Understanding the Mechanics of Soil Movement by Wind

Wind erosion is mainly a surface phenomenon and is influenced directly by wind velocity in various strata near the ground through which soil particles rise in saltation. Therefore, a brief discussion about wind structure near the ground would be necessary for understanding the mechanics of soil movement by wind.

Wind Structure Near the Ground:

The erosion causing wind is always turbulent; that is, its flow is characterised by eddies moving in all directions at variable velocities. Measurements show that zero velocity is somewhere above the roughness elements of the surface.

As shown in Fig. 4.3, the estimated zero velocity is at height Z₀ + d in which Z₀ refers to aerodynamic surface (surface roughness parameter) and d the height above Z₀, where velocity is zero (also called zero plane displacement).

The air is usually calm or slow moving in the Z₀ zone. The zero velocity at height Z₀ + d is obtained for impervious surfaces, such as the ground surface. Over porous vegetative surfaces, however, the velocity at Z₀ is greater than zero (shown by discontinuous line, Fig. 4.3 curve a).

The total surface roughness is actually Z₀ + d and depends upon the vegetation height, density and other characters of surface features, d is actually a conceptual tool which is used to make the wind profile over rough surface and conform to an ideal relation between wind, speed U and height Z above the surface, such as:

Where, U = wind speed at height Z

k = van Karman’s constant (≈ 0.4)

Ï„ = shear stress (the flux of horizontal momentum transferred vertically and absorbed by the ground)

ρ_a = density of air

H = crop height

At some point near the ground surface, usually less than 2.5 mm for a bare, relatively smooth surface, the wind velocity is zero. For a very short distance above this level, the flow of air is smooth and laminar and ends into turbulent flow at higher elevations.

It is this turbulent flowing air which produces the force to cause soil movement. As shown in Fig. 4.3 the average forward velocity of a turbulent wind increases exponentially with height above Z₀.

Wind Forces Causing Soil Movement:

Moving air exerts three types of pressure on a soil grain resting on the ground. One is a positive pressure against that part of the grain facing into the direction of movement. This pressure results from the impact of air against the grain and is called the impact or velocity pressure.

The velocity pressure causing soil movement varies directly as square of the fluid velocity. It is expressed as force per unit cross-sectional area of the grain normal to the direction of fluid in motion. The second type is a negative pressure on the lee side of the grain, known as viscosity pressure.

Its magnitude depends on the fluid’s coefficient of viscosity, density and velocity. The third type of pressure is a negative pressure on the top of the grain caused by the Bernoulli effect. According to Bernoulli law, wherever the fluid velocity is speeded up, as at the top of the soil .grain, the pressure is reduced.

The impact or velocity pressure on a soil grain over the ground is known as ‘form drag’, and the pressure due to viscous shear in the fluid close to the sur­face of the soil grain is called ‘skin friction drag’. The sum of the two forces is called the ‘total drag’ or simply drag.

The drag F_c, on the top of grain at the threshold of its movement is due to the pressure difference against its windward and leeward sides. The arrow marked by F_c in Fig. 4.4 indicates the general direction and the average level at which it acts.

Lift on the grain is caused by decrease in static pressure at the top of the grain as compared to that at the bottom. It is determined by the pressure dif­ference against the top and the bottom halves of the grain.

The threshold drag acting on a spherical grain can be expressed as:

Chepil (1959) observed that drag on the topmost grains on the bed acts at an average level of about one-third of the grain diameter below the top of these grains and the angle of repose of the topmost grains with respect to the mean level of drag is about 24°. Therefore, tan Ï•’ is equal to about 0.45 and the value of n’ may be about 0.2 and T_f as 2.5.

Lift and drag on soil grains change rapidly as the grains move up from the surface of the ground. Lift decreases with height and becomes almost negli­gible a few grain diameter heights above the ground.

The greater the ground roughness and drag velocity of the wind, the higher is the lift due to steeper velocity gradients. Drag on the other hand, increases with height just as wind velocity increases with height, and apparently is due mainly to the direct pres­sure of the wind against the grain.

Initiation of Soil Movement:

Movement of soil particles in wind erosion is initiated when the pressure by the wind against the surface soil grains overcomes the force of gravity on the grains. The wind forces result from impact or velocity, viscosity and static or external pressure over the surface of the ground. The impact or velocity results positive pressure against that part of the grain facing in the direction of wind.

The mag­nitude of force exerted is governed by wind velocity above the ground surface. Soil particles or other projections on the surface protruding into the layer of turbulent air absorb most of the force created on the surface.

If the particles are sufficiently large or attached to other particles, they may resist the force of strong winds. If, however, they are unattached and are not too heavy, wind may lift them from the surface initiating the soil movement.

Fine dust particles offer high resistance to movement by wind partly due to cohesion among the par­ticles (which raises the threshold value) and to the fact that the particles are too small to protrude above the laminar and viscous layer of air over the ground surface. Fine dust particles are, therefore, ejected into the atmosphere mainly by impacts of larger grains.

Above discussion reveals that a certain minimum velocity is needed to initiate the movement of soil particles. The velocity at which the movement of most erodible soil particle is initiated is known as the minimum fluid threshold velocity. This minimum fluid threshold velocity depends on the size and weight of soil particles and the friction provided by neighbouring particles.

This threshold velocity is the lowest for grains of 0.1 to 0.15 mm diameter. Observations show that surface winds exceeding about 3 km/hr velocities are turbulent and are responsible for initiating the process of wind erosion. In fact, the threshold velocity depends not on the average forward velocity but on the maximum momentary velocity of turbulent flow.

As wind velocity increases beyond this minimum, bigger size particles start moving till the movement of almost all size of particles in a soil containing only erodible fractions is initiated. This critical wind velocity is known as maximum fluid threshold velocity because it also initiates the soil movement by the impact force of moving particles in addition to that by wind pressure. In practice no maximum fluid threshold velocity exists because soils contain both erodible and non-erodible fractions together.

In the field, knolls, ridges and other most exposed or most erodible spots first start to erode and spread fanwise towards leeward side. The threshold velocity under the bombarding action from the most exposed and most erodible grains is known as the impact threshold velocity.

The fluid and impact threshold velocities are same for most erodible grains. However, the impact threshold velocity becomes lower than the fluid threshold velocity as the grain size and density increases.

The fluid and impact threshold drag velocities for dry grains greater than 0.1 mm in diameter can be expressed as (Bagnold, 1943):

where, V_t = threshold drag velocity, i.e., minimal drag velocity required to initiate soil movement,

D_s = diameter of the grain

ρs^t = immersed density of the grain,

a = coefficient whose value depends on the range of equivalent size of particles present on the eroding surface and on whether movement is initiated by direct pressure of wind or by pressure and bombardment from the most exposed and most erodible grains.

The determining factors of soil movement are thus those related to size and density of detachable soil particles and the turbulent force on them. With a given surface configuration or roughness the height of zero velocity is constant for all turbulent winds.

Turbulent flow occurs for all winds of speed over 1.6 to 3.2 km/hr. However, the height of zero velocity is greatly influenced by the surface roughness which is dependent on ridging, presence of large size particles, vegetation, vegetative residues, or other barriers on the surface. This helps to protect the erodible particles from being picked up by air current.

The rate of increase of wind velocity with logarithm of height can be ex­pressed in terms of the drag velocity according to the relation:

where, v = drag velocity, cm sec

v_z = velocity at any height, z, above z₀

Equation (4.7) shows that drag velocity increases with wind velocity. The minimum wind velocity required to initiate the movement of the most erodible soil particle (of about 0.1 mm dia) is about 16 km/hr at a height of 30.5 cm.

The most practical limit under field conditions where a mixture of sizes of single grained material is present is about 21 km/hr at a height of 30.5 cm. Particles of about 0.1 mm in diameter have a size-weight relationship which is most conducive to initiate the movement.

Transportation of Soil Particles:

The lifting of detached soil particles from the surface depends largely upon eddies and cross-air-currents of various strata of air through which soil grain rises. In general, the soil transportation by wind is caused by three distinct types of soil movement viz. saltation, suspension and surface creep.

Saltation is caused by the direct pressure of wind on the soil particles and their collision with other particles. The soil particles skip or bounce along the sur­face of the ground. After being pushed along the ground surface by wind (Fig. 4.5), the particles suddenly leap almost vertically in the first stage of saltation movement.

Some grains rise only a short distance; others leap about 30 cm or more depending upon the velocity of rise from the ground. The higher the grain rise, the more the energy they derive from wind. This energy is liberated in bombarding action of the particles on the other particles.

Once saltation begins, the erosion is accelerated mainly by the saltating particles except on the windward edge where the direct presence of wind against the ground is the main eroding force.

The vertical rise appears to be due to the spinning of the grain and the steep velocity gradient near the ground. The variation in air velocity near the ground causes a substantially higher rate of air flow at the upper than at the lower surface of grain at rest on the ground surface.

Consequently, if the total dif­ference in pressure between the upper and lower surface is greater than the force of gravity, the grain will rise in a vertical direction.

After being shot into the air, grains rise vertically to about one-fourth to one-fifth of the horizontal length L (Fig. 4.6) traversed while falling, as a result of gravity and horizontal acceleration due to the force of drag. The downward and forward accelera­tions are proportioned uniformly so that the particle travels in a straight line as it descends to the soil surface forming an angle of about 6 to 12°.

The soil moved by saltation chiefly consists of fine grains ranging in diameter from 0.1 to 0.5 mm. Wind erosion studies have indicated that about 50-75% of particles are moved by saltation. More than 75% of the grains carried by saltation spin rise almost vertically, rotating at a speed of 200-1000 revolutions per second and travel 10-15 times of their height of rise before returning to the surface.

In brief, the whole process can be described as below.

After being rolled by the wind, soil particles suddenly leap almost vertically to form the initial stage of the movement in saltation. Some grains rise only a short distance, others, leap 30 cm or more, depending directly on the initial velocity of rise from the ground.

They also gain considerable forward momen­tum from the pressure of the wind acting upon them, and acceleration of horizontal velocity continued from the time grains began to rise to the time they struck the ground.

In spite of this acceleration, the grains descend in almost a straight line invariably at an angle between 6 and 12° from the horizontal. On striking the surface they either rebound and continue their movement in salta­tion, or loose most of their energy by striking other grains, causing them to rise and sinking themselves into the surface or forming part of the movement in surface creep.

They also break down clods and crusts. Most of the dislodged particles in the size range from 0.5 to 1.0 mm roll and move by surface creep.

The concentration of saltation grains increases with distance downwind till it becomes the maximum that wind of a particular velocity can sustain or a bar­rier is encountered. The impacts of saltating grains initiate movement of larger and denser grains and of dust particles.

During collision, the saltating grains also cause disintegration of the grains involved which exhibit different degrees of mobility and sort into different erosion products, such as lag sands, lag gravels, dunes, and deposited dust (loess).

Suspension:

Transportation of very fine dust particles (less than 0.1 mm dia) by wind occurs in true suspension in the turbulent air-stream when the soil is bombarded by the saltation movement.

Normally, soils composed of fine dust particles are ex­tremely resistant to erosion by wind because the presence of dust in the soil greatly influences the threshold velocity and the intensity of erosion for a given wind. In saltation, the dust is ejected in the turbulent air-stream in the same manner as dust is kicked up by travelling vehicles, animals, etc.

The upward eddies of erosive wind having a velocity more than 3 km/h are capable of lifting silt and very fine sand particles to heights greater than 3-4.5 km. Suspension may move 3 to 36 per cent of soil loss by wind erosion.

One can see this kind of movement during dust storms. The atmosphere has a large capacity to transport the soil particles. Estimates show that the potential carrying capacity of 4.2 km of the atmosphere is about 114.3 million kg of soil depending upon the wind velocity.

The soil particles carried in suspension are deposited when the sedimenta­tion force is greater than the force holding the particle in suspension. This occurs with decrease in wind velocity resulting in decrease in the drag velocity gradient.

The particles are also deposited in the depressions and behind the ridges and other obstructions. The coarser particles may settle in the windward edge of the field while finer particles settle near the leeward side. The surface roughness is smoothened with the deposition of soil particles.

Surface Creep:

Surface creep is the movement of soil particles due to impact of particles des­cending and hitting during saltation (Fig. 4.6). The movement of particles by surface creep causes an abrasion of soil surface resulting into breakdown of non-erodible soil aggregates due to impact of moving particles.

Quartz grains of about 0.5 to 1.0 mm dia are too heavy to be moved by saltation but are pushed along the surface by the impact of particles in saltation to force surface creep. Laboratory studies have shown that about 7 to 15% soil may be moved by sur­face creep.

The three types of soil movement described above usually operate simul­taneously. Chepil (1945) reported the proportion of soil loss by the three processes in coarse to fine soil textures as shown in Table 4.2.

In general, wind moves the finer and lighter particles faster than the coarser and denser ones despite the fact that the finer particles are less erodible.

This results into sorting or separation of particles into the following distinct grades:

(a) Residual soil materials containing non-erodible clods and massive rock materials

(b) Lag sands, lag gravels and lag soil aggregates which are semi- erodible grains moved primarily by surface creep.

(c) Sand and clay dunes formed by accumulation of highly erodible grains moved primarily in saltation.

(d) Loess comprising of dust particles lifted off the ground by saltation, carried in air by suspension and deposited in uniform layers both near and far from dunes.

However, there are no distinct demarcations of size between the various grades of wind-sorted materials. The composition of freshly deposited dust is similar to that of the loess laid down in the Pleistocene age. Almost complete removal of surface soil by wind is associated primarily with loess as a result of non-selective removal.

On the other hand, on soils developed from glacial till, residual material, mountain outwash, and sandy soils of various origins there occurs a selective removal of silt and clay leaving behind sands and gravels.