DYNAMICS OF WIND EROSION

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Abstract
Because of rapid spinning of the grains moving in saltation and a steep velocity gradient, there appears to be a considerable vertical component of wind force near the ground. On account of these effects the grains rise steeply and descend very obliquely toward the surface. As the downward acceleration, due to gravity, and the forward acceleration, due to wind pressure, are approx. equal, the grains fall in almost a straight line. They strike the surface at an angle of 6 to 12[degree]. Movement in suspension and in surface creep is a result of movement in saltation. The whole program of wind erosion control should therefore depend on methods designed to reduce or eliminate saltation. The intensity of soil movement depends not so much on the force of the wind acting on the ground, as on its pressure against the grains as they leap in saltation. Soil movement is therefore dependent not on velocity at any fixed height but on the velocity distribution to the height of saltation. Dust in suspension does not affect the general character of the movement in saltation or in surface creep, but the presence of dust in the soil increases the minimum velocity required to initiate erosion and decreases the intensity of erosion for a given erosive wind. Once lifted off the ground, fine dust is carried to great heights and distances from its original location and thus may be considered a complete loss to the eroding area. The soil moved in saltation and surface creep, on the other hand, usually remains within the eroding area, especially when the erosive winds are from different directions. The max. diam. of soil particles carried in suspension is on an average slightly greater than 0.1 mm. The proportion of the 3 types of movement varies widely for different soils. In the cases examined, between 55 and 72% of the wt. of the soil was carried in saltation, 3 to 38% in suspension, and 7 to 25% in surface creep. Coarsely granulated soils erode mainly in saltation, and finely pulverized soils, in saltation and suspension. The trapping capacity of stubble or ridged strips depends on the relative receptiveness of the surface and the length of jump of particles in saltation. The data presented supply information on the approx. % of soil grains that may be caught by trap strips of different widths. The effectiveness of a particular type of trap depends also on the height and density of the obstruc- tions and on the resistance of these obstructions to the abrasive action of wind-borne grains. Soils vary greatly in resistance to abrasion, but grain stubble is virtually unaffected.

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