218 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



We will not attempt here a complete discussion of this equation along with numerical 

 factors but will discuss each of the factors in a quaUtative fashion because it appears that 

 many of these could be of importance in any case of wind resuspension. 



The soil and knoll erodibility consists of two terms. The soil-erodibility index is 

 related to the percentage of dry aggregates greater than 0.84 mm in diameter. The higher 

 this percentage, the lower the soil erodibility. Conversely, the higher the percentage of 

 aggregates less than 0.84 mm in diameter, the greater the erodibility. The knoll erodibility 

 is expressed as the percentage of the level-ground erosion that occurs at various slopes of 

 the knolls in the field. This factor ranges from for a level field to about 650% at the top 

 of a knoll having a slope of 10% and about 360% from that portion of the windward 

 slope where the drag velocity is the same as the top of the knoll (about the upper third of 

 the slope). A surface crust stability factor is usually ignored because the crust 

 disintegrates rapidly as a result of abrasion once the wind erosion starts. 



The soil ridge roughness is a measure of the surface roughness other than that caused 

 by clods or vegetation. Ridges of 2 to 4 in. in height have been found to be the most 

 effective in controlling erosion; the erosion for ridges of this height is about 50% of that 

 over a smooth surface. 



The wind-erosion chmatic factor includes the influence of wind speed and moisture. 

 The rate of soil movement varies directly as the cube of the wind velocity. In this factor 

 the mean annual wind velocity, corrected to a standard height of 30 ft, is used. Since 

 atmospheric wind velocities are normally distributed, the probability of obtaining high 

 winds is higher with liigher mean velocities. The rate of soil movement varies 

 approximately as the square of effective soil moisture. The wind-erosion climatic factor 

 has been given for a number of locations by the U. S. Department of Agriculture (1968) 

 for each month of the year. To illustrate the differences in this factor from one place to 

 another, I have given rough ranges for four locations. For tlie State of Washington, the 

 chmatic index varies from about 1 to 50; the higliest values are for the months of March 

 through May. For eastern New Mexico, along the northeastern border, the climatic factor 

 remains the highest in the nation throughout the year; values range from about 70 to 300. 

 For the State of Ohio, the factor ranges from 1 to 10, and for the State of Georgia, about 

 1 to 5. Thus there are widely differing wind and moisture factors throughout the country; 

 the east, in particular, has low factors as compared with the west. It could be predicted 

 that the resuspension of contaminants from the soil will also be lower in the areas of low 

 climatic factor for erosion. 



The field-length factor again has two parts: the distance across the field and the 

 sheltered distance. The distance across the field is measured along the prevailing 

 wind-erosion direction. On an unprotected field, the rate of soil flow is zero on the 

 windward edge and increases with distance downwind until, for a large field, the flow 

 reaches a maximum that the wind of a given velocity can maintain. The sheltered distance 

 is that distance along the prevailing wind-erosion direction that is sheltered by any 

 barrier. 



Vegetative cover is an important factor in controUing wind erosion. Three different 

 factors are included in the equivalent quantity of vegetative cover. The first is the 

 quantity of the vegetative cover expressed as clean, air-dried residue. The second denotes 

 the total cross-sectional area of the vegetative material. The finer the material and the 

 greater the surface area, the more it reduces the wind velocity and the more it reduces 

 wind erosion. The tliird factor is the orientation of the cover. The more erect the 



