The Physics of Blown Sand and Desert Dunes

I The Physics of Grain Movement.- 1 Sand and Dust.- 1 The Order of Size of Small Particles. Use of the Logarithmic Scale.- 2 The Definition of Grain Size.- 3 The Suspension of Small Particles. The Distinction between Sand and Dust.- 4 The Composition of Natural Sand. The Prevalence of Quartz.- 5 The Origin and Formation of Sand Grains.- 2 The Behaviour of Sand Grains in the Air.- 1 The Phenomenon of Sand-Driving.- 2 Susceptibility of The Grains to Wind Forces.- 3 Experimental Verification.- 4 The Paths Traced by Sand Grains through the Air.- 5 Experiments on the Bouncing Properties of Quartz Grains.- 6 Saltation.- Appendix I Calculation of the Susceptibility.- Appendix II Calculation of Grain Motion.- 3 Wind-Tunnel Observations.- 1 The Design of a Special Wind Tunnel for Sand Experiments.- 2 Method of Operation.- 3 Observation of the Grain Paths. Method of Illumination.- 4 Saltation Over a Loose Sand Surface, Initiated by Grain Impact.- 5 Saltation Initiated by Direct Wind Action.- 6 Saltation at Higher Wind Speeds.- 7 The Surface Creep.- 8 Surface Ripples.- 9 Saltation Over Hard Ground.- 10 Three Types of Grain Motion: Suspension, Saltation, and Surface Creep.- 4 The Surface Wind.- 1 The Measurement of Wind Velocity.- 2 The Surface Drag.- 3 Streamline and Turbulent Flow.- 4 The Transition Condition. the Reynolds’ Number.- 5 The Distribution of Wind Velocity with Height.- 6 The Surface Roughness.- 7 The Drag Velocity V*. V* Also a Measure of the Velocity Gradient.- 8 Change of Wind Speed Caused by Change of Surface Texture.- 9 The Boundary Layer.- 5 The Effect of Sand Movement on the Surface Wind.- 1 Changes in the Velocity Distribution.- 2 The Characteristic Grain Path.- 3 The Drag Caused by Moving Sand. Relation Between V’* and Rate of Sand Movement.- 4 Relation Between Wind Velocity and Rate of Sand Movement.- 5 Increase of Sand Movement Over Surfaces Containing Large Grains.- 6 Sand Movement Under Water Compared With That in Air.- 7 The Suspension of Sand Grains in Air.- 6 Confirmatory Measurements in the Desert.- 1 The Apparatus Used.- 2 Measurement of the Rate of Sand Movement.- 3 Results.- 7 Threshold Wind Speed and Size of Sand Grain.- 1 The Fluid Threshold for Uniform Sand.- 2 The Fluid Threshold for Very Small Grains.- 3 Immobility of Settled Dust and Loess.- 4 The Fluid Threshold for Natural Sand of Mixed Grain Size.- 5 The Impact Threshold Wind.- 8 Summary of the Physics of Grain Movement.- 1 The Forces Acting on Single Particles.- 2 The Size of Natural Particles in Terms of the Equivalent Spherical Diameter.- 3 The Turbulence of the Fluid.- 4 Suspension, and Transport Along the Floor. Distinction Between Dust (OR SILT) and Sand.- 5 Turbulent Flow Near the Bed.- 6 The Drag ?, and the Drag Velocity V*.- 7 Flow Over a Flat Surface Whose Texture Changes Across a Transverse Border. The Boundary Layer.- 8 The Fluid Threshold of Flow Required to Move Grains on the Bed.- 9 Sand Movement in Air and Water. The Saltation Drag.- 10 Sand Movement in Air. Wind Velocity Distribution.- 11 The Impact Threshold.- 12 Saltation and Surface Creep. Sand Flow in Terms of Wind Strength.- 13 The Mean Grain Path of the Saltation and the Wave-Length of the Surface Ripple in Air.- II Small-Scale Effects. Grain Size Distribution. Surface Ripples and Ridges.- 9 Grading Diagrams.- 1 Mechanical Analysis.- 2 The ‘ Percentage-Smaller ’ Diagram.- 3 The Block Diagram.- 4 The Differential Diagram.- 5 The Log Diagram.- 6 Symmetrically Graded Sands. Comparison With the ‘ Normal Probability Curve ’.- 7 Regular and Mixed Sands.- 8 Grain Diameter Defined by Sieve Aperture. a Method of Adjustment of Sieve Calibration.- 10 Grading Changes in Non-Uniform Sand.- 1 The Cycle of Sand Movement. Removal, Transportation, and Deposition. Three Modes of Deposition: Sedimentation, Accretion, and Encroachment.- 2 Grading Experiments, and the Representation of the Results.- 3 Grading Changes due to Wind Action During the Cycle of Sand Movement.- 4 Summary of Grading Changes in Regular Sands.- 5 Grading Changes Due to Mechanical Movement of Grains Down a Slip-face.- 6 How do Irregular Sands Become Regular ?.- 7 Incomplete Sands.- 11 Surface Ripples and Ridges.- 1 Sand Waves and Water Waves Not Analogous.- 2 Classification of Sand Ripples and Ridges.- 3 Instability of a Flat Sand Surface. Effect of Changes in the Angle of Incidence of the Descending Saltation.- 4 Surface Creep Dependent on the Angle of Incidence of the Saltation.- 5 Ripples Formed Under Equilibrium Conditions: (a) Factors Controlling the Wavelength; (b) Wind Strength and Characteristic Grain Path; (c) The Ripple Height and Shape Depend on the Surface Grading.- 6 Ripples during Deposition.- 7 Ripples and Ridges During Sand Removal: (a) Limited Removal—Coarse, Steep-Faced Ripples; (b) Removal Maintained by an Oncoming Saltation—Unlimited Growth of Ridges; (c) Ridges in Very Strong Winds.- 8 Ripple-Less Sand Mixtures. The Surfaces of Sand Sheets.- 9 The Transverse Alignment of Sand Ripples in Air.- 10 Sand Ripples in Air and Water Compared.- 11 Ripples in Snow.- 12 Ripples in Very Fine Sand.- III Large-Scale Effects. Sand Accumulation. Dunes. Internal Structure, Etc..- 12 Conditions for the Growth of a Sand Surface.- 1 Limitations to the Experimental Study of Dune Formation.- 2 The Supply of Material for Dune-Building. Nature of the Desert Floor.- 3 Sand Storage on a Pebble floor.- 4 Deposition by Accretion on an Existing Patch of Bare Sand.- 5 Explanation of Sand Accretion on a Flat Surface in Terms of the wind Velocity Picture. Distinction Between ‘ Gentle ’ and ‘ Strong ’ Winds.- 6 The Effect of Varying Wind Direction.- 7 Sand Drift Over a Uniform Surface is Unstable Across a Transverse Section. Sand-Strip Formation. Dune-Building by a Uni-Directional Wind.- 8 Surrounding Surface Not Immobile. Inhibition of Dune Growth by the Spread of Coarse Grains over the Sand Surface.- 9 Fluctuations in the Rate of Sand Movement Over a Sand Surface. The Minimum Size of Sand Dunes.- 10 The Influence of Vegetation.- 11 The Prevailing Sand-Driving Wind. The Long-Period Sand Movement as a Vector-Sum Of Movements During ‘ Strong ’ and ‘ Gentle ’ Winds.- 13 Sand Shadows and Sand Drifts. General Factors on Which Dune Shape Depends.- 1 Types of Sand Accumulation.- 2 Sand Shadows Behind Obstacles.- 3 Sand Drifts Between Obstacles.- 4 Sand Drifts Below Cliffs.- 5 The Influence of Cross-Winds on Dune Shape as Suggested By the Behaviour of Under-Cliff Sand Drifts.- 6 True Dunes as Deformable Wind Obstacles. General Considerations Affecting the Shape they Assume. The Longitudinal Cross-Section. (a) Gentle Winds; (b) Strong Winds; (c) Long-Period Mean Wind—Relation Between Rate of Sand Flow Over Dune, and The Shape of the Dune; (d) Formation of The Slip- Face; (e) The Effect of The Slip-Face on Dune Formation; The Rate of Advance.- 7 The Instability of a Long Transverse Dune.- 14 The Barchan Dune.- 1 The Crescentic or Barchan Dune. The Observed Wind Distribution Over It.- 2 The Shape and Position of the Slip-Face.- 3 The Minimum Height of The Slip-Face.- 4 The Trailing Wing-Tips. The Lateral Sand Movement Away from the Axis. Sand Leaves the Dune in Two Streams.- 5 Non-Uniform Distribution of Sand Supply. Complex Barchans. Fuljis.- 6 The Size and Height of Barchans, and Their Rate of Advance.- 7 The Effect of Winds from the Reverse Direction.- 8 Barchan Belts.- 9 Ice Barchans.- 15 The Longitudinal or Seif Dune. The Whaleback.- 1 The Transition from the Barchan to the Seif Dune Forms.- 2 The Seif Shape Varies According to the Long-Period Wind Régime.- 3 Movement of the Seif Dune.- 4 Growth of a Seif Dune. Grading of Sand Over the Cross-Section. Formation of a Plinth of Coarse Sand at the Base.- 5 Seif Dune Chains. Height, Length, Repetition Distance, and Alignment.- 6 Whalebacks or Sand Levees. The Egyptian Sand Sea.- 7 The Transverse Repetition Distance Between Seif Dune Chains.- 8 The Formation of Barchans in the Longitudinal Troughs between Multiple Seif Chains, where the Effect of Cross-Winds is Excluded.- 9 Dune Systems and Wind Direction.- 16 The Internal Structure of Sand Deposits.- 1 Consistency of the Deposited Material.- 2 Laminar Structure Revealed by the Differential Rate of Water Seepage through Layers of Varying Grain Grading.- 3 Structure of Beds Deposited by Accretion. Their Firmness. Similarity to Wind-Packed Snow.- 4 Grading Changes due to Shearing. Structure of an Avalanched Encroachment Deposit.- 5 Structure of a Barchan Dune.- 6 Structure of Seifs and Whalebacks.- 7 Structure and Growth of Sand Sheets.- 8 Water Retention and Vegetation Dependent on the Internal Structure.- 17 ‘ Singing Sands ’.- 1 ‘ Whistling ’ Beach Sand.- 2 The ‘ Song ’ or Booming of Desert Sands.- 3 What Controls the Note, and How Does the Sound Arise ?.