5.2.3  Wind Erosion and Mechanical Suspension


 Cowherd et al. (1985) define the steps for determining potential respirable particulate emission from wind erosion.  The soil particle size distribution, apparent roughness of the site, vegetation cover, presence of a crust on the soil, and presence of nonerodible elements (e.g., large stones) are used to define the potential for suspension.  Depending on the results of their procedure, the site is characterized as having 1) unlimited erosion potential, 2) limited erosion potential, or 3) no erosion potential.

 The methodology uses different formulations for the two cases with wind erosion potential.  Cowherd et al. (1985) suggest that if the site is completely covered with vegetation or if there is a thick crust (or a wet, saturated soil) and if no mechanical disturbances occur at the site, it can be assumed that no contaminants are suspended.  However, for certain contaminants, even very small suspension rates from well-stabilized surfaces may be significant.  These wind erosion formulations give results comparable to using resuspension factors over a range of surfaces from bare, unstabilized surfaces to well-stabilized surfaces (Whelan et al. 1989).

 In the module, the wind/mechanical emission rate for a surface is computed as the sum of the unlimited and limited erosion emissions.  The emission rate terms were not summed in the original methodology described by Cowherd et al. (1985).



where
 Limited Erosion Calculation The potential for wind erosion is quantified in terms of a threshold friction velocity.  The greater the value of the threshold friction velocity for a site, the lower the potential for particle suspension.  The threshold friction velocity for the contaminated area is determined by knowing the mode of the aggregate particulate size distribution (which is derived from the soil composition) and using a formula derived from the graphical relationship given by Gillette (1980):



where
The aggregate size distribution is estimated using



where    Psand is the percent sand (unitless).

From the viewpoint of increasing the potential for suspension, this relationship provides relatively realistic estimates for soils with greater than 75% sand content.  For other soils, the relationship provides relatively conservative estimates that are more typical of disturbed soils than undisturbed soils.

 The correction factor, fne, in Equation 5.39 allows for the effects of any non-erodible elements in the contaminated area.  This correction factor depends on the fraction of surface coverage and is estimated from graphical results given by Cowherd et al. (1985), derived from wind tunnel studies by Marshall (1971).  As the silhouette area of nonerodible elements increases, so does the threshold friction velocity.  If the threshold friction velocity is less than 0.75 m s-1, the area has unlimited erosion potential; otherwise, the area has only limited erosion potential.

Once the threshold friction velocity has been determined, the critical wind speed at a given height above the surface can be determined using the equation (a)



where
The critical wind speed is one of the parameters used below to define the erosion potential.  The value of href recommended by Cowherd et al. (1985) is 7 m.  The surface roughness length of the site, Lr, is related to the size and spacing of the roughness elements in the area.  The von Karman constant is equal to 0.4.

 For estimating particulate emissions from a contaminated area having limited wind-erosion potential, the following equation is used to predict potential emissions:



where
In Equation 5.42, the factor (8.76x10-4A) is needed to convert Elim from the units used in Cowherd et al. (1985) (mg m-2 hr-1) to those used in this report (g yr-1).  The frequency of disturbances per month, fd, is defined as the number of actions that could expose fresh surface material.  If the entire area is not disturbed, this frequency should be weighted to reflect the actual area exposed.  A disturbance could be vehicular traffic, plowing or turning of the soil, mining, or construction.  However, vehicular traffic is accounted for by separate expressions described in the next section.  The erosion potential, Epot, depends on the maximum wind speed, Umax, so that



where Umax is the maximum wind speed at a reference height above the soil surface (m s-1).

The vegetation fraction, fv, varies from 0 for bare ground to 1 for total coverage.  The Thornthwaite's PE Index, fPE, is used as a moisture-correction parameter for wind-generated emissions.  Cowherd et al. (1985) provide a map with values of fPE for all regions in the contiguous United States.

Unlimited Erosion Calculation

   For unlimited erosion potential, the relationship for the surface emission rate is



where
In Equation 5.44, the factor (0.876 A) is needed to convert Eunlim from the units used in Cowherd et al. (1985) (g m-2 hr-1) to those used in this report (g yr-1).

 The vertical flux of particles smaller than 10 mm in diameter is assumed to be proportional to the cube of the horizontal wind speed.  This relationship was originally developed from measurements made by O'Brien and Rindlaub (1936) in studies at the mouth of the Columbia River and later measurements made by Bagnold (1941) in the Egyptian desert.  Chepil (1951) found this same relationship using results from wind-tunnel experiments.

 The function, F(Uc, ) , comes from the cubic relationship of the vertical transport of particles and the wind speed.  It is defined in graphical format by Cowherd et al. (1985).  This relationship can be broken into the following discrete parts:



where    x = 0.886(Uc/) .


 
(a) Units for the surface roughness length are listed as meters (m) for consistency with equation 5.39; the common units for this variable are centimeters (cm).