FLOOD09931 - Laserfiche WebLink

Home Browse Search

Estimating Soil Losses from Wind Erosion Wind erosion is a dominant conservation problem in the Great Plains States. Crops are often damaged or destroyed in a short time by abrasion from wind-blown particles. Blowing soil can cause traffic problems, fill drainage ditches, block roads, bury fences and equipment, and create dust and health problems in residential areas. Wind erosion occurs when a wind of adequate velocity blows across a soil surface that is smooth, bare, loose, dry, and finely granulated. Soil particles start to move when the forces of wind overcome gravity. The minimum wind speed required to start movement de- pends on the size and weight of soil particles and the friction provided by neighboring particles. Where very small particles are present, soil movement begins under field conditions when wind speed is about 12 to 15 miles per hour at a height of I foot above the soil surface. - Wind erosion usually starts at some critical location such as an exposed knoll, tracks or paths made by implements or animals, and in corners or turnrows where the soil has been excessively pulverized. As the soil moves downwind, increasing numbers of soil parti- cles are set in motion. Soil flow is zero at the windward edge of an eroding area, but the rate increases leeward until it reaches maximum flow. The more erodible the soil, the shorter is the distance to reach maximum flow. THE WIND EROSION EQUATION The five factors which influence wind erosion from a given field can be expressed as an equation: E = f (IKCL V) where, E = the computed average annual soil loss in tons per acre. F = a function of I = the soil erodibility index. It is expressed in tons of soil loss per acre, per year where the C value is 100 percent. It is indicated by the soil aggregates larger than 0.84 mm in diameter and the percent- age of land slope. The least erodible soils by tex- tural class are the silts, silt loams, clay loams, and silty clay loams with a clay content of 20 to 30 percent. The most erodible soils are the very fine, fine, and medium sands, and dune sands. Table 2 lists the erodi- bility of Colorado soils based largely on textural clas- sification. In most instances, this listing should be ade- quate. K = The soil surface roughness factor. Ridges and de- pressions formed by tillage alter the wind speed at the soil surface. In most cases, during construc- tion activities, the soil surface will be relatively smooth. If the soil is chiseled or plowed leaving a ridged effect, erosion calculations should be based on the ridged surface charts. C = The climatic factor. Wind erosion increases as wind speed increases. Conversely, wind erosion decreases as soil moisture increases. For con- venience, windspeed and soil moisture are con- sidered together as a local wind erosion climatic factor, based on official weather records. The charts included in this publication forestimat- ing wind erosion are based on a "C" value of 50 which will include most of the areas where urban development is significant in Colorado to calcu- late soil loss from wind in areas with "C" values other than 50, contact the local Soil Conservation Service Office for assistance. L = The unsheltered distance in feet across an area along the prevailing wind erosion direction. V = The soil cover factor. Three conditions are con- sidered in determining the factor "V", They in- clude the quantity, kind and orientation (flat or standing) of the residue. The charts used to de- termine wind erosion are based primarily on pounds of flat small grain residue on the soil sur- face. Refer to Figure 12 to determine the effects various kinds and amounts of residue have on wind erosion. Example Problems Problem I An unvegetated construction site exists in Arapahoe County. The unprotected width of the area perpendicu- lar to the prevailing wind direction is 900 feet. The surface is smooth, and the soil type is Buick Loam. Compute the soil loss based on these facts. Step I Determine the factor values for the site. I 56 or WEG 5 - from Table 2. K smooth - final operation with motor grader C 50 - from Figure 12. L = 900 - site investigation V 0 - site investigation indicated no residue on the soil surface. Step 2 Refer to the applicable section of Table 3 and estimate the soil loss. Using the above fac- tors, the estimated soil loss from wind is 20.9 tons/Ac/Yr. 32