Entrainment From the Surface of a Homogeneous Powder Layer

DOE-HDBK-3010-94

4.0 Solids; Powders

displacement of material from soil surfaces as a function of surface stress show three types of

behavior - saltation, surface creep and suspension.

Saltation is defined as a mode of soil/sand particle movement where particles have alternate

contact with air and ground in a layer close to the surface and affects particles in the size

range of approximately 100 to 500 m in diameter. The size range of particles suspended is

shown in Figure 4-21 reproduced from Martin et al. (October 1983) indicating that particles

in this size range are the most readily suspended (depends upon local wind conditions and

particle morphology). It is noted that the graph plots the square root of particle diameter

vice friction velocity and is not a direct indication of the air speeds or particle diameters.

This is the approximate size range for saltating particles. Saltating particles can initiate

surface creep and suspension upon impact and/or continue saltation.

Larger particles move by surface creep where particles always maintain contact with the

surface. Surface creep affects particles in the size range of 500 to 1000 m (0.5 to 1.0 mm)

in diameter and cover the behavior of particles that slide or roll across the surface pushed by

wind stresses and momentum exchange resulting from the impact of saltating particles.

Suspension is the mode of movement where particles do not come in contact with the surface

locally. Suspension affects particles in the size range less than 100 m in diameters and

cover the behavior of particles that tend to follow the air motions. The fraction of soil

eroded by the three modes varies greatly - 50 to 75 wt/o by saltation, 5 to 25 wt/o by surface

creep, and 3 to 40 wt/o by suspension.

Three tables (Tables 4-16, -17, and -18) reproduced from the referenced source are included.

Table 4-16 tabulates resuspension factors (mostly from soil or vegetated soil but unspecified

city surfaces are included) from wind stresses. The values range from ~ 1E-10/m to

5E-5/m. Table 4-17 tabulates the resuspension factors measured for mechanical stresses and

are almost completely for indoors situations. The values range from 1E-10/m to 2E-2/m.

Table 4-18 shows the resuspension rates from the published literature and all represent

outdoors conditions (a mix of soil erosion and experimental data). The values ranges from

4E-9/hr to 4E-1/hr. The latter value, 4E-1/hr, is for corn pollen and thus is extreme. Pollen

has been naturally selected over millennia to be readily driven airborne (low density, large

surface area) and is not representative of the particles of interest in accidents at nonreactor

nuclear facilities. Not all the relevant factors (e.g., windspeeds, particle size distribution of

soil or pollutant, local surface conditions) are given in the data nor is it always clear whether

the relevant factors are known.

Sehmel (1980) provided a comprehensive review of literature on the resuspension/suspension

(erosion) of soil. Resuspension is defined as re-entrainment of material deposited on the

Page 4-89