. Strong turbulence in heated condition can put large grains in suspension.

b)saltation:-(the term is coined by gilbert;derived from Latin word "saltare" meaning "to leap")

In this process an ejected grain is hurled and gathers momentum from the wind before descending back to the surface.The saltation grains are able to splash up other grains.

c)Creep:-

surface movement involving particles coarser than those of saltatin or reptating ones and involves 2 types of movement together:-

1)Saltation impact induced rolling;

2)gravity induced rolling into craters created by saltation impacts.

d)Reptation:-(latin word to "move slowly")

the distribution of grain velocities are strongly exponential and heavily weighted towards small velocities.the mean launch angle of the reptating grains is 60 degree - 70 degree.10 reptating gains are dislodged by 1 saltating grains as has been proved mathematically,theoretically.

there are 3 types of deposition work of the wind.there are certain criterion or factors which influence deposition:-

a)grain size;

b)composition;

c)type of structure obstructing the path of the wind and the direction of the wind;

Deposition occurs on both windward and Leeward sides of the fixed obstacle.Deposition may occur because of change in wind velocity.

Sand shadows are depositions occurring on either sides of the obstacles.Sand drifts are accumulations of sands between the obstacles.the 3 processes of deposition are as follows:-

a)Sedimentation:-

This occurs when the grains fall out of the air or stop creeping forward.there are 2 types of sedimentation dry and wet.

b)accretion:-

It occurs when the grains being moved by saltation hit the surface with such force that some grains carry on moving as surface creep but the majority come to the rest where they strike.

c)encroachment:-

It occurs when deposition occurs on the rough surface.