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The occurrence of wind erosion could be described under following three different phases. They are: 1. Initiation of Soil Movement 2. Transportation of Soil Particles 3. Deposition of Soil Particles.
Phase # 1. Initiation of Soil Movement:
Soil movement is initiated by the turbulence created by strong wind currents. The movement of soil particles may be described in three distinct forms, depending on the size of the soil particles.
The three types of particles movements are given below:
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i. Suspension:
It is the movement of very fine soil particles, generally less than 0.1 mm in diameter. According to the Stoke’s Law, the speed of freely falling body through a fluid is directly proportional to the square of the particle’s diameter. In this way, a small particle has low settling velocity and once lifted up, it remains suspended in the air for a long period of time by the effect of turbulence and eddy currents of the air.
Dusty storm is an example of this kind of movement, which has fine particles in suspension. In suspension process a large amount of soil is transported to a longer distance. From laboratory studies, it has been found that 3 to 38% soils are transported through suspension mechanism.
ii. Saltation:
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This process is most effective among all three forms of soil movement. Saltation is referred as the movement of soil particles in a series of low bounces over the soil surface. It is responsible, mainly for the medium size soil particles. The size range of particles is from 0.05 to 0.5 mm in diameter.
However, the most vulnerable range of particles for this type of movement is from 0.1 to 0.15 mm diameter. Fig. 7.3 presents the path of saltation process. The kind of particles movement, depending on the soil particle’s size is shown in Fig. 7.4.
From Fig. 7.3, it can be narrated that first the soil particles rise into the air up to some height, and then they return back to the surface, probably due to decrease in vertical movement, caused by gravity effect. At the same time the soil particles are also picked up by the wind currents moving in lateral direction.
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After rising of particles to a peak point which is only few centimeters above the ground, they start to fall down, but continue to accelerate laterally due to wind force, and so return to the land surface long flat glide path, striking the soil with high energy. Among all the types of soil movements, the saltation is responsible for transporting the maximum amount of soil particles along the ground surface by rolling and pushing actions, which varies from 55 to 72% of total soil erosion.
iii. Surface Creep:
Surface creep is referred as the movement of soil particles by rolling along the land surface, activated by the wind force and other particles moving with the wind. Theoretically, there is no upper limit of the size of soil particles, which are responsible for movement through surface creep, but most rolling particles are found in the size from 0.5 mm to 1 or 2 mm in diameter.
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Apart from above, the movement of soil particles is also initiated by several processes, which act separately or in combination. During collision of rolling grains and bumping over the land surface, few soil particles are easily bounced up and suspended into the air, depending on the particles size. This effect is due to pressure difference, caused by blowing wind over the soil surface (i.e., venturi effect).
Another factor, that can help to initiate the process of soil movement, is the vibration energy, which imparts two bodies in their path. The process behind this phenomena is that, the dry soil particles pick up a very high frequency of vibration energy from the wind, and when two such rapidly vibrating particles come in contact with each other, then one of them is flipped up into the air and initiation of soil movement is resulted at that moment.
Regarding initiation of soil movement by wind, there are several views given by different researchers, few of them are as under:
Chepil (1945) reported that the soil particles responsible to move under saltation process, are ascended about 1/4 to 1/5 of the total leap length. The particles leaping to a height of 5 cm or less, travelled about 7 times the height, in horizontal direction over the land surface. And the particles leaping more than 15 cm, travel about 10 times the height, in horizontal direction. About 57% load is carried in saltation by an erosive wind which leaping height is less than 5 cm; 93% soils move in leaps less than 30 cm height and less than 1% soil movement is observed in the leaps higher than 100 cm above the soil surface.
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Bagnold (1937) reported that the sand particles rise into the air under saltation process, because of the following reasons:
i. Stationary soil particles present over the land surface are knocked into the air by impact of descending particles.
ii. The saltating soil particles, which return to the soil surface, are mixed back into the air.
Free and Chepil stated that the force responsible to start the movement of soil particles in saltation, is the turbulent current of the air stream. A wind velocity in excess of 4.65 m/s is essential for creating turbulence, to lift a soil cube of 0.50 mm diameter.
Bisal and Nielson claimed that, the soil particles do not roll on the soil surface prior to get lift into the air stream, but they vibrate first and then jump vertically upward.
Particles Classification for Wind Erosion:
The classification of soil particles as per their size, which are susceptible to get erode by wind, is given as under:
(i) Larger than 1 mm in diameter – non erodible.
(ii) Between 1 to 0.5 mm – erodible only by very strong velocity wind.
(iii) Less than 0.5 mm – highly erodible
A soil, which contains large proportion of non-erodible particles is said to be less erodible soil.
Phase # 2. Transportation of Soil Particles:
The transportation of soil particles in wind erosion, is directly influenced by the particles size, gradation of particles, wind velocity and distance across eroding area. From field studies, it has been found that the quantity of soil moved through wind, varies as the cube of excess wind velocity over and above the constant threshold velocity. And also, it is directly proportional to the square root of the soil particles diameter. In addition, the transportation also gets increase as the gradation of the soil particle decreases.
In Fig. 7.5 the degree of wind erosion has been presented by two different proportions of erodible and non-erodible soil fractions, in which the distance between erodible to no-erodible soil fractions is indicated by ‘Y’. In Fig 7.5 (a) the erodible and non-erodible soil particles are widely separated from each other, are easily eroded, but in Fig. 7.5 (b) these two are close to each other, and are not easily eroded.
Thus, for well stabilized land surface, to reduce the wind erosion, the ratio of distance between (hem should be constant, regardless of particle size. However, it may be changed due to wind velocity, size and specific gravity of the soil particles.
i. Transportation Distance of Soil Particles:
The transportation distance of soil particles in wind erosion depends on diameter of the particles and maximum height up to which they are lifted, mainly.
The following formula can be used for computing the transportation distance of soil particles through blowing winds:
Where,
l = transportation distance (m)
hm = maximum height up to which soil particle is lilted (m)
t = temperature (°C)
g = 9.81 m/s2
α = constant
ii. Amount of Soil Blown and Deposited:
The amount of soil materials blown and deposited in wind erosion depends on the height up to which they are lifted, maximum wind velocity involved in the transport and transportation distance (i.e., the horizontal trajectory of the particles).
The following formula can be used for computing the amount of soil blown, transported and deposited in the shifting of sands:
G = 0.5 hm . b Vt …(7.32)
Where,
G = amount of soil materials transported (m3/s/m)
hm = height upto which material is transported (m), can be predicted by using the equation, given as under –
Vm = maximum wind velocity involved in transport of soil particles (m/s)
Vk1 = critical wind velocity at which particles begin to move on the soil surface. It can be calculated by using the equation 7.13.
g = 9.81 m/s2
b = relative width of sand drift (m)
Vt = transit velocity of moving particles (m/s), which is the difference of V and Vk i.e. (V – Vk)
Phase # 3. Deposition of Soil Particles:
Alter initiation and transportation of soil particles, the next phase is their deposition on the soil surface. Deposition of soil particles depends on the particle’s weight and wind velocity, in which particles weight is directly related to the gravitational force; has prime importance for deposition.
The deposition of soil particles occurs, when gravitational force is greater than the resisting force holding the particles in the air. The particles deposition can also takes place, when wind velocity gels decrease sufficiently near the ground surface either due to surface roughness or some other natural causes.
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