214 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



TABLE 1 Most Important Factors in Wind Erosion 

 of Agricultural Soils 



I. Air 



II. Ground 



Velocity 



Turbulence 



Density 

 Temperature 

 Pressure 

 Humidity 



Viscosity 



Roughness 



Cover 



Obstructions 



Temperature 



Topographic features 



*Based on data from Chepil (1945a). 



III. Soil 



Structure 

 Organic matter 

 Lime content 

 Texture 

 Specific gravity 

 Moisture content 



and settle by gravity until they strike the ground. Chepil (1945a) attributes this sudden 

 rise to the spinning of the grain as it rolls along the surface, with the Bernoulli effect 

 causing a difference in pressure at the top and bottom of the grain. The third method of 

 movement is, then, suspension of the small soil particles by the wind. In the last case the 

 particles must be small enough to be kept airborne by the turbulent forces in the 

 atmosphere overcoming the force of gravity. Since the turbulence varies with atmospheric 

 stability and, to some extent, with wind speed, one would expect that larger particles 

 would be suspended in strong turbulent winds and that these particles would settle out as 

 the winds decrease. However, the fme particles can remain suspended for long times and 

 can cover large distances. It is the suspension fraction, and particularly the smaller 

 particles, that is of interest in resuspension since resuspension is defined as the suspension 

 of a previously deposited contaminant. 



The airflow characteristics over the surface of the soil are important in transmitting 

 force to the soil grains and in determining the velocity at wliich they start moving. In a 

 neutral atmosphere (i.e., temperature decrease with height is adiabatic), the velocity 

 profile is logarithmic with height; so one can write (Prestley, 1959), 



K \zo/ 



(3) 



where Uz = wind speed at a height z 



zo = height at wliich the wind speed is zero 

 U:^ = friction velocity (or drag velocity) 

 K = von Karman constant, which has been found by integration to equal 0.4 



Zo is a characteristic of the surface, increasing as the roughness of the surface increases. 

 The friction velocity is of importance in determining the force exerted on any object 

 protruding above the laminar layer of the atmosphere and has been shown to be the 

 characteristic wind speed tliat affects soil movement (Bagnold, 1943; Chepil, 1945b; 

 1945c). 



Bagnold (1943) has shown that in severe erosion conditions the profile of wind speed 

 with height is changed by the momentum transfer to the particles in saltation. It has been 

 observed that the wind velocity must exceed some threshold value to induce movement 

 of the soil. Chepil (1945b) has studied the movement of particular sizes of grains and has 

 noted that the curve of grain diameter times the specific gravity vs. threshold friction 



