WIND EROSION: MECHANICS OF SALTATION AND DUST GENERATION
by UDAI BHAN SINGH, B.Engr., M.Engr., M.S.C.E
A DISSERTATION IN
CIVIL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
Approved
Accepted
DeaR of the Graduate School
December, 1994 -f3
ivio.)'r.
Copyright 1994, Udai B. Singh ACKNOWLEDGEMENTS
I express my sincere thanks and appreciation to Dr. James M. Gregory for his guidance, assistance, and encouragement throughout my course of study at Texas Tech University. Dr. Gregory has been a true mentor for me, and without his help and guidance, this work would have not been possible. I also express sincere thanks to Dr. Kishor C. Mehta for his support, guidance, and for providing an opportunity to be a part of the Cooperative Program in Wind Engineering funded by the National Science Foundation. I extend sincere thanks to Dr. Richard E. Peterson for his timely help and encouragement throughout the course of study. Sincere thanks to Drs. Cliff B. Fedler, Lloyd V. Urban, and Richard E Zartman for serving on my dissertation committee and teaching their respective courses. I also thank Dr. Ted M. Zobeck for his support and help in conducting the dust generation experiment. I also extend sincere thanks and gratitude to my colleague and team member. Dr. Gregory R. Wilson, for his friendship and help in doing wind erosion research. I thank Mr. Jim Snyder for his help in fabricating the controlled energy dust generator. Many thanks to the faculty and staff members of the Civil Engineering Department and the Wind Engineering Research Center for their cooperative and friendly nature. The help and encouragement from the friends at Texas Tech University have been invaluable. I thank the staff of the Allen Engineering Communication Center for their timely help and support in enhancing my oral and written communication skills. I express my sincere gratitude to my parents and family members for their sacrifice and moral support during my entire academic training. Their sacrifice has been the biggest source of inspiration to me, and it is all because of their moral support that I could complete the highest academic degree in my life. I dedicate this work to my parents and family members. I also express my sincere gratitude to the members of my host family from Floydada, Texas for their moral support and encouragement. One person who deserves the most appreciation is my wife, Sunita; I am indebted for her support and understanding during the last several months of my dissertation work.
ii TABLE OF CONTENTS
ACKNOWLEDGEMENTS ii ABSTRACT v LIST OF TABLES vi LIST OF FIGURES viii LIST OF SYMBOLS x CHAPTER I. INTRODUCTION 1 1.1 Scope of the Problem 1 1.2 Objectives 3
n. LITERATURE REVIEW 5 2.1 Wind Erosion and Maximum Transport Rate 5 2.2 Eolian Suspension 12 2.2.1 Length of Visibility Prediction 16 2.2.2 Suspended Particle Concentration with Height 18 2.3 Saltation 20 2.4 Dust Generation 22
UI. MECHANICS OF SALTATION 25 3.1 Description of Wind Tunnel used for Wind Erosion Studies 25 3.2 Mean Saltation Height and Maximum Transport RateDataCollection 25 3.3 Height of Saltation 27 3.4 Reference Concentration of Soil Particles in the Saltation Layer...34
IV. MECHANICS OF DUST GENERATION 37 4.1 Theory of Dust Generation 37 4.2 Design and Development of Controlled Energy Dust Generator 40
ill 4.3 Procedure and Data Collection for Dust Generation 43
V. RESULTS AND DISCUSSION 46 5.1 Mass Distribution with Height in the Saltation Zone 46 5.2 Effect of Kinetic Energy and Soil Texture on Dust Generation 56 5.3 Prediction of Dust Generation Rate Factor as a Function ofSand and Clay Content 63 5.4 Prediction of Dust Potential as Function of Particle Size Distribution 66
VL SUMMARY AND CONCLUSIONS 70
REFERENCES 73 APPENDICES A. WIND TUNNEL DATA FOR THREE UNIFORM PARTICLE SIZES 81 B. DUST GENERATION DATA FOR SEVEN DIFFERENT SOILS 104
IV ABSTRACT
Soil erosion by wind and associated dust generation cause major social and economic problems in many parts of the world. There is a high cost associated with on-site and off-site damages due to wind erosion. Wind erosion of top soil causes loss of essential plant nutrients and soil productivity, resulting in loss of agricultural production. Visibility reduction on highways during dust storms can cause severe accidents. Fine dust particles suspended in air deteriorate the environmental air quality. Particulate matter smaller than 10 jim in size (PM,o) are also recognized as health hazards to human and animals. Off-site costs of wind erosion are much higher than on-site costs. The reference saltation height and reference concentration in the saltation zone must be determined to accurately predict dust concentrations and visibility at different heights in the atmosphere. The rate of dust generation depends on kinetic energy from saltating particles during the wind erosion process and dust potential in natural soil. The mechanics of saltation and dust generation and the relationship between kinetic energy of saltating particles and dust generation are not very well understood. An experiment was carried out in a wind tunnel to collect mean saltation height and maximum transport data for three different uniform particle sizes and a physically based model was developed to predict mean saltation height as a function of particle size, wind velocity, and material properties. Data on soil particle concentration with height in the saltation zone were analyzed to develop a general equation to predict soil particle concentration with height as a function of friction velocity and particle size. A methodology was also developed to connect dust generation to the wind erosion process. A controlled energy dust generator (CE/DG) was designed and developed to relate dust generation to the wind erosion process. The device was tested with seven different soil types to investigate the relationship between dust emission and kinetic energy from abrasion during wind erosion. LIST OF TABLES
3.1 Mean saltation height as a function of friction velocity data collected in the wind tunnel 28
3.2 Intercept and slope for the mean saltation height and friction velocity relationship 30
3.3 Threshold friction velocity and angle of repose for
materials tested in the wind tunnel 31
4.1 Percent sand, silt, and clay for seven soils 44
5.1 Coefficients Zg, Z9, Z15, and Z,6 and coefficient of determination (R^) 55 5.2 The percent sand, silt, and clay and the values of coefficients Dp, D„ Wf, and coefficient of determination (R^) for the soils tested with CE/DG 60
5.3 Percent clay content, dust generation rate factor, and crushing energy data for the soil tested in CE/DG 64
5.4 Particle size associated with the ratio of measured dust potential
coefficient to total sample size (500 gram) 66
A.l Wind tunnel data for 0.15-mm size loamy sand 82
A.2 Wind tunnel data for 0.30-mm size glass spheres 90
A.3 Wind tunnel data for 0.37-mm size sandblasting sand 97
B.l Dust generation data for sandy loam-1 soil 105
B.2 Dust generation data for silt loam soil 108
B.3 Dust generation data for sandy loam-2 soil Ill
B.4 Dust generation data for sandy loam-3 soil 114 B.5 Dust generation data for loamy sand-1 soil 117
B.6 Dust generation data for loamy sand-2 soil 120
vi B.7 Dust generation data for clay soil 123
VI1 LIST OF FIGURES
2.1 Mass transport rate as a function of friction speed. Data from Svasek and Terwindt( 1974) 13
2.2 Mass transport rate as a function of friction speed. Data from Nickling( 1978) 14
2.3 Mass transport rate as a function of friction speed. Data from Wilson (1994) 15
3.1 An isokinetic sampling unit used in the wind tunnel 26
3.2 Relationship between the mean saltation height and friction velocity 29
3.3 Relationship between intercept at threshold and (U,tV2g)tan 3.4 Measured versus predicted mean saltation height for all three particle sizes 35 4.1 Wind erosion and dust generation process 38 4.2 The controlled energy dust generator 41 4.3 Particle size distribution curves for seven soils used in the dust generation experiment 45 5.1 Mass distribution with height for 0.15-mm sand moving in saltation 47 5.2 Mass distribution with height for 0.30-mm glass spheres moving in saltation 48 5.3 Mass distribution with height for 0.37-mm sand blasting sand moving in saltation 49 5.4 The relationship between H/HS and MHa/MT for surface creep data 51 5.5 The relationship between H/HS and MHa/MT for saltation plus surface creep data for 0.15-mm sand 57 Vlll 5.6 The relationship between H/HS and MHa/MT for saltation plus surface creep data for 0.30-mm glass spheres 58 5.7 The relationship between H/HS and MHa/MT for saltation plus surface creep data for 0.37-mm sand blasting sand 59 5.8 Effect of generator rotations or number of impacts and soil type on dust generation (sandy loam-1, silt loam, and clay soils) 61 5.9 Effect of generator rotations or number of impacts and soil type on dust generation (sandy loam and loamy sand soils) 62 5.10 Relationship between measured and predicted dust generation rate factor for all soils 65 5.11 Particle size distribution curves showing percent of soil mass associated with a 26 |im particle size 68 5.12 Measured and predicted dust potential coefficient for all soils except the clay soil 69 IX LIST OF SYMBOLS PM,o = particulate matter smaller than 10 micro meter in diameter, \xm = micrometer (10'^ meter), q = mass movement per unit width per unit time (kg/m-s), cl = constant, d = diameter of sand particle (mm), D = particle diameter of a standard 0.25 mm sand. Pa = air density (kg/m^), g = acceleration due of gravity (m/s^), U, = friction velocity (=VT/pJ(m/s), X = surface shear stress (kg-m/s^), X = rate of soil movement (kg/m-hr), Pbs = soil bulk density (kg/m^), c2 = constant, c3 = constant, c4 = constant, U., = threshold friction velocity (m/s), D50 = average or mode diameter of soil particles (mm), D75 = particle diameter at the 75 percent location on the cumulative size distribution curve (mm), Dr = reference diameter of soil particles ( 1 mm), S = a cover factor, which expresses the energy transfer from the wind profile to the soil surface (dimensionless), Gf = a gust factor to adjust the dynamic threshold for wind gust in the field (dimensionless, Gf=1.5), L = length of filed (m), E = erodibility of soil (kg/J), A = abrasion factor, P = the fraction of light that penetrate the material. C = the concentration of sediments (kg/m^), p = particle density (2650 kg/m^), L = the thickness of the material (m), Pj = total light penetration through all particle sizes for a unit length of material, C, = concentration of i class size of sediments (kg/m^), D, = the particle diameter of i class size of sediments (mm), n = number of size classes of sediments, Dgff = the effective average diameter (mm), Cy = the total concentration of suspendible particles at a given height (kg/m^), Ly = length of visibility (m), Cz = concentration of suspended particles at any height Z (kg/m^), C, = concentration of suspended particles at reference height Z^ (kg/m^), Vj = terminal fall velocity of particles (m/s), H3 = mean saltation height (m), Djc = diameter of incoming particles striking the surface (mm), DQUJ = diameter of outgoing particles emitted due to energy transfer (mm), Al = constant, A2 = constant, A3 = constant, O = emptying angle of repose (degree), W = width of eroding surface (m), Q = air flow rate (m^/hr), U^ = wind velocity at height HS, Us = sediment velocity at height HS, dD = change in generated dust fraction, dN = change in number of impact, Wf = dust generation rate coefficient, D^ = available dust fraction in aggregate form in the natural soil system, D = potential dust fraction in aggregate form, D = initial dust fraction in primary particle form in the soil, xi CE/DG= controlled energy dust generator. MHa = mass per unit square area per unit time at height H (kg/m^-hr). MT = total mass transport rate per unit width per unit time (kg/m-hr). H = height at which concentration is calculated (m). HS = mean saltation height (m). z, = constant. z. = constant. Z3 = constant. s = standard deviation. X, = measured values of MHa/MT, x„ = predicted values of MHa/MT, K' = coefficient of determination. K, = constant. K2 = constant. K3 = constant. X, = percent clay content. x^ = percent sand content. Z4 = constant, Z5 = constant. z^ = constant. Z7 = constant. Zg = constant. z, = constant. Zjo = constant. Zn = constant. 7 = constant. Zn = constant. ZM = constant. Z,5 = constant. z,. = constant. XI1 CHAPTER I INTRODUCTION Soil erosion caused by wind (wind erosion) and associated dust generation cause major social and economic problems in many areas of the world. The major areas of agricultural land subject to wind erosion are m North Africa, the Near East, central, south, and eastern Asia, Australia, southern South America, and parts of North America (United Nations, 1960). In the United States of America (USA), the agricultural areas most prone to wind erosion are the Columbia River Basin, the muck and sandy soils around the Great Lakes, the Gulf and Atlantic sea boards and the Great Plains (Beasley et al., 1984). Wind erosion can occur where low vegetation, high winds, and loose dry soil exist, but it is primarily a natural hazard in the arid and semi-arid climatic regions of the world, where vegetative cover tends to be sparse due to lack of rainfall and recurring drought. 1.1 Scope of the Problem Erosion of topsoil by wind causes loss of essential plant nutrients and soil productivity, resulting in loss of agricultural production. Sand blasting of plants, exposure of plant roots, and burying of plants are other on-site damages caused by wind erosion. Visibility reduction on highways and community areas during dust storms, air and water pollution due to sediments, sand deposition in irrigation canals and on highways and railroads, damage to machinery and electrical system, and respiratory ailments of human and livestock are the major off-site damages due to wind erosion. Off-site damage costs are much higher than the on-site damage costs. Davis and Condra (1985), and Huszar (1985) studied the on-site and off-site costs of wind erosion respectively in New Mexico, USA and provided the data base from which Dregne (1988) estimated that the off-site costs are over 45 times greater than on-site costs. Soil erosion can be defined as the removal of topsoil, which is best suited for agricultural crop production, from one area and transportation and deposition to 1 another location by an eroding agent such as wind or water. The wind erosion process consists of three distinct phases: detachment or initiation of movement, transportation, and deposition. Detachment or initiation of soil particle movement is caused by wind force exerted against the ground surface. After detachment, particles are transported from one location to other locations by saltation, surface creep, or suspension, depending on their sizes in relation to the velocity and turbulence of the wind. When the shear force exerted by the wind exceeds the gravitational force of soil particles, the soil particles are lifted from the surface. Soil particles rise almost vertically, travel 10-15 times their height of rise and return to the ground surface. This process of bouncing or jumping over the ground surface is called saltation. Most of the erodible soil mass is moved by saltation. Soil particles too heavy to be lifted by the wind force are pushed or rolled due to the impact of saltating particles. The rolling or sliding of the soil particles along the soil surface is called surface creep. Wind can also entrain fine soil particles from the eroding surface into air and carry these fine soil particles (dust) to substantial heights and long distances. This mode of transport is known as suspension. Although, most soil particles are moved by saltation and surface creep, the most noticeable mode of transport is by suspension in the form of blowing dust and dust storms. Blowing dust directly affects visibility and highway transportation safety, and reduces environmental air quality. Fine dust and particulate matter smaller than 10 ^m in diameter (PMJQ) are recognized as hazardous to human health and environmental air quality (Pope, 1989; Pope et al., 1991; Hall et al., 1992). There is a 26 percent higher risk of premature death in cities polluted with ambient particles (Leutwyler, 1993). These particulates include carbon, hydrocarbons, all kinds of dust (<10 pirn) including agricultural dust, acid aerosols, and sulfates. Fine particles (PM,o) pass through the filtering system of the human body and cause respiratory ailments. They immediately affect respiratory response (Pope et al., 1991), and the long-term effects are significant increase in respiratory illness and cancer (Churg and Wiggs, 1985; Hatch et al., 1985; Attfield and Morring, 1992; Hall et al., 1992). The most common respiratory problems that can develop from exposure to these pollutants are chronic obstructive pulmonary disease, cardiovascular disease, and asthma. The majority of these air pollutants come from a variety of sources including construction work, vehicles driving over dirt and paved roads, wind erosion, tobacco smoke, fireplace smoke, and others. Because of the serious health hazard, the Environmental Protection Agency (EPA) has established an upper limit of 150 ng/m^ for a 24-hour period and a monthly average not to exceed 50 ^lg/m^ Some states have their own standards for the upper limit of PMJQ concentrations for a 24-hour period and long-term average. For example, California has established an upper limit of 50 |xg/m^ for a 24-hour period and 30 ng/m^ for the monthly average. Wind erosion from agricultural lands can contribute significantly to the amount of air pollutants and PM,o in the atmosphere because the fine silt and clay fraction of soil are in the PM,o range. In arid and semi-arid climates, dust from agricultural fields can be a major source of atmospheric pollution and PM,o. While fine particles exist in a natural soil, particles in the PM,o size range are usually bonded to other particles because of their large surface area to mass ratio. Energy is needed to break the bonds between smaller size particles to generate dust. The dust potential in natural soils is controlled in part by the soil particle size distribution. All particles small enough to be suspended are potential dust. Particles greater then 80 \xm rarely go into suspension because of their high settling velocity. 1.2 Obiectives In the literature, there is an information gap and a need for further research to determine the concentration of the various size fractions and particle size distribution of the soil mass moving in the saltation and suspension zones. The reference saltation height must be determined to accurately predict dust concentrations and visibility at different heights in the atmosphere. Much of the dust production occurs as a result of saltation during the wind erosion process. Kinetic energy is applied through the saltation of particles, causing abrasion of large-size aggregates into smaller particles, including fine dust. Thus, the rate of dust generation depends upon kinetic energy from saltating particles during the wind erosion process and the dust potential in natural soil. The mechanics of saltation and dust generation and the relationship between kinetic energy and dust generation are not very well understood. Therefore, the objectives of this research were to (1) develop a physically based model to predict mean saltation height as a function of particle size, wind velocity, and soil properties, (2) analyze saltation data collected for uniform particle sizes in a wind tunnel, (3) develop a methodology to connect dust generation to the wind erosion processes, (4) describe the design, development, and testing of a controlled energy dust generator (CE/DG), and (5) study the effect of kinetic energy and soil type on dust generation CHAPTER II LITERATURE REVIEW Background information and review of previous work performed on wind erosion and dust generation are presented in this chapter. A literature review related to wind erosion and maximum transport rate is presented in section 2.1. Section 2.2 contains a literature review on eolian suspension, and section 2.3 contains a literature review on saltation of soil particles. The literature review on dust generation as a byproduct of wind erosion is presented in section 2.4. 2.1 Wind Erosion and Maximum Transport Rate Wind is a natural force that detaches and transports soil particles. Wind erosion is not a new problem. It has been a natural hazard in the dry climatic regions of the world where vegetative cover tends to be sparse due to lack of rainfall and recurring drought. The major dust storms of the 1930's in the western part of the Great Plains area of the USA were convincing evidence of the severity of wind erosion on agricultural soil. The subject of wind erosion was studied early in the twentieth century by Free (1911). Free was among the first people who introduced the term "saltation" to describe the motion of soil particles by a series of bouncing and jumping. He reported that 0.1 mm diameter particles are the smallest particles carried in saltation. Soil particles smaller than 0.1 mm in diameter have lower terminal fall velocity than the vertical component of fluctuating turbulent wind so these particles are carried in suspension and travel long distances in the direction of wind. Soil particles larger than 0.1 mm in diameter and smaller than 0.5 mm are usually moved by saltation. The basic research on the subject was not initiated until about 1940. Bagnold (1941), an army engineer by profession, was one of the first scientists who initially advanced research on wind erosion by his pioneering work on the study of eolian sediment transport. Bagnold reported that sand particles subjected to the direct force of the wind, first start to roll along the surface then begin to bounce or jump. Soil particle transport by rolling along the surface was termed "surface creep". Soil particles larger than about 0.5 mm in diameter and smaller than 1 mm are usually transported by surface creep. Particles greater than 1 mm in diameter are considered too large to be moved or transported by ordinary wind (Chepil, 1941), although particles larger than 1 mm theoretically can be moved by high winds. Bagnold introduced the term "static" or fluid threshold and "dynamic" or impact threshold. The static threshold is the wind speed at which sand particles move due to direct shear of the wind, whereas dynamic threshold is the wind speed required to sustain an initial movement of sand particles. Soil particles can move in saltation due to direct shear of the wind or due to impact of other particles. Bagnold presented a comprehensive framework of theoretical, experimental, and field work and described the basic mechanics of wind erosion. According to Bagnold, the rate of sand transport by wind is a function of the square root of particle diameter and the cube of the friction velocity. His equation is of the following form: d Pfl,r3 (i=ci. u: (2.1) N^^ where q = mass movement per unit width per unit time (kg/m-s), cl = a constant varying from 1.5 for uniform size sand particles to 2.8 for a wide range of particle size, d = average diameter of sand particles (mm), D = particle diameter of a standard 0.25 mm sand, P3 = density of air (kg/m^), g = acceleration due to gravity (m/s^), and U, = friction velocity derived from the wind profile (m/s). Bagnold's work on sand transport by wind provided guidelines for later work in the field of wind erosion. Wind erosion research was further advanced by W. S. Chepil, an agricultural scientist at the Soil Research Laboratory, Dominion Experimental Station, Swift Current, Saskatoon, Canada. After his initial work in Canada, he joined the United States Department of Agriculture (USDA), Agricultural Research Service (ARS) team at Manhattan, Kansas. Chepil and his associates studied wind erosion on agricultural soils, which is a more complex matenal than sand. The influence of surface roughness on the intensity of drifting of dune matenal and cultivated soils was studied by Chepil and Milne (1941). They concluded that the rate of soil flow under a wind force varies inversely with the roughness of the surface. They also determined that the rate of soil flow is reduced by ridging cultivated soils. Reduction in the average wind velocity for some distance above the average surface and the trapping of soil on the leeward side of the ridges were the factors identified as responsible for reducing the rate of soil flow. They also associated greater turbulence and eddying at higher wind speeds with increased rate of soil flow at the crest of the ridges. Chepil (1945a) investigated the dynamics of wind erosion on agricultural soils. He measured wind erosion on different types of soil with varying degree of surface roughness both in the open field and in a portable wind tunnel. He observed that soil particles first roll along the surface and then suddenly jump almost vertically to a certain height depending on the initial velocity of rise from the soil surface. Soil particles gain forward momentum from the horizontal wind velocity and travel 10 to 15 times their height of rise before returning to the ground surface at an angle between 6 and 12 degrees from horizontal. He reported that in addition to soil movement by saltation and surface creep, a substantial proportion of the soil is carried in suspension; however, the transport of soil by wind is similar to that of dune sand described by Bagnold (1941). Chepil concluded that the mechanism by which fine particles are lifted off the soil surface is different from the mechanism of saltation. Based on the threshold friction velocity for fine particles, he stated that the movement of fine soil particles in air is mainly the result of the movement of soil particles in saltation. This observed relationship probably implies that detachment of fine particles that go into suspension is a result of abrasion from saltating particles. Chepil (1945b) analyzed the effect of particle size and soil type on the minimum wind velocity required to initiate the soil movement under field conditions. The size of soil particles was found to be the greatest single factor influencing the threshold velocity. He found that threshold velocity was least for soil particles ranging from 0.1 mm to 0.15 mm in diameter. Above this size range of soil particles, threshold velocity increased with the increase in soil particle size, and below this range, threshold velocity increased with a decrease in particle size. He reported that the threshold velocity is a function of the square root of the soil particle diameter for particles larger than 0.1 mm. Similar results had been reported by Bagnold (1941) for dune sand. As reported by Bagnold (1941) for dune sand particles, Chepil (1945c) also verified that the total soil transport rate varies with the cube of the friction velocity of the v^nd. The total soil movement is not affected much by the change in air density due to change in air temperature, pressure, and relative humidity. It was in the fall of 1947, when funds were provided to the USDA to carry out extensive research on wind erosion. The Wind Erosion Research Laboratory (WERE) at Manhattan, Kansas, provided excellent opportunities to focus on the wind erosion problem and its control. Initial efforts of scientists at WERE were to design and fabricate equipment to study wind erosion. A laboratory wind tunnel (Zingg and Chepil, 1950), a portable wind tunnel and dust collector (Zingg, 1951), and a rotary sieve (Chepil, 1952) were designed and fabricated to carry out experimental work on wind erosion research. The wind tunnel and rotary sieve were used to study the effects of various soil properties on wind erosion. The effects of surface roughness (Chepil, 1950a), dry aggregate structure (Chepil, 1950b), apparent density (Chepil, 1951), soil texture (Chepil, 1953), organic matter (Chepil, 1954), soil moisture (Chepil, 1956), and tillage (Chepil et al., 1952; Woodruff and Chepil, 1956; Woodruff et al., 1957) on wind erosion were evaluated separately. Field wind tunnel tests (Zingg et al., 1953; Chepil et al., 1955) were combined with studies of the effects of soil properties on wind erosion to estimate the erodibility of soil by wind (Chepil and Woodruff, 1959). The data on wind velocity (Zingg, 1950), climatic factors (Zingg, 1953a), and shelterbelts (Woodruff and Zingg, 1953; Woodruff, 1956) were incorporated with the results on estimation of soil erodibility by wind to develop a universal wind erosion equation (Niles, 1961). An overview of research on wind erosion till early 1960's was given by Chepil and Woodruff (1963). 8 Woodruff and Siddoway (1965) consolidated the results of the effects of prevailing wind direction (Chepil et al., 1964a), and rough and level terrain (Chepil et al., 1964b) on wind erosion and produced an updated version of the universal wind erosion equation. This equation has been used to determine potential wind erosion and the field conditions necessary to reduce wind erosion to an acceptable level. The research on wind erosion then continued to refine the effects of residue on wind erosion (Woodruff et al., 1965), wind barriers on microclimate (Skidmore et al., 1966), tillage implement on soil erodibility by wind (Lyles and Dickerson, 1967), and wind erosion damage on crop growth (Fryrear and Downes, 1975). The wind erosion research accomplishments until 1975 were reviewed by Fryrear (1977). He emphasized the need to develop a single event wind erosion soil flux model. A need for more reliable and accurate wind erosion measuring equipment and the identification of the impact of long term continuous wind erosion on soil productivity were reported. The benefits of emergency tillage and optimization of wind barrier influence on an eroding field were also suggested. The wind erosion equation (Woodruff and Siddoway, 1965) is still in use. However, the USDA, ARS, is in the process of developing a more physically based wind erosion prediction system (Hagen et al., 1988, Hagen, 1991) to replace the original wind erosion equation. Wind erosion research was initiated in the Department of Agricultural Engineering, Texas Tech University in 1986. The major emphasis of research efforts was to develop a process based wind erosion and dust generation model. Gregory and Borrelli (1986a) identified and described the basic physical concepts for wind erosion modeling. They used dimensional analysis to develop a single event soil detachment function for wind erosion. Basic fluid mechanics principles were reviewed and applied to describe the wind velocity profile for field conditions. A wind velocity profile equation coupled with soil detachment function resulted in a wind erosion transport equation. The effect of surface cover (Gregory, 1984a) was also considered in the development of the final wind erosion equation. Soil shear strength was used as a variable in the detachment function so the equation worked for both cohesive and non-cohesive soils. Gregory and Borrelli (1986b) reinvestigated and analyzed the length effect for soil erosion by wind. The length effect function was originally developed by Gregory (1984b). Based on the amount of energy initially used to detach soil particles compared to that required to re-detach, a mathematical equation was developed to explain the length effect in the wind erosion process. Change in abrasion with field length was also considered. Soil detachment (Gregory and Borrelli, 1986a) and length effect (Gregory and Borrelli, 1986b) functions combined with a description of the wind velocity profile were used to derive a single event wind erosion equation (Gregory and Borrelli, 1986c). Physically based relationships for turbulent wind shear, soil detachment, fraction of cover, and length effect were combined to develop a wind erosion equation. The model was physically based and could be used for different soils and a variety of locations. Arika et al. (1986) developed a ridge and clod wind erosion model and found that clods were more important in controlling wind erosion than ridges. The horizonal velocity at half of the ridge height was estimated to be 5.1 times the friction velocity. Abtew et al. (1989) developed a procedure to calculate displacement height and aerodynamic roughness from the geometry and fraction of cover of rigid surface roughness elements. These parameters are needed to describe the wind velocity profile for wind erosion studies. Borrelli et al. (1989) reviewed the literature on wind breaks as a control measure to protect fields from wind erosion. They developed an alternative equation to predict the relationship between lee and upwind velocities as a function of the ratio of distance leeward to wind break height. The effect of height, spacing, and porosity of wind breaks was also studied. Effect of soil moisture content on threshold friction velocity was considered by Gregory and Darwish (1990). A theoretical equation was developed to predict threshold friction velocity as a function of soil particle size, soil moisture content, and 10 soil moisture associated with clay content. This equation was verified with wind tunnel data collected by Darwish (1991). Wilson and Gregory (1992) developed an equation to relate soil particle detachment and kinetic energy of impacting particles. Soil erodibility was defined as the soil bulk density divided by soil shear strength, which is dimensionally equivalent to the inverse of crushing energy per unit mass of soil. The relationship between erodibility and clay content was also established. Gregory et al. (1993a) further considered the effect of particle size, particle size distribution, relative humidity, and soil shear strength on detachment and maximum transport rate of soil particles during the wind erosion process. Using dimensional analysis, an equation was developed to predict detachment and maximum transport rate as a function of wind speed, soil particle size, soil moisture, soil cover factor and soil erodibility. The equation is as follows: ^=I^^P«<7T§r)'^<^ ^csJ^^)(SUU^?)VM{L^rA) (2.2) Where X = rate of soil movement (kg/m-hr), pi,j = soil bulk density (kg/m^), c2 = coefficient obtained from calibration (0.004), c3 = coefficient obtained from calibration (125), c4 = minimum particle diameter for saltation process (0.08 mm), U,t = threshold friction velocity (m/s) which depends on particle size and relative humidity, D50 = average or mode diameter of soil particles (mm), D75 = particle diameter at the 75 percent location on the cumulative size distribution curve (mm), D, = reference diameter of soil particles (1 mm), S = a cover factor, which expresses the energy transfer from 11 the wind profile to the soil surface (dimensionless), U, = friction velocity for the wind (m/s). Of = a gust factor to adjust the dynamic threshold for wind gust in the field (dimensionless, Gf = 1.5), L = length of field (m), E = erodibility of soil (kg/J), and A = abrasion factor (dimensionless). The term contained in the square bracket in Equation 2.2 is the maximum transport rate for a given wind speed and soil conditions. The maximum transport equation was calibrated with measured wind tunnel data reported by Zingg (1953b), Belly (1964), and Williams (1964). The equation was also verified with field data from Svasek and Terwindt (1974) shown in Figure 2.1 and from Nickling (1978) shown in Figure 2.2. Wilson (1994) conducted a wind tunnel study to collect more accurate data on sediment transport by wind and verified the effect of particle size and moisture content on maximum transport rate of soil (Figure 2.3). 2.2 Eolian Suspension Visibility reduction is the most commonly observed effect of suspended soil particles in the atmosphere. Light is blocked and scattered by the surface of suspended soil particles, which reduces the amount of light penetration through the atmosphere. The visibility depends on particle size and concentration. Concentration depends on entrainment mechanics. When the vertical lift of wind on a particle is greater than its weight, the particle moves up into the air. The turbulent fluctuations of wind near the ground surface provide the vertical lift responsible for entrainment of particles into the air. Soil particles are detached, moved, and lifted into air due to the force of wind exerted against the surface of the ground. The wind force is characterized by the friction velocity (U.) which is the square root of the ratio of surface shear stress (x) to air density (pj. The value of friction speed can be estimated as a function of 12 10000 T 1 1 I I Measured (Svasek and Terwindt) •Predicted (Equation 2.2) X! 1000 100 (0 o w 10 CO % 0.1 Friction speed, U^ m/s Figure 2.1 Mass transport rate as a function of friction speed. Data from Svasek and Terwindt (1974). 13 1000 -I r- Measured (Nickling, 1978) Predicted (Equation 2.2) 100 CD CO RH= 20%- O 10 RH- 30% CO _i I I 1 0.1 Friction speed, U^ m/s Figure 2.2 Mass transport rate as a function of friction speed. Data from Nickling (1978). 14 2500 f 2000 E \ U) 6 1500 LU I- ^ 1000 o 500 - < a: 0 0. 2 0. 4 0. 6 0.8 1.0 FRICTION VELOCITY (m/s) Figure 2.3 Mass transport rate as a function of friction speed. Data from Wilson (1994). 15 horizontal wind speed at any height, using surface roughness, and displacement height (Abtew et al., 1989). When wind speed over an eroding surface exceeds the threshold friction velocity of the soil particles, the particles begin to move. Soil particles that are too heavy to enter into suspension bounce or saltate along the ground surface. They rise almost vertically, travel horizontally 10 to 15 times their height of rise and return to the ground surface at an impacting angle of 6 to 12 degrees. Soil particles too heavy to be moved initially by wind force are pushed or rolled by the impact of smaller soil particles in saltation. The soil particles once lifted from the ground surface can enter into suspension and travel a long distance, if their terminal fall velocity (VJ is smaller than the vertical component of turbulent wind speed, which is approximately equal to the friction speed. Energy is also transferred to the soil surface by saltating particles; therefore, the rate at which small soil particles are ejected from an eroding surface to enter into suspension depends upon the saltation process. 2.2.1 Length of Visibility Prediction The length of visibility in the atmosphere depends on the amount of light penetration through the material in question, i.e., small droplets of vapor or suspended fine dust particles. Visibility is routinely recorded by the National Weather Service, but there is no published relationship connecting soil movement at the surface to dust concentration and visibility. Greeley and Iverson (1985) and Anderson and Hallet (1986) also emphasized the need to develop a theoretical basis to link dust concentration in air and visibility to soil particle movement in the saltation layer. Gregory (1987) derived an equation to predict visibility as a function of dust concentration and particle size. The fraction of light that penetrates the atmosphere filled with suspended dust particles can be predicted by the following equation: -1500^^ (23) p=e P^ where P = the fraction of light that penetrates the material, C = the concentration of sediments (kg/m^). 16 p = particle density (2650 kg/m^), L = the thickness of the material (m), and D = the particle diameter (mm). Equation 2.3 applies to one particle size; however, it can be used to develop a general procedure for a mixture of particle sizes and their respective concentrations. Light penetration through a system of various particle sizes can be predicted by evaluating the probability of light penetration through all concentrations of the various particle sizes in the system. The total light penetration for a unit length of material can be predicted with the following equation: -ISOolEi:?^ (2.4) Pj=e where Pj = total light penetration through all particle sizes for a unit length of material, Cj = concentration of i class size of sediments (kg/m^), D, = the particle diameter of i class size of sediments (mm), and n = number of size classes of sediments. The effective average particle diameter at a given height can be determined by rearranging Equation 2.4. D^=-1500l-% (2.5) '^ p \X\PT where D^ff = the effective average particle diameter (mm), and Q^ - the total concentration of suspendible particles at a given height (kg/m^). The length of visibility (Ly) at a given height for a standard light penetration (P=0.02) can be predicted with the following equation: 17 L^=[Jn^]^. (2.6) '^ ' 1500'C^ To calculate the length of visibility at any height by Equation 2.6, the concentration of each size suspendible particles at each height is required. Thus, there is a need to link soil movement at the surface to soil movement in suspension and the concentration of soil particles in the saltation layer is of prime importance in the prediction of visibility. 2.2.2 Suspended Particle Concentration with Height Anderson and Hallet (1986) developed a general model of eolian sediment transport by both saltation and suspension. The general equation to predict suspended particle concentration with height is as follows: V, Cr-Crlf) '•*"• <'•'> where C^ = concentration of suspended particles at any height Z (kg/m^), C, = concentration of suspended particles at reference height Z^ (kg/m^), and V3 = terminal fall velocity of particles (m/s). Equation 2.7 was also derived for concentration profiles of suspended particles by Budd (1966), Shiotani and Arai (1967), Gillette et al. (1972), and Kind (1989). The settling velocity of each particle determines whether the particle enters into suspension or whether it moves solely by saltation. If the terminal settling velocity of the particle is less than the friction speed, the particle moves in suspension. The value of the terminal settling velocity is different for various soil particle sizes, hence different size particles will have varying concentrations with height. 18 Equation 2.7 can be used to estimate concentration profiles of each suspendible soil particle size in the eroding surface and the total concentration of suspendible particles at any height will then be the sum of individual concentration profiles. In order to calculate concentration profiles of suspended particles with height, the reference concentration (C,) and reference height (Z,) in the saltation layer should be determined for each particle size. The shape of the concentration profile is fixed by equation 2.7, but the magnitude of the concentration profiles depends on boundary conditions such as reference concentration (C,) and reference height (Z,) for each particle size, which should be related to the saltation process near the ground surface. The research need to define and calculate reference concentration (C,) and reference height (ZJ was emphasized by Owen (1964), Budd (1966), Shiotani and Arai (1967), Gillette et al. (1972), and Kind (1989). A fixed value for reference height was used by these researchers and the soil particle concentration in the reference layer was estimated by empirical methods. The process used was very empirical and judgmental. Kind (1992) considered the suspension of fine particles due to high wind blowing over a large surface of loose particles such as tailings disposal sites, bare dry fields, or snow-covered areas. He concluded that if the wind is strong enough to move soil particles to produce suspension, the saltating particles will eject suspendible particles at a constant rate. Despite the fact that the terminal settling velocity of very fine particles is almost zero and the power law relationship of particle concentration with height predicts uniform concentration profiles for very fine particles, he assumed that concentration profiles with height for fine particles will be of a logarithmic form. The value of reference level height was assumed arbitrarily above the saltation layer and reference concentration was calculated with an empirical relationship. Kind identified that soil erosion caused by wind at the ground level should be connected to dust concentration above in the air. He also recommended further research to define reference level height and reference concentrations during the wind erosion and dust suspension processes. The saltation process plays an important role in determining reference height and reference dust concentration. In the presence of mixed soil particle sizes m the 19 field, impacting of particles due to saltation eject small soil particles which enter into suspension. Therefore, the rate at which particles are ejected from an eroding surface depends on the saltation process. Sediment concentration in the saltation layer and height of saltation layer are the key factors in predicting concentrations of suspended particles and visibility with height above the ground surface. 2.3 Saltation Wind blowing over a soil surface induces motion of soil particles when particle weight is less than the uplifting force caused by wind shear. The soil particles ejected from the soil surface that are too heavy for suspension follow unique trajectories under the effect of gravity and air drag. This motion is known as saltation. Saltating soil particles are too heavy to enter into suspension because their terminal fall speed is higher than the fluctuating wind shear. They rise until their kinetic energy is converted to potential energy and travel a distance 10 to 15 times their height of rise and return to the ground surface where they either rebound or embed themselves in the soil surface and eject other particles into saltation and suspension depending upon their size in comparison to the shear force of the wind. The height of rise of saltating particles depends on the initial upward velocity with which the grain leaves the surface. After the comprehensive work of Bagnold (1941), Zingg (1953b) studied the relationship between the mean saltation height, friction velocity, and soil particle size for various sand size ranges and reported that mean saltation height is given by the following relationship: where H^ = mean saltation height (m), d = soil particle diameter (mm), and U. = friction velocity (m/s). Owen (1964) further investigated the theoretical framework of eolian saltation. He examined the interaction between the motion of uniform saltating particles of sand 20 and soil and the turbulent wind. He concluded that the effect of the moving soil particles on the fluid above the region of saltation is similar to that of solid roughness height, which can be compared with the height or depth of the saltation layer. Owen attempted to answer two main questions: (1) what was the effect of saltation on the airflow at heights above the soil surface, and (2) what determines the concentration of particles engaging in the saltation process. He emphasized the need of research to define the concentration of soil particles near the soil surface and declared that if the concentration of soil particles in the saltation layer is known, then the amount of fine dust carried from the soil surface into suspension could be calculated. Assuming that the saltating particles travel in identical trajectories, Owen developed solutions to wind velocity profiles, in and outside the saltation layer, and for the fluxes of saltating grains. According to Owen's solution, the mean height of saltation for uniform particles is given by the following equation: H =-^ (2-9) Owen (1980) considered the effect of particle impacts on the soil surface during saltation and suggested an improvement over the Equation 2.9. The improved equation is of the following form: Based on the energy transfer to the soil surface by impacting particles, Gregory et al. (1991) derived the following relationship to estimate mean saltation height: D, -im^ (2.11) where D,, = diameter of incoming particles striking the surface (mm), and 21 D^^,, = diameter of outgoing particles emitted due to energy transfer (mm). From a wind tunnel study Wilson (1994) found that for uniform size particles the mean saltation height is a linear function of friction velocity not the square of friction velocity. Therefore, there is a need to further investigate the relationship between mean saltation height and friction velocity. 2.4 Dust Generation During the wind erosion process, much of the dust production occurs as a result of saltation. While the suspendible dust particles exist in natural soil, particles in the silt and clay size range are usually bonded to other particles. Energy is needed to break the bonds between smaller size particles to generate dust. The dust potential in natural soil is controlled by the soil particle size distribution. All particles small enough to be suspended are potential dust. Particles larger than 80 |im rarely go into suspension because of their high settling velocity. Kinetic energy is applied through the saltation of particles causing abrasion of large size aggregates into smaller particles including fine dust. When the large size saltating particles return to the ground surface, they often hit aggregates causing abrasion and emission of fine particles. The literature on dust generation as a byproduct of the wind erosion process is limited. The size distribution of aerosols produced from soil was studied by El-Fandy (1953), Chepil (1957), and Chepil and Woodruff (1957). Although, wind erosion was recognized as an important mechanism that produces fine dust particles, only a few studies have been conducted to relate wind erosion to dust particles in the air. As early as 1970, Hidy and Brock (1970) produced an inventory of the global sources of tropospheric aerosols and it was estimated that dust from wind erosion of soils contributed 9.3 percent of total suspendible aerosols and 21 percent of the total mass of the aerosol source. Gillette et al. (1972) investigated the size distribution of aerosols measured for different wind and soil conditions in fields subjected to wind erosion. They compared 22 the size distribution of dust particles to the size distribution of the soil in the fields. They reported that the size distribution of aerosols in the range of 2 |im to 12 |im in diameter was expressed by a power law curve and the size distribution of aerosols in the range of 0.6 |im to 2 [xm in diameter was expressed by a flatter curve. Prediction of wind erosion by empirical methods and observed vertical fluxes of aerosols were related qualitatively. They also identified that soil moisture, wind velocity, surface roughness, and vegetative residues were important parameters to determine the availability of dust particles for movement during wind erosion. Gillette (1977) reported that fine dust particles are produced by the sand blasting effect of saltation particles in the wind erosion process. He measured the emission of fine particles less then 0.02 mm in diameter and total soil movement (horizontal soil flux) on an eroding field. He concluded that the ratios of fine particles emitted to total soil movement were dependent on soil texture, wind speed, mineralogy, and soil moisture-wind history. Saltating particles emit fine particles by breaking the bond between aggregated fine particles on the surfaces of larger particles. He reported that fine textured soil produced more fine particles per unit soil movement unless the structure of the soil aggregates was highly resistant to breakage of bonds. Gillette and Walker (1977) analyzed the particle size distributions of measured airborne particles generated by wind erosion. They reported that the fine airborne particles (2 -20 |im in diameter) consisted mainly of clay minerals and larger particles were exclusively quartz grains, which contained coatings of clay. From the findings of particle size distributions analysis, they concluded that smaller size particles were derived from the exposed soils by sand blasting during wind erosion. Continuous sand blasting by saltation removed the clay particles aggregated to surfaces of quartz grains. Fine textured soil having a higher percentage of clay yielded higher percentage of fine airborne particulates. Evans and Cooper (1980) prepared an inventory of particulate emissions from open sources. Empirical equations were developed to estimate emissions from open sources, i.e., paved and unpaved roads, agricultural tilling and wind erosion, construction site activity, mining, and forest fires. Dust flux was expressed as a 23 constant fraction of the total soil erosion flux. They concluded that in 1976 the total open source emissions of particles in the U.S. amounted to over 580 x 10^ ton and particulate emissions from unpaved roads and agricultural wind erosion accounted for 86 percent of the total emissions from all the open sources. Gillette (1986) reviewed the method used by Evans and Cooper (1980) for estimating total mass flux of eroding soils. He reported that the method over estimated the erosion for areas outside the region for which it was developed. Evans and Cooper expressed the dust flux as a constant fraction of the total soil erosion flux so it was concluded that the dust flux was over estimated. Based on the effect of field length on wind erosion, he reported that dust flux does not come to equilibrium with field length so dust flux estimates were in error if they were calculated by a constant fraction of total soil loss from fields. Gillette (1988) developed a model to estimate dust emission caused by wind erosion. The model was based on the summation of the expected dust production caused by wind erosion for individual sampling units of the detailed soil and land use inventory of the National Resources Inventory compiled by USDA. A dust emission function was theoretically derived and calibrated by experiments. However, the effect of field length on wind erosion described by Gregory (1984b) was ignored in the model development, so this model is also an empirical method to predict dust emission from wind erosion. Hence, there is a need to develop a physically based equation to predict dust generation as a function of field length and kinetic energy due to saltating particles. 24 CHAPTER m MECHANICS OF SALTATION 3.1 Description of Wind Tunnel used for Wind Erosion Studies A suction type wind tunnel was designed and fabricated at Texas Tech University to study wind erosion on agricultural soils. The wind tunnel is 0.5 meters wide and 1.0 meter high. Originally, the wind tunnel was 5 meters long and used by Darwish (1991) to study the effect of moisture content on threshold friction velocity of soil particles. This wind tunnel was expanded to a length of 10 meters by Wilson (1994) to conduct a wind tunnel study to collect more accurate soil transport data in which particle size, particle size distribution, and friction velocity of wind were controlled. A unique isokinetic sampling unit (Figure 3.1) was also designed and fabricated by Wilson (1994). The unit is comprised of eight, 1.26-cm^ vacuum driven nozzles placed at heights of 1.5, 3.2, 4.9, 6.6, 11.0, 16.5, 25.5, and 50.0 cm in the sampling unit to collect the sediment transport data at different heights. In order to remove the particles from the air stream, each sampler was connected to a 10-cm diameter cyclone (Zobeck, 1989) in series with a 0.90kW wet/dry vacuum cleaner via a 2.5-cm diameter gate valve to control the air flow rate. Two Prandtl-type pitot-tubes were equipped with each sampler, one inside the sampler nozzle and other at 12-cm from the aerodynamic sheath of the sampling unit. All the pitot-tubes were connected to a manometer table. The gate valves on the vacuums were used to adjust the velocity in each nozzle to match the velocity in the matching pitot-tube in the wind tunnel. At the surface of the test section, a 1.0 cm x 0.5 cm non-isokinetic creep sampler was also installed to collect the creep data. 3.2 Mean Saltation Height and Maximum Transport Rate Data Collection The wind tunnel was used to carry out an experimental study to collect more accurate sediment transport data (Wilson, 1994). The isokinetic sampling unit was tested for its efficiency before the actual maximum transport rate data were collected. 25 Figure 3.1 An isokinetic sampling unit used in the wind tunnel (Source: Wilson 1994). 26 The particle size, particle size distribution, friction velocity, and threshold fnction velocity were controlled in the wind tunnel to collect maximum transport rate data for uniform particle size sand blasting sand. The effects of relative humidity and particle size on maximum transport were also studied by Wilson (1994). The mean saltation height and mass distribution with height in saltation data were collected for three different uniform particle sizes and material. These included: 0.15 mm uniform sand, 0.30 mm glass spheres, and 0.37 mm uniform sand blasting sand. A total of eight runs were made for 0.15-mm uniform sand with friction velocity ranging from 0.32 m/s to 1.09 m/s. Seven runs were made for 0.30-mm diameter glass spheres with friction velocity ranging from 0.35 m/s to 1.13 m/s. For 0.37-mm diameter uniform size sand blasting sand, a total of seven runs were made with friction velocity ranging from 0.37 m/s to 1.08 m/s. The wind profile was allowed to develop over 4.0 meters of fixed roughness in the wind tunnel similar to the roughness of the material in the test section. The remaining 6 meters length of wind tunnel was used as a test section. Mass of particles moved at different heights was collected in cyclones connected to an isokinetic sampling unit. A methodology and procedure developed by Wilson (1994) was used to calculate maximum transport rate by integrating the area under the mass distribution with height curve. The mean saltation height was determined as the height at which half the mass moved in saltation. The data on mean saltation height as a function of friction velocity are presented in Table 3.1. A more detailed summary of data is presented in Appendix A. 3.3 Height of Saltation Height of saltation for uniform soil particles can be calculated by setting the potential energy of the particles at the top of its trajectory equal to the kinetic energy of the particle at shear velocity or friction velocity (U,), and neglecting the friction in the system. The height of saltation layer can be calculated by the following equation: mgH^=^mUl (3.1) 27 Table 3.1 Mean saltation height as a function of friction velocity data collected in the wind tunnel. S. No. 0.15 mm Sand 0.30 mm Glass Spheres 0.37 mm Sand U, (m/s) MSH (m) U. (m/s) MSH (m) U. (m/s) MSH (m) 1 0.32 0.006576 0.35 0.009801 0.37 0.013566 2 0.42 0.006816 0.44 0.011669 0.46 0.013131 3 0.50 0.006905 0.52 0.014498 0.51 0.014220 4 0.66 0.007787 0.64 0.017738 0.71 0.016439 5 0.70 0.008474 0.78 0.022890 0.90 0.020302 6 0.79 0.008856 1.00 0.035096 0.99 0.021177 7 0.84 0.008961 1.13 0.037034 1.08 0.023974 8 1.09 0.011061 U. = Fnc tion VelocityJ MS H = Mean Saltation Height rf (3.2) ^8 Equation 3.2 is similar to the theoretical development of Owen (1964) who assumed that saltating particles travel identical trajectories. However, experimental measurements made in the wind tunnel, for 0.15 mm uniform particle size sand, 0.30 mm glass spheres, and 0.37 mm uniform size sand blasting sand, do not follow the relationship given by Owen (1964) and Equation 3.2 (Wilson, 1994). The mean saltation height appears to be a linear, not a square, function of fnction velocity. The mean saltation height as a function of friction velocity for three different materials is shown in Figure 3.2. Each line in Figure 3.2 has a different intercept and slope. The values of intercepts and slopes are given in Table 3.2. 28 0.05 o 0. 15 mm SAND E R^2 = 0.95 0.04 -- + 0.30 mm SPHERES X R^2 = 0.98 ••• ^^ * 0. 37 mm SAND X 0.03 R^2 = 0.97 y" • y/^* < 0. 02 - < *^i--lj^^^ 0.01 - < UJ 1 1 1 1 1 1 0.2 0.4 0.6 0.8 1.0 1.2 1. 4 FRICTION VELOCITY Cm/s) Figure 3.2 Relationship between the mean saltation height and friction velocity. Data from Wilson (1994). 29 Table 3.2 Intercept and slope for the mean saltation height and friction velocity relationship. Particle Size and Material Intercept Intercept at Slope threshold 0.15 mm sand 0.00427 0.00540 0.0058 0.30 mm spheres -0.00505 0.00369 0.0380 0.37 mm sand 0.00705 0.01111 0.0145 1 These intercept values correspond to zero friction velocity. But there will not be any saltation if friction velocity does not exceed the threshold friction velocity of the particles. Therefore, the lower boundary condition of saltation height is controlled by the threshold friction velocity. The value of the intercept for 0.37 mm sand blasting sand is higher than the value of the intercept for the 0.15 mm uniform sand. The value of the intercept for the 0.30 mm size glass spheres is the lowest and is negative. One reason for the difference in intercepts could be a difference in threshold friction velocity for different size particles and materials. If the friction velocity does not exceed the threshold friction velocity, there will not be any movement of particles, hence there will not be any height of saltation. Therefore, the intercept for mean saltation height is controlled by threshold friction velocity. The threshold friction velocity for 0.30 mm size glass spheres is higher than the 0.15 mm size sand, but the value of intercept is smaller. This relationship can be explained by the difference in material properties, such as angle of repose. If a particle is perfectly spherical, frictionless, and lying on a smooth surface, it should move by rolling along the surface and should not saltate. However, all the materials (even spheres) have a property of stacking due to angularity, shape of particles, and stacking arrangement, which is commonly defined by angle of repose. Therefore, soil has a higher intercept than glass beads because of a higher angle of repose. The values of threshold friction velocity were calculated by the equation developed by 30 Gregory and Darwish (1990) and the emptying angle of repose for each tested material was measured. The values of threshold friction velocity and angle of repose for the three different materials tested are given in Table 3.3. Table 3.3. Threshold friction velocity and emptying angle of repose for the material tested in wind tunnel. Size and material Threshold friction velocity Emptying angle of repose (m/s) (degree) 0.15 mm sand 0.21 37.33 0.30 mm spheres 0.23 27 0.37 mm sand 0.28 38 If the effects of particle size and material property are considered, then the intercept at threshold can be explained by the following function: U 2 Interceptan„,j^=A^ (3J) 28 where O = emptying angle of repose (degree). Figure 3.3 shows the relationship between intercept at threshold and (UtV2g)tan 31 -go. 020 a _j So. 015 I/) UJ a: X 1- to. 010 •CEP T £o. 005 0.001 0.002 0.003 0.004 0.005 ^tan* Cm) Figure 3 3 Relationship between intercept at threshold and (U^V2g)tan 32 ^D,^m (3.4) r where D = particle diameter (mm), and D^ = reference particle diameter (0.08). The particle diameter associated with minimum saltation particle size is taken as 0.08 mm. Particles smaller than 0.08 mm will not return to the surface quickly enough to transfer significant energy to the eroding surface. The angularity of particles will affect the nature of collision in the saltation process. The glass spheres being perfectly smooth, will cause near perfect elastic collisions and will tend to rise to a greater height in saltation. The equation to describe the slope of mean saltation height and friction velocity relationship can be written as follows: where A2 and A3 are calibration coefficients, and (U. - U.J is used as the independent variable. The combined equation to predict mean saltation height as a function of particle size and material properties will be of the following form: H=Al^mQ>^(42-AStan^){^^^^^^^){U^-U^). (3.6) 28 JD^ A computer program MERV (Gregory and Fedler, 1986) was used to calculate coefficients Al, A2, and A3. The calibrated equation to predict mean saltation height as a function of particle size and material properties is of the following form: ^ 3.36^tan^+(0.30-0.32tan4>)(^^5^^^)(t/.-l/J. (3.7) 28 JD, 33 Equation 3.7 predicted mean saltation height for all three particle sizes tested in the wind tunnel with an R^ of 0.99 and was statistically significant (a = 0.001). Figure 3.4 shows the relationship between measured and predicted mean saltation height. 3.4 Reference Concentration of Soil Particles in the Saltation Layer The dust concentration is defined as the mass of soil divided by the volume of air in which the soil is suspended. An alterative definition for dust concentration can be expressed as mass per unit time divided by volume per unit time. The flow rate of air can be estimated by the product of the average friction speed (reasonable estimate of wind velocity in the saltation layer) of the wind over the saltation depth m which the fine material is suspended and the field width: (?=5.1 (iyj(C/J(ff036OO (3.8) where Q = air flow rate (m^/hr), and W = width of eroding surface (m). The reference concentration (kg/m') in the saltation layer is calculated by multiplying the maximum transport rate (X) by the width of the eroding surface (W) divided by the air flow rate: C=^. (3-9) ' Q Thus, the value of C^ can be established from the sediment concentration in the saltation layer and the air flow rate in the saltation layer. The concentration in the saltation layer consists of different particle sizes. Particle size distribution analysis of eroded mass can determine the fraction of sediment concentration contributed by each particle size. Thus, the concentration for specific size ranges can be determined by multiplying the total concentration by the fraction of matenal that is in the specific size range. 34 ^ 0-05 UJ G. 04 K G- 03 h < (jy G. 02 < UJ R*2 - 0.99 Q 0.01 LU U o LU 01 G G. 01 G. 02 G. 03 0.04 G. 05 Q- MEASURED MEAN SALTATION HEIGHT Cm) Figure 3.4 Measured versus predicted mean saltation height for all three particle sizes. 35 In the development of Equation 3.9, it was assumed that the sediment particles were moving at the same average velocity as the wind speed. Specifically, it was assumed that the average particle velocity at the mean saltation height is the same as the wind velocity at the average saltation height. If the wind velocity is higher than the particle velocity, then Equation 3.9 will under predict sediment concentration This problem can be eliminated by multiplying by the ratio of wind velocity to particle velocity: C -^^^^ (3.10) ' Q u, where U^ = wind velocity at height H^, and U = sediment velocity at the same height. 36 CHAPTER IV MECHANICS OF DUST GENERATION 4.1 Theory of Dust Generation Dust generation from agricultural soil is a byproduct of the wind erosion process. Soil is formed by the conversion of parent material to a new product with properties different from the parent material. A soil system is often composed of a mixture of different sized particles bound together by clay and organic matter. Coarse fragments and soil separates are the two broad groups of mineral particles in the soil. Coarse fragments include subclasses such as stones, cobbles, and gravel. Soil separates are the sand, silt, and clay fractions that are called primary particles and most commonly subjected to the wind erosion process. According to the USDA system of particles size distribution, sand particles range from 0.05 mm to 2.0 mm in diameter, silt particles range from 0.002 mm to 0.05 mm in diameter, and clay particles are less than 0.002 mm in diameter. Much of the soil mass is moved by saltation and surface creep, but the soil particles can also be injected into the air if their terminal settling velocity is less than the vertical component of the turbulent wind speed. While suspendible particles do exist in natural soil, particles in the PMJQ size range are usually bound to other particles. Fine silt and clay particles bond to other soil particles to form larger size aggregates. These bonds must be broken for the fine silt and clay to be released as dust and PMJQ. External energy is needed to break the bonds between smaller size particles to generate dust. All the particles small enough to be suspended are potential dust. Energy required to break these bonds can be applied by crushing or abrasion. During the wind erosion process, kinetic energy is applied through abrasion by saltating particles and much of the dust production occurs as a result of saltation (Figure 4.1). 37 SUSPENSION CREEP Figure 4 1 Wind erosion and dust generation process. (Source: Greeley and Iverson, 1985) 38 The number of particles moving in saltation increases with length downwind on an eroding field. Therefore, the amount of kinetic energy and the release of dust per unit area also increases with field length. The amount of dust generation is a function of kinetic energy and amount of dust available in aggregate form in the soil. The change in detachment of dust particles with number of impacts from particle abrasion can be expressed with the following equation (Gregory et al., 1993b): dN ^^ where D = generated dust fraction as primary particles, N -- number of impacts, Wf = dust generation rate coefficient (related to soil texture, soil moisture, and weathering), and D = available dust fraction in aggregate form in the natural soil system. The available dust fraction in aggregate form can be expressed as the difference between potential dust fraction in aggregate form (Dp) and dust fraction already released (D): D^=D^-D. (4.2) Substituting the value of D^ from Equation 4.2 into Equation 4.1 results in a general function relating the change in detachment of dust particles to the number of impacts from particle abrasion. The equation is of the following form: ^ = -WlD-D). (4.3) dN ^ ^ 39 Assuming the Wf is constant. Equation 4.3 can be rearranged and integrated from zero to the final dust potential fraction and from zero impacts to the final impacts to obtain: D-D \X\(-£—) = -W/^. (4.4) P Taking the exponential of both sides of Equation 4.4 and with rearrangement results in the following form: D-DpiA-e"^^). (4.5) Equation 4.5 can predict dust generation as a function of the potential dust fraction in the soil and number of impacts from particle abrasion. If there is an initial dust fraction in primary particle form in the soil caused by tillage, mechanical action, or weathering, it can be considered with the following form of the equation: D=DJi^-e~^^yD^ (4.6) where Dj = initial dust fraction in primary particle form in the natural soil. 4.2 Design and Development of Controlled Energy Dust Generator In order to simulate the saltating process during wind erosion and to study the relationship between kinetic energy and dust generation, a controlled energy dust generator (CE/DG) was designed and developed (Figure 4.2). An empty 55 gallon barrel, 86 cm long and 56 cm in diameter was mounted with its axis horizontally on 7.6-cm diameter caster wheels located on each of the four comers of a 107 cm X 76 cm X 81 cm angle-iron frame. The angle-iron frame was fitted with 3.8 cm caster wheels so the whole unit could be moved easily from one place to another. A single- phase (1.1-kW, 9.6-AMP, 1725-RPM) electric motor was mounted on the frame. The motor slowly rotates the barrel using a V-belt arrangement for speed reduction. The 40 Figure 4.2 The controlled energy dust generator (CE/DG) motor has a variable speed control device that allows the operator to control the speed of the barrel and the rate of energy applied. Three stationary blades (86 cm long and 6 cm high) were placed at equal distances inside the barrel. A fixed half-barrel section (86 cm long and 44 cm in diameter) was installed inside the barrel and was attached to U-shaped fixed-frames outside the barrel on the main frame. As the barrel rotates, the half section holds the soil sample against the bats until the sample is at the top of the barrel. A 0.90-kW wet/dry vacuum was used to blow air into the barrel. The air was blown through a 2.54-cm diameter pipe installed at 6.35 cm above the base of the barrel. Holes spaced 2.0 cm apart on the pipe provided air to the base of the barrel with sufficient velocity to suspend dust particles. The exit air was sucked into a 2.54- cm pipe in the center of the barrel with holes spaced 5 cm apart. The air was routed through a cyclone (Zobeck, 1989), which collects most of the dust particles being sucked by the vacuum system. The cyclone is connected to the suction side of the wet/dry vacuum, which makes it a completely closed and recirculating system. The final trap for dust particles was the paper bag filter on the wet/dry vacuum. Two valves were installed at the inlet and outlet of the piping to control the flow of air in and out of the system. The air velocity was measured with a red-oil manometer connected to the piping system on the suction side. The whole CE/DG unit was placed in a wooden frame room covered with a plastic sheet to control air humidity. The apparatus was designed to simulate an impact velocity associated with a medium to severe wind erosion event. The impact velocity of soil particles falling from a height (h) can be given by V2gh, so the impact velocity from the fall height of 50 cm in the CE/DG is equal to 3.12 m/s. The impact velocity for saltating particles is the resultant of horizontal and vertical components of the friction speed (U,). The horizontal component of the friction speed is approximately equal to 5.1U. and the vertical component is equal to U.. The resultant of these component is given by V(5.1U.)^+U.^ Thus, the impact velocity from saltation for a friction velocity of 0.6 m/s IS also equal to 3.12 m/s. 42 4.3 Procedure and Data Collection for Dust Generation A total of five soils (three sandy loam and two loamy sand) were collected from the agricultural fields around Lubbock, Texas. A silty loam soil was acquired from Pullman, Washington, and a heavy clay soil was obtained from Temple, Texas. Soil samples were collected from the top 15 cm of the soil profile. All soils were air dried. A particle size analysis was performed for each soil using the standard hydrometer method to determine the percent sand, silt, and clay particles in the soil. The sand fraction was further analyzed for the percent fraction from very fine sand to coarse sand. The particle size distribution curves for all the soils are shown in Figure 4.3 and the percent sand, silt, and clay values are given in Table 4.1. Air-dried samples were sieved through a 2 mm sieve before they were placed into the CE/DG. A 500 gram sample of soil was placed into the barrel and the barrel was rotated at a speed of 12 rpm for a total of 80 minutes. The 80 minute time was associated with 1458 impacts, which is equivalent to the number of impacts for particles in saltation to travel 185 meters. The dust release was measured every minute for the first 5 minutes, every 5 minutes for the next 15 minutes, every 10 minutes for the next 20 minutes, and every 20 minutes for the remaining 40 minutes. Air circulation to remove dust was employed during the rotation of the CE/DG and the following 2 minutes after the CE/DG was stopped. The 2 minute period after rotation ceased was used to remove the suspended dust. Since the barrel had three stationary blades inside, one-third rotation and 2 minute suction, and another one-third rotation and 2 minute suction were carried out to completely collect all the free dust in the soil. The generated dust was captured in a plastic bag inside the cyclone and weighed after each time interval. Finer dust, which was not collected by the cyclone was collected on the vacuum filter bag. After a run of 80 minutes, the remaining soil in the barrel was collected and weighed. Moisture content samples were taken before each run to determine the soil moisture content at the time of the experiment. Relative humidity was also measured before and during each run. All runs were made under low relative humidity conditions, which varied from 10 to 50 percent. All measurements were replicated three times. The data are presented in Appendix B. 43 Table 4.1 Percent sand, silt, and clay for seven soils. Soil texture Percent Sand Silt Clay Sandy loam-1 58.8 23.5 17.7 Silt loam 28.6 66.0 5.4 Sandy loam-2 63.3 25.0 11.7 Sandy loam-3 68.4 22.1 9.5 Loamy sand-1 80.1 11.9 8.0 Loamy sand-2 84.3 9.7 6.0 Clay 8.0 41.9 50.1 44 0.001 0.010 0.100 1.000 Particle Size, mm Sandy Silt -*— Sandy Sandy Loam-1 Loam Loam-2 Loam-3 Loamy Loamy -»- Clay Sand-1 Sand-2 Figure 4.3 Particle size distribution curves for the seven soils used in the dust generation experiment. 45 CHAPTER V RESULTS AND DISCUSSION 5.1 Mass Distribution with Height in the Saltation Zone Data obtained for mass distribution with height in the saltation zone for varying friction velocities and three particle sizes are shown in Figures 5.1, 5.2, and 5.3. Only three data sets (highest, lowest, and mid value of friction velocity) are plotted in Figures 5.1, 5.2, and 5.3 to show the trends in mass distribution with height in the saltation zone. Data from the surface creep sampler are not plotted, but they are considered in the analysis of mass distribution with height for surface creep and saltation together. The maximum amount of mass movement occurs near the surface, and the amount of soil mass movement increases with an increase in friction velocity for all three particle sizes and materials. Similar results were reported by Fryrear and Sal eh (1993) for field conditions. The measured mass per unit square area per unit time at a particular height (MHa) is divided by the total mass transport rate (MT) measured in mass per unit width per unit time to calculate the ratio of MHa/MT for each friction velocity value. The height of mass measurement (H) is divided by the mean saltation height (HS). Figure 5.4 shows the MHa/MT versus H/HS curve for surface creep data for all three particle sizes, which is explained by a single non-linear function, except for two points that appear to be outliers. The non-linear function to describe MHa/MT versus H/HS relationship for the surface creep data is of the following form: MHa_^,.=Z,0-e ^"^^^^' "^^ H) (5.1) MT where MHa = mass per unit square area per unit time at height H (kg/m'-hr), MT = total mass transport rate per unit width per unit time (kg/m-hr). 46 30000 L f 25000 o U* = 0. 32 m/s E + U* = 0. 66 m/s cr \ 20000 * U* = 1. 09 m/s en ^x UJ 15000 < H- 10000 o CL 2 5000 < Q: 0.05 0.10 0.15 0.20 0. 25 0. 30 HEIGHT Cm) Figure 5.1 Mass distribution with height for 0.15-mm sand moving in saltation. 47 40000 h. 35000 I E cr 30000 2> 25000 UJ 20000 - < °^ 15000 h- g 10000 I 5000 0.05 0.10 0.15 0.20 0. 25 0. 30 HEIGHT (m) Figure 5.2 Mass distribution with height for 0.30-mm glass spheres moving in saltation. 48 5 40000 1 o U* = 0. 37 m/s ^ 35000 + U* = 0. 71 m/s \ 30000 * U* = 1. 08 m/s U) ^ 25000 UJ t 20000 a: 1- 15000 CL 10000 J^ 5000 1- n "•"•"•p a 0.05 0.10 0.15 0.20 0. 25 0. 30 HEIGHT Cm) Figure 5.3 Mass distribution with height for 0.37-mm sand blasting sand moving in saltation. 49 H = height at which concentration is measured (m), HS = mean saltation height (m), and ^1. ^2, Z3 = constants obtained from calibration. The function fits the data with an R^ of 0.86 and is statistically significant (a=0.001). The calibrated, non-linear function for surface creep data is of the following form: M£=26.22(1-."""^'^"'). (5.2) MT The line in Figure 5.4 is the model fit to the data points. Equation 5.2 has three coefficients determined from the experimental data. If this equation is used to calculate an estimate of the mean value for MHa/MT at a given H/HS, an average standard deviation can be determined with the following equation: 5=Ei^^L^ (5.3) n-Z where s = standard deviation, X, = measured values of MHa/MT, and X„ = predicted mean value of MHa/MT, n = number of data points. Both points initially judged as outliers are more than three standard deviations away from the equation line; therefore, there is statistical evidence that the two points in question are outliers. Because surface creep data are difficult to measure (especially in a field situation with a rough surface). Equation 5.2 provide some understanding about the process. First, it implies that the height of measurement used to obtain surface creep data determines the fraction of the total movement that is defined as surface creep. Also, because the variable HS increases with friction velocity, the fraction of moving material defined as surface creep decreases as friction velocity increases. These results imply that reported fractions of movement defined as surface creep are of no use 50 30 o 0. 15 mm LOAMY SAND 25 h* 0.30 mm SPHERES « 0. 37 mm SAND R*2 - 0.86 20 ^15 10 - 5 - 0.2 0. 4 0. 6 0.8 1.0 H/HS Figure 5.4 The relationship between H/HS and MHa/MT for surface creep data. 51 without accompanying data on friction velocity, particle size, height of saltation, and height of measurement. Figures 5.5, 5.6, and 5.7 show the relationship between H/HS and MHa/MT for saltation plus surface creep data for 0.15-mm sand, 0.30-mm glass spheres, and 0.37- mm sand, respectively. The saltation data for each particle size appear to collapse into one non-linear function. The next step is to develop an equation to describe the observed relationship. The saltating particles in the wind erosion process experience a transition from all kinetic energy to all potential energy back to all kinetic energy at the retum impact. In route the particles will experience drag and may experience a collision with another particles. The process is complex; however, the major factors should be potential and kinetic energy components. The potential energy of saltating particles at any height H (PEH) is given by the following equation: PEg=mgH, (5.4) If the energy is held constant, then the mass can be expressed as a function of height as follows: Z m= 4 (5.5) H where Z4 = a constant. Similarly the mass at mean saltation height can be expressed by the following equation: Z m = 5 (5.6) ' HS where Z5 = a constant. From Equations 5.5 and 5.6, the ratio of m/m, can be expressed by the following equation: 52 m _ ^A Equation 5.7 is undefined when H goes to zero; however, the same approximate shape can be obtained with stable boundary conditions using the following equation: -^(4) -^=Zee ^^ (5.8) where Zg and Z-, = constants. The mass movement at height H is measured as mass per unit square area per unit time (MHa) and mass movement at mean saltation height is defined as the half of the maximum transport rate (MT) measured as mass per unit width per unit time. Therefore, the ratio of m/mj can be converted to a ratio of MHa/MT by the following equation: g MHa _^ -^^Hs^ (5.9) MT ^ where Zg and Z9 = constants. The kinetic energy of saltating particles at any height H (KEH) is given by the following equation: J^^=lmt/,2. (5.10) Equation 5.10 can be rearranged to calculate mass (m) at any height which can be given by following equation: (5.11) ^ where Zio a constant. 53 According to Wilson (1994), a linear relationship exists between friction velocity (U.) and height of saltation, so Equation 5.11 can also be rewritten as follows: m-^ (5.12) where Z,, = a constant. Similarly, the mass at mean saltation height can be calculated by the following equation: m=-^ (5.13) 'S im 2 where Z12 = a constant. From Equations 5.12 and 5.13, the ratio of m/m^ can be estimated by the following equation: m ^11 '' ^i2(^r HS^ The approximation of Equation 5.14 can also be written as follows: -z. r * ^2 m _y ^ ^*^W (5.15) where Z,3 and Z,4 = constants. The ratio of m/m, can be converted to MHa/MT by the following equation: MHa_y '^^is^ (5.16) MT ^^ where Z,5 and Z,6 = constants. 54 Thus, the MHa/MT for saltation process can be expressed by a combination of Equations 5.9 and 5.16: ^-(z^-''^\z,^-'^^\ (5.17) The single non-linear function to describe the MHa/MT for surface creep plus the saltation data together will be of the following form: i^.Z,(1-e-«^>-^)(Z3. - The coefficients Z„ Zj, and Z3 are already calculated for surface creep data. Using the computer program MERV, the coefficients Zg, Z9, Z,5, and Z,6 are estimated for each particle size. The function fits the data very well with an R^ of 0.98 for 0.15- mm sand, 0.76 for 0.30-mm glass spheres, and 0.91 for 0.37-mm sand. All results are statistically significant (a=0.001). The calibrated values of coefficients Zg, Z9, Z,„ and Z,6 and the corresponding coefficient of determination (R^) for each material and particle size are given in Table 5.1. Table 5.1 Calculated coefficients Zg, Z9, Z^j, and Z,6 and the coefficient of determination (R^). Particle size and Zs Z9 Z,5 Z,6 R^ material 0.15 mm SAND 0.59 0.32 0.65 0.11 0.98 0.30 mm SPHERES 0.45 0.32 0.41 0.32 0.76 0.37 mm SAND 1.24 0.79 0.12 0.01 0.91 Coefficients Zg and Z9 are linear functions of particle diameter and coefficients Zj5 and Z16 are inversely related to particle diameter squared Based on these 55 observations, the following generalized non-linear function is developed to describe the MHa/MT data as a function of H/HS for all three particle sizes. ^=26.22(1 -."•^'^'^•"")(3.&fe"^i>.M^,^'^>V <^-l') Equation 5.19 fits the data very well with an R^ of 0.98 for 0.15-mm sand, 0.68 for 0.30-mm spheres, and 0.91 for 0.37-mm sand blasting sand and is statistically significant (a = 0.001). The value of R^ does not change for 0.15-mm sand and 0.37- mm sand blasting sand but drops from 0.76 to 0.68 for 0.30-mm spheres. The line shown in Figures 5.5, 5.6, and 5.7 illustrate the model fit. While Equation 5.19 appears to fit all three data sets reasonable well, more data for different particle sizes are needed to accept calibration coefficients. 5.2 Effect of Kinetic Energy and Soil Texture on Dust Generation Dust generation data collected with the CE/DG are analyzed to test Equation 4.6. The MERV program is used to determine the unknown coefficients. The data matches the derived equation with an R^ of 0.99 or better for all seven soils and are statistically significant (a =0.001). The values of coefficients Dp, D„ Wf, and the coefficient of determination (R^) for all seven soils are given in Table 5.2. The effect of generator rotations on dust generation for three soils (silt loam, sandy loam-1, and clay) is shown in Figure 5.8. Figure 5.9 shows the effect of generator rotations on dust generation for four coarse textured soils (sandy loam-2, sandy loam-3, loamy sand-1, and loamy sand-2). As the number of generator rotations increases, amount of dust generation increases but the rate of dust generation decreases. The dust generation levels off to the maximum dust potential of the soil, which is described by coefficient Dp in Equation 4.6 and Table 5.2. Thereafter, there is no effect of generator rotations on dust generation. 56 30 o 0-15 mm SAND R^2 = 0.98 afiMM 20 30 40 H/HS Figure 5.5 The relationship between H/HS and MHa/MT for saltation plus surface creep data for 0.15-mm sand. 57 30 25 0.30 mm SPHERES R^2 = 0.68 20 20 30 40 H/HS Figure 5.6 The relationship between H/HS and MHa/MT for saltation plus surface creep data for 0.30-mm glass spheres. 58 Figure 5.7 The relationship between H/HS and MHa/MT for saltation plus surface creep data for 0.37-mm sand blasting sand. 59 Table 5.2 The percent sand, silt, and clay and the values of coefficients Dp, D„ Wf, and coefficient of determination (R^) for the soils tested with CE/DG Soil texture Percent The values of coefficients in Equation 4.6 and coefficient of determination Sand Silt Clay Dp D. Wf R^ Sandy Loam-1 58.8 23.5 17.7 211.52 0.00 0.0019 1.00 Silt Loam 28.6 66.0 5.4 337.16 9.11 0.0028 0.99 Sandy Loam-2 63.3 25.0 11.7 202.93 0.16 0.0022 0.99 Sandy Loam-3 68.4 22.1 9.5 195.62 0.00 0.0017 0.99 Loamy Sand-1 80.1 11.9 8.0 123.70 0.00 0.0014 0.99 Loamy Sand-2 84.3 9.7 6.0 95.30 0.00 0.0012 0.99 Clay 8.0 41.9 50.1 297.53 0.00 0.0007 1.00 There is a difference in amount of dust generation for different soils at a particular level of kinetic energy provided by the generator rotations. The difference depends on the amount of energy required to break bonds between aggregated particles. Silt loam soil produces the highest amount of dust at a particular level of kinetic energy. Sandy loam-1 soil produces a lower amount of dust than the silt loam soil. Higher levels of kinetic energy are needed to break the bonds between clay particles, so the clay soil produces the lowest amount of dust for a given level of kinetic energy, even though the clay soil has a higher potential of dust generation (Figure 5.8). Eventually, after a large number of rotations, the clay soil should produce more dust than the sandy loam soil. 60 500 a SILTY LOAM SOIL CS.4Z CLAY 2B.BS 8MCD _* SAtOr LOAM SOIL ai.7X CLAY B8.aS SAND) R*»«.0O • CLAY SOIL eSDL IZ CLAY •. OK SAMB R*2-l. GO 200 400 600 800 1000 1200 GENERATOR ROTATIONS Figure 5.8 Effect of generator rotations or number of impacts and soil texture on dust generation (sandy loam-1 silt loam, and clay soils). 61 200 400 600 800 1000 1200 GENERATOR ROTATIONS Figure 5.9 Effect of generator rotations or number of impacts and soil texture on dust generation (sandy loam and loamy sand soils). 62 From Table 5.2, it is obvious that soil texture has a major effect on the values for the variables Dp and Wf. There is a difference in dust generation between soils dominated by sand, silt, or clay. The silt loam soil with approximately 72 percent of fine silt+clay fractions and 28 percent sand particles produces the highest amount of dust. The sandy loam-1 soil with 41 percent of fine silt+clay fractions and 59 percent sand particles produces a lower amount of dust than the silt loam soil. The clay soil has stable aggregates, so even with 92 percent fine silt+clay fractions, this soil produces the least amount of dust. Higher levels of kinetic energy are needed to produce the potential dust for soils high in clay content (Figure 5.8). As the amount of sand increases, the potential for dust decreases because the amount of fine particles is limited in the soil (Figure 5.9). At the same time, if there is an ample amount of fine silt and clay in the soil, the dust potential increases as sand increases (clay soil and silt loam soil) because sand particles acts as an abrader in the energy/abrasion process. The amount of clay content increases potential dust generation, but the rate of increase decreases with increasing clay content. The difference in the results in Figure 5.9 appears to be primarily due to difference in clay fraction. 5.3 Prediction of Dust Generation Rate factor (Wf) as a Function of Sand and Clay Content Measured values of dust generation rate factor, clay content, and crushing energy for the soils tested in CE/DG are given in Table 5.3. Except for the loamy sand soils, the dust generation rate factor decreases inversely with clay content because clay particles form aggregates with bigger size particles such as sand and more energy is needed to break these bond to release fine dust particles. A similar trend is observed between crushing energy and the dust generation rate factor. Crushing energy is also related to percent clay content in the soil. The crushing energy data were obtained by crushing air dried clods for each soil using Soil Aggregate Crushing Energy Meter (Skidmore and Powers, 1982). 63 Table 5.3 Percent clay content, dust generation rate factor, and crushing energy data for the soils tested m CE/DG. Soil texture Percent clay Wf Crushing energy content (J/kg) Silt loam 5.4 0.0028 7.21 Sandy loam-2 11.7 0.0022 47.86 Sandy loam-1 17.7 0.0019 49.28 Clay 50.1 0.0007 105.32 Loamy sand-1 8.0 0.0014 11.89 Loamy sand-2 6.0 0.0012 - Sandy loam-3 9.5 0.0017 22.64 The dust generation rate factor increases with an increase in sand content because sand particles act as abrader. The non-linear function to determine the dust generation rate factor as function of clay and sand content is of the following form: -jCjOOO-Xg) Wj-i )(1-e ) (5.20) X1+X2 where Wf = dust generation rate factor, X, = percent clay content, X2 = percent sand content, and K,, K2, K3 = calibration coefficients. The calibration coefficients are determined using MERV and the calibrated general non-linear function to calculate the dust generation rate factor as a function of clay and sand fraction is given by the following equation: (5.21) Figure 5.10 shows the relationship between measured and predicted values of dust generation rate factor. The relationship has an R^ of 0.82 and is statistically significant (a=0.05). 64 3.0 0 0.5 1-0 1-5 2.0 2.5 3.0 MEASURED Wf (1/lGGO) Figure 5.10 Relationship between measured and predicted dust generation rate factor for all soils. 65 5.4 Prediction of Dust Potential Coefficient (Dp) as a Function of Particle Size Distribution The particle size associated with ratio of measured dust potential coefficient. Dp, over 500 gram sample mass, determined from the soil particle size distnbution curve for all seven soils is given in Table 5.4. Table 5.4 Particle size associated with the ratio of measured dust potential coefficient to total sample size (500 gram). Soil texture Measured Particle size Percentage of Predicted value of associated soil associated value of potential dust with measured with 26 \im potential dust coefficient (Dp/500)* 100 particle size coefficient (g) (^m) (%) (g) Sandy loam-1 211.52 22 44.6 223.00 Silt loam 337.16 20 72.4 362.00 Sandy loam-2 202.93 25 39.8 199.00 Sandy loam-3 195.62 30 16.3 81.50 Loamy sand-1 123.70 30 20.9 104.50 Loamy sand-2 95.30 34 32.4 162.00 Clay 297.53 3* 92.5 462.50 _ '^^ •- With the exception of the clay soil, the percentage of the soil associated with the ratio of Dp to total sample size occurs at a particle size of approximately 26 |im in diameter (Figure 5.11). The clay soil appears to have stable aggregates that act like sand. The particle size removed by the CE/DG was expected to be less than 80 jim; therefore the CE/DG works as designed. The size of particles removed can be regulated by adjusting the air flow in the recirculation line. At low circulation rates. 66 only the fine particles will be removed. The suspension of 20-30 nm diameter particles is very reasonable for a friction velocity of 0.6 m/s during field conditions, therefore the current airflow adjustment in the CE/DG appears to be at an appropriate setting. The dust potential coefficient for a friction velocity of 0.6 m/s can be predicted by reading the percentage of soil associated with 26 jim particle size. The relationship between measured and predicted Dp is shown in Figure 5.12 for all soils except the clay soil. The relationship has an R^ of 0.93 and is statistically significant (a=0.001). Therefore, it is concluded that particle size distribution analysis is a right step towards the prediction of dust potential for non-clay soils. Clay soils can be considered by evaluating the water stable aggregate fraction; however, insufficient data are available to make this analysis. 67 0 0 000 0.001 0.010 26 [xm 0.100 1.000 Particle Size, mm Sandy Silt Sandy Sandy Loam-1 Loam Loam-2 Loam-3 —^— Loamy Loamy Clay Sand-1 Sand-2 Figure 5.11 Particle size distribution curve showing percent of soil mass associated with a 26 |im particle size. 68 500 a.400 Q"300 Q UJ u 200 - a LLI ^ 100 0 0 100 200 300 400 500 MEASURED Dp Cg) Figure 5.12 Relationship between measured and predicted dust potential coefficient for all soils except the clay soil. 69 CHAPTER VI SUMMARY AND CONCLUSIONS An experiment was carried out in a wind tunnel to collect mean saltation height data at varying friction velocities for three different uniform particle sizes. Mass movement at different heights in the saltation layer was also measured using an isokinetic samphng unit in the wind tunnel. A physically based model was developed and verified to predict mean saltation height as a function of particle size, wind velocity, and material properties. The data on soil particle concentration with height in the saltation zone were analyzed to develop a general non-linear function to predict soil particle concentration with height as a function of friction velocity and particle size. A theoretical equation was developed to relate dust generation and kinetic energy of saltating particle with field length. A simple and convenient device Controlled Energy Dust Generator (CE/DG) was designed and fabricated to develop a methodology to connect the wind erosion and dust generation processes. The device is easy to operate. The apparatus was used to study the relationship between kinetic energy from abrasion and the effect of soil type on dust generation from seven soils. The following conclusions are drawn from the present work: (1) The mean height of saltation for uniform soil particles can be predicted accurately as a function of particle size, wind velocity, and material properties. (2) The mass distribution with height in the surface creep plus saltation zone is a non-linear function of maximum transport rate and mean saltation height. The function is general and can be used to determine the mass movement at any particular height, which can be used to determine concentration of soil mass at any particular height. (3) Dust generation is a function of kinetic energy of saltating particles and is controlled by the amount clay and sand fractions in a soil. While dust (including PM,o) generally is not a separate entity in soil, 70 when energy is applied, a considerable amount of dust can be released, especially under dry conditions. (4) Dust generation increases with increased impacts, but the rate of dust generation decreases. The dust generation levels off to a maximum dust potential for the specific soil. (5) The type of soil and its sand and clay fractions affect the dust potential and the rate of dust generation. (6) As the amount of the sand fraction increases, the potential for dust decreases because the quantity of finer particles is limited in the soil. If there is an ample amount of fine silt and clay in the soil, the rate of dust generation increases with an increase in sand fraction because sand particles act as abraders. (7) The dust potential increases with an increase in clay content because of the availability of more fine particles but the rate of dust generation decreases with increase in clay content because clay particles form aggregates with bigger particles such as sand, and more energy is needed to break the bonds between primary and secondary size particles. (8) The potential dust generation for a soil generally can be determined from the particle size distribution analysis. The exceptions are soils with high clay content. (9) The dust generation rate can be determined as a function of clay and sand fractions of the soil. Recommendations for future work include the following: (1) More data should be collected in a wind tunnel for uniform as well as mixed size particles to further verify the saltation height model and mass distribution with height function. (2) The physics of stochastic soil particles and soil particle bed interactions should be understood to develop a model to predict mean saltaticn height as a function of inter-particle collision in the saltation process. 71 (3) The non-linear function to predict mass distribution with height should be verified with field data. (4) Dust concentration with height data should be collected in the field to verify the visibility prediction model. (5) The CE/DG should be used to conduct experiments to test the effect of abrader amount on dust generation. 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Transactions of ASAE 32(4):1316-1318. 80 APPENDIX A WIND TUNNEL DATA FOR THREE UNIFORM PARTICLE SIZES 81 3 VO o o O o o o ^0 o cd Q u B 3 CS > E op op 00 'S c o 'S > • »-* •3 c (J u I 6 S Si o c 00 u OS •c u 3 H H > B O X> u jo -c I u u 0) n c CQ u m o I .CO 3 c CQ CO u 13 u Q •c czi a 2 B oo '00 oo oo o ON 00 ON NO ON OO o ON NO ON C i vi NO B s o a or t :a d V)9- *o oo NO oo oo o O o ON o O Z s NO O o o ed ra n O ed o O o o cd td H vo o o o O o o o «r> V) T3 C/} C o •o ON NO Vi V) O o NO NO" vi o B B ^ o Vi 3 OS o C/3 CQ •«-» "^ -r* < -C ed OH 82 4> fS t^ NO 3 «p»4 rs ON NO r^ OO cd /•^ 'T' •«-» M ed UI ) Q .^^ /"-V _^ Redu c Variabl e Bul b Temp . (°F ) Bul b Temp . (T) spor t Rat e (k g m ' h r ishol d Frictio n Veloc : ilacemen t Heigh (c m ac e Roughnes s (cm ) tiv e Humidit y (% ) t i Saltatio n Heigh (cm ) CO • .1 J fictio n Velocit y ( m s' ) 1 fc* el a 1 H b Q CZl We t a Pi H 2 it y o o ON t^ v^ o m "Ju "„-, CO <-H •^ i"^/- ^ S"? O v^ O VI o o o o o OO vo v> Tt fS NO o o ed CO (J o m O «N "* o o o CQ •s m VJ "^ f»H o o o o ""c ed "a. 0. 0 0. 1 0. 2 0. 0 0. 0 0. 0 0. 0 Q 0. 0 O C/5 Mas s T i 1 (k g 1.26 : Ui c«5 3 o OH *w v> VI Vi cn c o v» CQ Ii u < ^^ o 83 en 3 o ON "3 ed *- "^ ed crt Q e. T3 /—N ^ Redu c Variabl e ea n Saltatio Heigh t (cm ) hreshol d Frictio n Velo c r y Bul b Temp . (°F ) rictio n Velocit y ( m s' ) isplacemen t Heigh (c m urfac e Roughnes s (cm ) elativ e Humidit y (% ) ranspor t Rat e (k g m ' h r H b Q c« We t Bul b Temp . (°F ) Q oi H 2 ^ 1 - oo •^ r^ NO o _^ oo en "» "M 00 en •«1- oo ••- /—N ^"^ o O o O o o o o o "* ->* "* NO NO oo ^ o ed fS en o "<1- Tf Heig h H o CA CA CQ Lo a e t N u rticl e CM O u - 84 "* r>- t^ u ^M NO oo o ed td 'co Q e^ (U I T3 ^ /—s -' ) u ^-^ o ^-N e 2 3 u ^ g /—V _ .^ Re d Variabl Frictio n V e Temp . (°F ) Rat e (k g m elocit y ( m s Temp . (°F ) len t Heigh i oughnes s (c i > oi ol d Saltatio n Heig h ul b o a 3 »or t cd en _o CQ OQ tiv e Humidit y ( % S" c Me a Surf ! Rel a We t Dr y Dis p Fric t Thr e ^ en ON o v> ON 'Wi Vi t^ oo OO ON i^ CQ Vi o Vi vo NO Vi r*-" cm ' ^ 00 CA .wm ••- u 6 ^9 Vi NO r^ oo < u - CS en -"^ No . a Shee t Numbe r a n Particl e Siz nit s o f terial : Loam y S Sample r 1 ^ .5 ^ P p S S 85 o "«1- 0 T^ 3 ^^ 0 Vi Vi (N •3 ed ••-• "^ cd c» Q 6^ p ^^ ^ Redu c Variabl e Threshol d Frictio n Velo c Surfac e Roughnes s (cm ) Frictio n Velocit y ( m s' ) Displacemen t Heigh (c m We t Bul b Temp . (°F ) Dr y Bul b Temp . (°F ) Relativ e Humidit y (% ) Transpor t Rat e (k g m ' h r Mea n Saltatio Heigh t (cm ) ^ 8 ^ P-4 r>> Vi UI O 0 0 0 0 0 0 UB . &"? 0 0 0 (N 0 •* 0 0 1.2 6 0" '-<' 0" 0" 0" 0" 0" 0" as s T i "3. ^ 5 00 c/5 cd T3 NO ffJ a 3 O T3 C •*-* Vi o 0 Vi V) 0" Vi (N ON NO 0 o •a >N.!S NO en ">* NO" vi 0" (cm ) ^^ Vi Heig h 0.0 - H CA (A \-i Cd -c -a pL, Ji CX 0 —' rsi en •^ Vi NO r^ 00 o ^ "C - u CA rt iJ § 1 CO CO c 1 tS ed a> 1 1 1 p 86 u NO NO ON ON Vi •o33 ed /—\ ••-< tn ed UI ) Q ^^ p •' ) 'cfl 6^ 'B Redu c eigh t (cm ) n Velo c igh t (c m s (cm ) f 00 Variabl e hreshol d Fricti o rictio n Velocit y isplacemen t H e urfac e Roughn r y Bul b Temp . elativ e Humidit y (% ) lea n Saltatio H UH CZ3 We t Bul b Temp . H Q Q OJ Transpor t Rat e ( k 2 it y J - _1 r*- O ^iH ^^ NO oo NO TS ~»i cd Vi '* rt NO Vi NO Vmi o t-^ 1.2 6 c m Ii •••a^ -C cd OH o < Vi ^ "C - 1 2 U — 87 o es w ^^ Tf o ^^ o NO _3 cd /'-N -^-t cd CA Q e. (U I ^.^ jr- 4) .^ o U u /—N B /""s 3 U o o & /—\ _h .^ Re d Variab l ci B io n Heig h at e (k g m " ocit y ( m s emp . (°F ) rictio n V e u midit y ( % b a len t Heigh ( oughnes s (c i H H oi E > a Pi Salta t ul b lol d a ul b >or t CO ^^ o r>» VI r^ "^ NO t^ Vi "fc- 1 - es Tf o f^ CO • "^ U ^•^ 6 NO •^ OO ^ oo NO CN NO Z. i*"^ Vi Tf NO -^ Vi Vi ON en cd a ed o NO O •* NO en o o H vo ON "* OO en —" o o o d "o. lo 05 o ran ! :!? 6. 5 o 1 en ^ NO CN Vi 0. 0 as s T S VI t^ oo < u - 88 u oo NO f-^ OO _3 ON o NO "3 ed /-^ •*-• *rC A ed Q TS V /-N ^ 3 Ji '^ (c m •—N t(cm ) Blo c B /•"v O C9 > 'B D^ C s 00 en ^ CO a t 00 eig h > hreshol d Fricti o rictio n Velocit y isplacemen t H e urfac e Roughn r y Bul b Temp . elativ e Humidit ; •anspor t Rat e ( k ea n Saltatio H H b Q !/) We t Bul b Temp . Q Pi H 2 ^^ O o Vi NO Tf NO NO CN J- —H "S "on Cd o o r>- OO r^ o > en ^^ m oo CA +- /^S i^b d &>» 00 00 00 o oo •^ "«d- TT NO 0.0 - Vi CA 1 o i2 CA (d t^ B o o< 6 u -^ CN en rr Vi NO r>- oo rt « § CA tS cd 89 u m ed 4-> ed tn Q B -0 /•~\ Variabl e e u H Velocit y ( m s' ) Roughnes s (cm ) I d Frictio n Veloc : 5men t Heigh (c m x> i b Temp . (°F ) ! Humidit y (% ) rt Rat e (k g m ' h r altatio n Heigh t (c i a 3 3 sh o io n lac ( CQ sp o Surf i Thr e We t Fric t Dis p Dr y Rel a Tra n Mea i ^ •^ 0 0 ON en Vi t~^ en "a1S '^-* ed •^ Tt PV| 1-H ON ^H Vi »-H Vi en NO 0 f>*4 ON f-^ NO ^ 00 ••- y^\ 6 NO "^ 00 rt ;_• J: ^"^ Vi CA 3 ^ Vi Vi ^ CT) N 0" Vi (N ON NO p p .2P 6 1 ^1^ NO VI 0" S 'S "c^ 0 "^ cn •^ "O .-^ CN Vi l-^M 3 0" CA •:5 ^ « CA -^ rh -3 (d l-i ?S ^ ^ CU 0 — 90 NO rt ON m "^ r«» "^ OO NO CN Tj- wm ^^ 00 ON 'r~. ^ ^ o Vi NO Vi NO Vai l o" o" o CN ^^ cd r-^ 4-> ed M Q S, Reduce d Variabl e Frictio n Velocit y Temp . (T ) elocit y ( m s' ) len t Heigh (cm ) oughnes s (cm ) Temp . (°F ) Rat e (k g m" ' hr' ) > S or t ul b lol d ii 3 Saltatio n Heigh t (cm ) a CO •^ o CO m CQ tiv e Humidit y (% ) CA e u c el a hr e is p ^ ^ 2 We t H b Q »3 Q Pi H Mea i ^^ oo o O NO ,^ ^~i NO o CS 1—H ^M 1 ^ CS oo rr t^ oo NO r^ en NO ON o Vi rl- CN ^ CN fS 't oo TT ON OO Vi r^^ ^ "^' vi vi vi K t»-" od oo" B ^ o 00 +- U is 6 Vi Vi Vi o O Vi Vi o Z (=s*':i 'B en NO Vi oo oo »-H ON U 0. 0 CA CA i-i Od > i CA td 3 « '5 H o S S P 91 u ON oo en CS V) CS en ON 3 CS Vi o CN Vi o Vi en NO oo o" rr ed td Q -a (U B CJ S u 3 Ji B 3 (J x: "O > B op (U 3 tn CO c op v> 00 3 0d o 3 ii t^ ••a CX CX X G u B B O c a CO •*3 •c 3 ii O H ii Pi > B H a Pi Xi X) 3 e u ii > o CO CO 2 (i 3 OH ,C0 CQ 3 ••3 EA c 3, *-> CQ CO •c a 3 oo I OO Vi NO ON Cd oo oo Vi CS en oo rt- > Vi CN NO CN NO o" NO" NO" oo" ON" B vi o oo d O Vi Vi Vi Vi v» o o o Vi rt Z NO Vi rt 2 o OO ON oo V) v> oo CN Cd Ji CN •«d- o ON NO o o rt H vo CN rs NO O o o M 92 u NO CN 00 rt 3 cn r^ CN vO cn CN NO r- en ^^ en o t*-. ^ NO Vi cn t^ Vai l o" o o r^ o" V90i ^^ rt /^^ •«-• rt CA Q s ^-^ /—V .^ /—s B Redu c eigh t (c i igh t (c m s (cm ) n Veloc : £ 00 Variabl e Velocit y Roughn e I d Fricti o b Temp . l b Temp . 1 Humidi t rt Rat e ( k iltatio n H 2 c cemen t H e a 3 3 o c/5 o OQ •S _o 3 CQ CA e Surf i We t Thr e Fric t Dis p Dr y Rel a Tra n Mea i ^ o CS t*- Vi OS t^ cn ON NO ON t^ Vi 'Ir^ -"^M •^ oo oo oo -5 rt rt NO NO oo rN-O OO o CN p "O ^^ a o ^^ o" CS ^ c ^ rt NO" NO" t-*-" ON" -^ ^^ -^ oB ^ 00 4- /—S U o rt CS 0. 0 CA CA '^ O "^ Cd CS 93 CN u Vi o cn oo ON 3 CN cn ON CN t^ CN rt CN ^^ o NO" OO NO NO NO CN Vai l o" o" CS o r^ o rt CN" a /"^ 4-> rt CA Q S^ Reduce d Variabl e ul b Temp . (°F ) n Velocit y ( m s"' ) tol d Frictio n Velocit y cemen t Heigh (cm ) e Roughnes s (cm ) ul b Temp . (T ) lor t Rat e (k g m ' hr' ) Saltatio n Heigh t (cm ) JS CX OQ OQ tiv e Humidit y (% ) CJ Vi .2 3 Vi Surf i We t Thr e Fric t Dis p Dr y Rel a Tra n Mea i ^ •g ^ NO rt ON NO Vi oo Vi Vi ed Vi rt m OO •—< oo ON en "* 00 NO r- t-^ oo rt ON ^ p r- rt -". ,^' ^ cn cn ( m s" ' -H n d Vei l NO" 00 B vi K •^ ^^ --" u ^ 00 /—v U i"3 6 Vi O cn o _H rt VI Vi Z CN Vi Vi <—1 ON NO NO —< cd jj ei 2 " cn r-4 o Vi ON OO cn cn o "S H vo CN cn ON CN oo cn •-H o ^ Vi CS ON NO p p ^ c/5 N .3»? 1 ^•H rt \D <> vi o" o X ^ cn" CN Vi H 3 ^ 0. 0 CA CA '^ O "" Cd CS UI u « .^ o* ^•^ O < ;^ 2 ^ CN en rt Vi NO r-> oo ^ "C - U ^ CA (U C§O 2 rt ^ i (Zl '5 i P 94 O ii rj NO 3 cn o ON "3 CN o Vi — Vi >—1 Vi oo > o" cn NO OO rt 0. 1 3.5 1 " 267 : ei /—\ "T >-ei> CA Q e^ ^^ •' ) it y 'i Redu c g m ' h r eigh t (C l n Velo c igh t (c m s (cm ) P y (% ) Variabl e Humidi t Velocit y Roughn e I d Fricti o l b Temp . b Temp . r t Rat e ( k o c cemen t H e ii ^ o Saltatio n H 3 CX .o u CQ tiv e •S 3 OQ Vi a 1 Surf i Fric t Dis p We t Dr y Rel a Tra n Mea i ^ oo CN r^ NO t^ cn -^ oo 1 - 00 NO OO Vi 4> 'i-^ ed oo NO rt o r^ rt rt t-^ OO cn 1—H NO ^ CN O 00 »-H o" cn vi vi B "*" NO" NO" oo' ^^ •—1 ^^ -H a ^ o 00 /—V ^o "^b 0.r. o CA • _ rt 00 00 oo rt NO rt o Z O NO CN oo o cn OO o Cd jj rt 2 o Vi t** t^ l-H cn oo rt CS Vi Vi (T^ ei H vo CN •^ ^ o rt — ^ (A 1 ; 0.3 1 rt T3 <3 ^-^ C^ a 3 o <«u E1 o C • • _C2 ^-^ '•+2 CA G O 1. 5 6. 6 3. 2 4. 9 11. 0 16. 5 50. 0 25. 5 (cm ) Heigh t 0.0-0. 5 CA CA e9 Glas s Sp ] e t Numbe r rticl e Siz <• o u f-^ CN en rt Vi NO t^ 00 Ji No . Xi Sample r c Mea n Pa i Material . en Dat a Sh e p H 95 O CN rt 3 cn cn o rt oo cn "3 CN •—' r»- t-H oo Vi o CN o > o cn NO OO rt rt o" o en cn rt *-' ™ es va Q 6 ^3 ^ ^ /—N 4> ^^ B /—V ^ o 3 ii o B t3 x> Vi ? /•~\ — 4.^ CO > s x: B (A Qfi G Op 00 O '5 u > X X ci. CX u u B ii c ii B •1..* o •c o^ a a CO •a a o H > B Pi 3 u Xi Xi t: "o c ii "eo o a u 3 o CA o CQ > cn •a 3 ,C0 OQ o "El •a a 73 CO 1 c 3 l-l B H 1 PH ^» 8 ^ NO CN 1^ ^^ Vi rt ON NO ed Vi cn oo ON ON cn O rd- rt CN 'd- CN p ON cn ^ rt rt oo CN" vi NO oo" c vPi NO" NO" oo' <—> 1—H ^H ^4 s ^ o /^N i3 d OO NO NO CN CN CN 00 rt 1-H CN 1.^ o oo r«» o NO Cd Z 2 « NO r—* —H OO r^ rt "^ rt td H vo o O NO o cn CS Vi (3 "o. o CA 96 r- Vi NO 4> oo r^ NO cn NO NO CS cn m o CN o Vi Vi a NO rt 1-^ ^'•^. oo cn cn > o o" en ^—' CN en 4^ en (73 Q § j:r .^ ^—N i o ce d u ^^ B ^-^ 3 u _o — u g ,—. _!s •^^ 13 J3 -o u op ab l > s f u^ e Oei Var i B ii Frictio n Rat e (k g elocit y ( m s Temp . C lumidit y len t Hci g oughnes s tatio n H e > a5 Pi Sai l ol d ul b o ii ul b a CO >y^ CQ tiv e I "6 _o ispor t c •s C3 § is p fcr el a Vi Me a Surf i We t Pi Fric t Thr e &0 a c c '••-• CA it y rt o o o NO Tt —^ Vi o 'u. S ^ CS IM CN ON NO Cd rt rt oo M f^ cn NO ON rt Vi o ON ^ NO CN 'd- OO t^ ON ON NO •^" vi vi vi r-" ^^ oo" oo" B andb l 1^ CA o ^ J«oSo B CA B . m4 1 -*- u 6 0. 3 i"^ &r< o NO rt 00 rt oo O NO b> rt Vi rt NO rs NO ^^ t** z ,o rt CN rt cn Vi CN en NO Vi ed C*-i NO ON Vi cn o rt -^ O c a"Q . rt r-- cd Vi CN -^ -* o p o O 1.2 6 cm " o" o" o o" o" o" td -^ m IS S Tran s o" Q C/§3 5 00 C/3 . . rt ,o oi CM nne l d 3 07 8 o tin g mm ) Ui >>.^ CXtn i-i Vi CN ON NO ^ in d 1. 0 ran i 5. 5 6. 5 -O .2P e -0. 5 cn rt ^ CN VI ^ 0. 0 as s T I t Num l S cn" t-a 4> articl e Siz 1 : San d Bl a u o Vi NO r^ 00 u *"* CN cn rd- CA No . c Samp l abl e A 4- Mea n P Materia ! H Dat a S h 97 o r^ oo 3 oo NO —* V, vO en CS "^ f^. ON Vi cn NenO ON" Val o" o o r^ cn Vi cn ^"^ cn "^ rt /^\ ••-» "7* en 75 Q E^ T3 ^,^ y-^ (% ) ^ 'c« 'B B o Reduc e eigh t (c m n Velocit y igh t (cm ) s (cm ) 00 Variable r y Bul b Temp . hreshol d Fricti o rictio n Velocit y isplacemen t H e Lufac e Roughn elativ e Humidit ; ranspor t Rat e ( k [ea n Saltatio H H b Q C/l We t Bul b Temp . Q Pi H 2 ^ 5 ON CS .^ Vi rt Vi ^^ ON 15 "v* ed t^ cn rt O ""d- O NO OO ON CN CN CN OO o o ,—4 ^ "^ Vi NO_ rt •T3 3 cn oo o" ^" C NO" NO" NO" ON" -H -H B vi oo' u ^ 00 +- /-*\ i"o.r^. rt NO CS O rt NO rt o NO rd- o NO Vi t^ NO Cd cm " rt ran s o ON Vi oo CS CN 90 NO H vo cn ON rt CN rs -^ o o ^ td M Cv| a. Q M , o" o" o" o" o" o" o" o" CO ^ "•4-- •^^ Vi CA Vi Vi G o" Vi CN ON NO P p O 1 NO" vi -: cn rt" NO" o (cm ) leig h Z^ CN hM o Vi o" CA CA t^ C/3 .H Cd en l-l < u O CN rt Vi NO 00 U — cn r- CA J) t^ Xi . 'c en H 1 p 98 < cn Table Con tinued. Xi C/3 o o Data eet Numb o ^ en 1 2 (/) . •o <^ Materi SandBl i^' Mean rticle Size r^ II Pi Q rt ^ rt rt Q >- duced rt en ^^ 5 0 E—1 vo ^ •0 s > - J* ~c« ti "ia CO ^ CA • CO S S "^ Vi •_ B"^ 0 .a ^ > .0 B C i ^ > CO Samplei Heigh C CO ••- alue No. (cm) •—N CJ Vi 0" rt rt t*- CN H •7 > e Cd 0" CN 0 c 0.0-0. hreshold Frictio elocity e 93 1-H rt cn NO 0 vi t*^ cn cn 0 UH 'vi 0" Vi B 9 1.5 riction Velocity rt -t-* rt Vi Q g .^^ ^^ JT UI i (% ) ^ 'vi /—^ s B o Redu c eigh t (c : n Velo c Igh t (c m S (cm ) 00 Variabl e Fricti o Temp . elocit y Rat e ( k Elen t H e oughn e Temp . alio n H > a Pi or t ul b lol d ii 3 Sal t JS a iX _o X. tiv e Humidi t Vi 3 CO oa Vi c S ^ •*-» 33 a CO "H, ^ u Su r Re l Fric t i Di s ^ Tr a H 2 ^ r^ oo cn ON t^ _< CN oo Ir"aS -"v- ed Vi 1-H <-H 1— Vi o ON ON Vi en rt CN cn Vi r^ oo Si "c O '-' O ON o ^ CN" r4 G ^ NO" K oo" K — '-^ -^ — oB ^ 00 4- iS o S"? O CS rt o CS NO NO o Z CN Vi O NO Vi -^ Vi NO rt 1 s Vi Vi "—< ^H ON CN oo t^ Cd l-H a H so ON 00 ON oo r^ oo CN ^ en Vi CN O NO rt CN o o "a. Q o" CN" —" o" o" O o" o" C/3 Mas s ^ (7) . . rt (k g 1. 2 ffj cS (1> o c g c 3 o C CA v^ • — Vi O. ^ rt Vi Vi CA c o" Vi CN ON NO o ^^ o (U ^ CQ N HM H •—• CA Cd en l-l ^ 13 d. B O CN rt Vi NO < o, 6 U —' cn t^ oo CA CO « ii i CZl 'c en td «« *i H 1 100 rt CN CS CN u 00 O en rt O CN ON Vi en en Vi Vi Vi 3 o o NO 00 cn rsi cvi > en Q c u S •J 3 O TD .2 13 s J5 •*-^ 00 j= CJ c op Vi 00 •c o Vi CO '5 u > "3 S B G 00 E o •c G 3 ii CO b 13 ii O H a > B .0 3 a o 3 o c CJ u 3 o 3 .>; o. 3 ,co CQ CQ •4-* Vi CO CJ 3 (3 t^ CO § I •c 5 c« 13 O 2 00 cn 00 rt cn Vi Vi I ON ON Vi 1-H cn cn Vi Vi m 00 P O > rt cn o I-H NO vi NO NO" o 00" cn .S 00" u 00 U 6 cn NO o cn Vi 00 Z Vi O rd- 00 rt cn ON Vi Vi Cd V) 00 Vi 2 « CS rt NO ON O a rt H NO o cn rt CX CA 101 o CN CN oo ON ON 3 CN Vi "eS CN ON —^ NO Vi rt Vi «/-i NO OO :> o" ""*' CN" cn 2. 1 5 193 : rt /-^ -*-< "7 en CO Q B, T3 >^ ^^ ^^ Vi ^ . (cm ) Velo c abl e gh t (c m h t (c m h^ (% ) 'B B 75 O Redu c M» c c VI \__y >-, 00 3 eo > r y Bul b Temp . hreshol d Fricti o rictio n Velocit y isplacemen t H e urfac e Roughn elativ e Hmnidi t ranspor t Rat e ( k lea n Saltatio H H IJU Q W3 We t Bul b Temp . a ai i— 2 ^ r^ NO ON rs Vi cn m rt 13 ''a, ed Vi ^H r^ en ON rt rt cn Vi ON ^-H ^^ CN O ON ON oo "c O t^ cn CN rt" vi r^" s NO K oo" oo" -™ •-H 1-H <—i s ^ o oo -•- o .£ Q''^ 6 CO •_ NO cn ^H f-H *—4 OO ON o Z NO CN OO cn cn rt ON CS cd en 2 " rt —^ rt oo rt P>H ^H r~ OO CN Vi ^H Vi 04 a t^ en H «o cn o ~C (A 102 CJ o Table A.3. c tinued. Q Xi ita Sheet Num T: 084 ^ . — w OQ N C/) . •T3 c^ aterial: Sand '. asting §^' ean Particle Si (mm) 1^ ps; Q rt ^ eS rt Q ••irf rt duced en .2P B 3 ^ 2 _3> .5 M vo U CJ •^ ^ S > ! j3 "oo iS Vi CS ^ CXr. > B s 8 ^ > eo Sampler rans c 09 ••- d. alue No. cm" u o" Vi o" (*»• Vi NO o "^ H > /-v C es o" CN o a e 0.0- tA 1 hreshold Frictio elocity Vi Vi" NO NO o O NO" en Vi NO m b 'vi ^ o o B 103 1 riction Velocity CN en CN en —^_ ON ON CN K Tt NO t^ -^ Q CN" •isplacement He ight (cm) "^t o en -^ ON CN" CN ON o NO ON V*H o t^ Vi cn o" o Vi CO o B . Luface Roughne 603 -^ NO" NO --H" r<; o o o o Tt ^ o NO OC • Wet Bulb Temp. Vi ^ o o" O O CN •^ O CN r\ e O o • ry Bulb Temp. NO NO" Vi o" en \D NO Vi" NO NO ON Oi elative Humidity {% t t*- Vi" Vi o" en CN K o o t^ "^ CN NO Ov Vi ON o 0 E 1 •ansport Rate (k o Vi o O "^ ON CN r^ 2 CO CN 1 ean Saltation H eigh t (cm) 3974 P «M C/3 -p "H. a Z U • ^m V7 1 L. 6 nits of Mias s In iport foi s kg cE Urn APPENDIX B DUST GENERATION DATA FOR SEVEN DIFFERENT SOILS 104 ON rt Vi "^ > V^ NO 6 9 60 1 P , o 10.5 1 16.6 4 130.8 0 159.2 2 Cumulativ e dus t generatio n (g ) 4.3 1 22.1 4 27.1 2 83.3 6 48.2 9 67.3 9 1 i £ P 0) ^ c Xi Xi *^ ffl PQ -5 o Vi +-• ..^ rt ^ Q OisJ Tr-t o o 19.1 0 15.9 7 5.3 0 6.1 3 21.1 7 26.3 3 28.4 2 Amoun t o f dus 4.3 1 6.2 0 4.9 8 21.1 1 ^^ generate d (g ) 1 en o ^ r^ ^—^ en ^—so' Vi k- (1) 00 •<-> a (*^ ei c^ NO CN VI •*-» 105 Vi -O 00 o Tt NO o -2 CN NO Tt >^ .i^ ^ Vi Vi Tt O Vi 3 (U en O o Tt Vi oo o o NO CN CN" s c NO o Vi NO NO en NO --^ 3 o" CN NO" NO U oo Vi" CN Tt oo E £"| (u S c ^ ^ ^ Vi 3 -D XI ^ -a PQ PQ -S 00 •- ..^ rt (U ^ Q »: c 3 r<-i NO Tt oo o (U — O O Vi 00 00 ON Tt en CN Vi Tt o Vi „M en c NO 00 Tt ON" ON" r--" ON NO od »—• ' ' 00 Vi" Vi" Vi fN CN fN P_^ ..-—».. en :J) CA ^—^ c b« _o /^•"^ fl) CO •»-> rt <^ « 2 Urn B .> k- Urn 3 en o -O 3 o CJ Vi 3 2 rt NO oo ON O CN x:•*—• en NO ON CN Vi CN 00 Tt NO ON en 1) T3 00 o U 00 V) NO CN en Tt ON c x: CU en " —' 3 c: c _C 1) o '••-» •4—> C c/i rt ^..i '?) 3 o . . crt c Ji T3 U O —, 106 -«-» Vi 3 ^_^ "O 00 o (U ^^ ro o *^ > Vi f>^ c ati o NO NO r-- X J^ "i" en en CN 0 1— NO •^ CN c^ Tt Vi ON CN S 0" 1. 8 ene r 3. 0 9. 5 3. 1 4. 3 9. 0 0 en NO 1^1 Cumulat i 00 en ON —^ — fN Tt NO t-- 1 9) Oi G Vi Bu i Bu i (g ) itiv e fd u We t Dr y Rel a 00 i-^ 1—^ jrate d 00 Vi 0 0 Tt Tt r-- 00 ON t-^ CN Tt 00 ,—1 G ON m ^ Vi r- r-- 00" od r~- od CS ^ 00 Amoun t o 00 en Vi" en NO" -"^ "-^ '—< '-^ CN CN en 1 0 — r«H 0 ^~^ Vi f^ ^ G 0 . . k- '+-> 00 ^ en •*-• 0 ^^ <*z > kx g B o •4^ 0 o rt t3 -^ 3 V) k- ^ CS -B ^ 3 G — c/i O rt >•< . . 00 C B^ 0Vi a^ u j= x: soi l ru n I N o ont e ^M U^ -H- C4-1 0 0 •— O 4) 0 PQ 107 PQ CN Q -a fH ^ C3 •4-> •4-» Vi 3 0 u G ^^s Z "oc f"H ^*'^1-^ : : uoi JO _o*^ ^ H3 O Vi'^ O —1 splical Wet Bui D. Q-a T): 7 O O SJ Vi o o W -^-t CX ~^ eight • Vi Dry Bui °F): 8: 9 9 3 0 ON oistur Rela itive (%): C4-1 'c' i2 u ) ) UI XJ 0) 0 k- c 3 •*- E 3 3 CA Vi 0) 3 Tim Amoimt 0 fdu Cumulati > '•»-• ^«^ ^ -•— -4— 0 en 1.2 Tt Tt" —t Vi Vi CN Tt 0 CN 0 5.7 ~M NO CTN" ON en 0 en NO ON Tt en CN 1.5 — Tt NO 0 "^ en ON CN Tt r- ON 0 5.6 ,_M Vi CN 0 t> en NO CN _ -^ 7.7 ON Tt ON CN 0 ON r-- t^ en CN 0 CN en 1.7 NO NO ^ 'O 3 ~ C/l _cu 0 C:^ c • —•*-> 0 V) 0 "^ Tt ^-V •*-> ^- ' "C! 0 V) ~0 ^ -C r- <" 0) a .>- E 0 ^ Weig bar . k- en Weig •4-^ cuu •4-» C/i 3 ^»«^ T3 00 CN (U ^^ m Amoun t o 00 Tt C«^ CN Tt en Vi en Vi "" en CN ^^ Vi G — 3 O .. k- •S 00 ^ rt •4-> ^ ^ o > hi fc E o J3 3 o ilat i ato r Vi 7 k« -o^ 3^ 00 ON _ >^ rN E NO r-- o CN 3 (U en NO ON CN Vi CN 00 Tt NO ON en t^ 00 r-> CN en V) ^ CN en Tt ON CJ 00 ^^ """ r^ • ~* •4-> 3 ct: c G o 3 CJ . . c« c Vi T3 0) (U •"^ 4—' soi l I N o ont e ri m gcg CJ CN o O O PQ rt -4-* — (U ^ ^ a x: B 00 00 E O o o o a •JS 00 w D. '53 o H Vi V) Vi ^" CN CN Ta b 109 Xi H ^ PQ CN U en 0) o ntinued •-^ an"Si 9* '53'5 CJ -c2 en -^^ O —^ :ion N 3 Wet Bui emp. ( Vi f».. Wi (^en rt 4_> of soi mple (g • o 500 Dry Bui emp. ( e cont (%): 2 RelaLtiv e midity CN H C4_ •^ u 3 '•4-» ) ) E UI O _> u "O E •4^ 3 •4-» •4—• ••-« o Vi 3 •4-* •^—' ^_^ 0 - 0 en NO ^1 CN "^ NO Tt ^ en 0 Tt ON ON Vi ^ 0 t^ en Tt" ON CN Tt ON" ON V) CN Tt 0 m Tt V) "t 0 •^ Vi Tt O m NO a\ r- f-^ "^ NO CN ^ NO o ^ Tt ON O od v> en Tt CN NO ON NO CN o r- f«-) ^ 0 Tt ON en od ON c. o ON r- CN CN K ON 0 en Tt 0 • mm— '5 -2 •fc- -4—• -S "^ 4-> sE5 -^ /—- 0) Vi t^ — 0 en '—v o — 0) a> 0 o t! 0) 1 C/i ^ ^ .i Vi v^ CN r--<^ . crtc Replical ion No Wet Bui « CJ 1^1 3- "^ s^ CN >-^ o Vi o o ed .4-> Weight of soil Ci ^ Dry Bui Moisture conte Relative Vi > «-4 u 3 1 ) ) lU -o O •4-> c •4-> E 3 > Vi Vi a> 3 Time run Amount of du Cumulati > '••-' ^_^ '—' td •4-> 0 (U Um o o en O G Vi 0 enerated (g) 0 (U c atio G 0 en od '-^. Vi od V) CN NO od >—1 en ^— NO CN o en C?N NO CN CN 0 f4-i '^. O Vi CN NO" c^ 1—1 CN ON Tt ^H 111 NO Vi NO" CN NO en Vi" NO t^ Vi CN r-" ^ NO CN ^ o\ Tt V) NO <-4 Vi en CN r- NO" ON en '^ 0 t>- j—i ON Vi ON NO CN Tt 0 1—1 ON" 1—1 0 -^ r—» O od CN r-- Vi en NO CN NO Tt '^' ON o as en CN 1—4 O Tt NO Tt ON O CN CN O r- ON r^ Vi t^ en CN ^^ en CN 0 CN o CN O 0 en ON r^ o 0 CN CN CN (^ <*m -C n; c/5 tt: c • "4—> 1 i •£ "^ Vi '-' CN ^ r- ^v o ON o 4—» ^ 2i o c/i TJ D <0 O 3 (U o t E OJ rt Weight 0 ^ . k- <*z o •- Weight CJ c/> 3 ^^ T3 00 o oo o "^ > c Tt NO o ••-• -2 rt 00 ON Vi kH 3 Vi ON NO NO rn 00 en Vi NO t^ ON en en ON 5 . o E ^ CN NO" o o" NO ON Vi" en o CN NO 00 CN CN NO" 00 U 00 ON" Tt NO £ E "S ^ ^ K c/5 Xi X) "^ 3 -o PQ PQ -.S C4-1 w •4-> ^ rt o TD -4-' (U ^ Q ftS •4-> c en Tt NO Vi Vi 3 NO CN 00 kN Vi ON Tt en Tt CN ON Tt Vi O CN Tt « Tt ^ E o en 3 o 3 Xi a Vi 3 2 D cn Tt E 2 NO 00 ON O CN U: > >k "^ NO ON CN Vi CN 00 Tt NO ON en en C JS a CJ 00 CN en Vi NO CN en Tt ON 3 ^ CN c C a o •4-* a >^ G c ~ c/5 O CN E ^-^ C/5 XJ U en 4-' 1^ . . C/3 C O _- 112 H .^ PQ en CJ ^ (U o ntinued — 2P" £^^ •r; O ^ •? . c/5C Wet Bui emp. (' Tj" Vi NO rt"^ 'oo ^ crt o ^ 4-< • Vi 500 Dry Bui emp. 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