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Wind Erosion: Mechanics of Saltation and Dust Generation

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Wind Erosion: Mechanics of Saltation and Dust Generation 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<I) 32 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.
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