4/7/2020 DBCA, SULCORNA Lect. No. 3 - PowerPoint Presentation

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4/7/2020

DBCA, SULCORNA

Lect. No. 3

Wind Erosion-Principle, mechanics, types of soil movement

Course Teacher: Er. Vinod T. Atkari

Course No. : ENGG-121

Course Title: Soil & Water Conservation Engineering

Credits: 2 (1+1)

Semester: II

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DBCA, SULCORNA

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:

Soil is loose, finely divided & drySoil Surface is smooth & bareWind is strong enough to detach the soil particles from the soil surface4/7/2020DBCA, SULCORNA

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.4/7/2020DBCA, SULCORNA

Factors affecting Wind Erosion

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.

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.Vegetation: It creates direct obstruction in flow path of blowing wind & deflects the wind currents

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Mechanics of Wind Erosion:

Phases of wind erosion –

Initiation of soil movement

Transportation of soil particlesDeposition of soil particles4/7/2020DBCA, SULCORNA

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.4/7/2020DBCA, SULCORNA

(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. Once these small particles are picked up by the particles in saltation and enters the turbulent air layers, they can be lifted high into the air by the upward eddy currents. 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.

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(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). They 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.   As the soil particle leaves the surface, it moves nearly vertical direction. The horizontal distance through which the particle continues to rise is about 1/5th 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

.

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(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. The concentration of particles carried by the wind is greatest just above the surface of the ground and decreases rapidly with height. 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. 4/7/2020DBCA, SULCORNA

(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.

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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 5-10

0 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. The whole dune is liable to move forward by transfer of sand grains from windward to leeward side, with the action of high wind. In this way a dune may move several meters in a single windstorm. High dunes move very slowly while low dunes may shift at a fast rate. Large dunes are very destructive when they invade areas of valuable land, buildings, crops and forests.

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Types of sand dunes:

Active dunesCrescent dunes

Linear dunesPyramidal dunes

B) Fixed Dunes4/7/2020DBCA, SULCORNA

Thank you for kind attention…

4/7/2020

DBCA, SULCORNA