11.1 Wind erosion, transport, and deposition

Wind Erosion Mechanisms

Primary Erosion Processes

Wind erosion works through three distinct mechanisms, each removing and breaking down sediment in a different way:

• Abrasion wears down rock surfaces through the impact of wind-blown particles. Think of it as natural sandblasting: sand grains carried by wind strike exposed rock, gradually sculpting and polishing it over time.
• Deflation is the direct removal of loose, fine-grained particles from the surface by wind. As the wind sweeps away unprotected sediment, the land surface lowers. Over time, this can leave behind a desert pavement, a lag surface of coarser pebbles and gravel too heavy for the wind to move.
• Attrition breaks particles down during transport. As grains collide with each other mid-flight and on impact, they chip and fracture into smaller, more rounded pieces. Angular quartz grains, for instance, become progressively smoother and more spherical through repeated collisions.

Factors Influencing Erosion Intensity

Wind velocity is the most direct control on erosion intensity. Higher speeds can entrain and transport larger particles. The relationship isn't linear, though: the force wind exerts on a grain scales with the square of velocity, so small increases in wind speed produce disproportionately greater erosion.

Surface roughness shapes the near-surface wind profile. Smooth surfaces like flat desert plains allow wind to maintain speed close to the ground, making them highly susceptible to erosion. Rough surfaces with boulders, ridges, or other obstacles create turbulence that disrupts the flow and can reduce net erosion.

Soil moisture significantly limits wind erosion. Capillary forces in moist soils bind particles together, raising the threshold velocity needed to dislodge them. Dry, loose soils erode far more easily.

Vegetation cover protects against erosion in two ways:

• It reduces wind velocity at the surface by increasing aerodynamic roughness
• Root systems physically bind soil particles together
• Examples range from grass cover on prairies to scattered shrubs in semi-arid regions

Wind Transport of Sediment

Modes of Sediment Transport

Wind moves sediment in three modes, distinguished mainly by particle size and how the grains travel:

1. Suspension carries the finest particles (less than ~0.1 mm diameter) high into the atmosphere within turbulent eddies. These grains can travel enormous distances. Saharan dust storms, for example, regularly transport particles across the Atlantic Ocean to the Caribbean and Amazon Basin.

2. Saltation is the dominant mode of aeolian transport, accounting for roughly 75% of total sediment flux in most systems. Sand-sized particles (0.1–0.5 mm) are lifted briefly by wind, travel in short parabolic hops, and strike the surface on landing. Each impact can eject other grains, creating a chain reaction that sustains the process.

3. Creep (or surface creep) moves the largest particles (greater than ~0.5 mm) by rolling or sliding them along the ground. These grains are too heavy for the wind to lift directly. Instead, they're pushed forward by the impact of saltating grains hitting them from above.

Transport Characteristics and Dynamics

The boundaries between these three modes aren't rigid. Whether a given particle travels in suspension, saltation, or creep depends on its size, the current wind velocity, and turbulence intensity. The Hjulström curve, originally developed for fluvial systems, has been adapted to illustrate the critical wind velocities needed to erode, transport, and deposit particles of different sizes.

Two related concepts describe what the wind can carry:

• Competence refers to the maximum particle size the wind can transport at a given velocity
• Capacity refers to the total amount of sediment the wind can carry

Both increase with wind velocity. Bagnold's equation formalizes this by relating sediment flux ($q$) to wind shear velocity ($u_*$), showing that flux scales roughly with the cube of shear velocity.

The vertical distribution of sediment concentration follows a logarithmic profile: concentrations are highest near the surface and decrease exponentially with height. Most saltation occurs within the first meter above the ground.

Aeolian Landform Formation

Sand Dune Formation and Types

Sand dunes form where wind velocity drops or where obstacles cause transported sediment to accumulate. The type of dune that develops depends on wind regime, sand supply, and local topography.

• Barchan dunes are crescent-shaped with horns pointing downwind. They form where sand supply is limited and wind blows from a consistent direction. Common in the Sahara and coastal Peru.
• Transverse dunes are long, linear ridges oriented perpendicular to the prevailing wind. They require abundant sand supply and a dominant wind direction. Found in the Namib Desert and parts of the Sahara.
• Star dunes are complex, pyramidal forms with multiple arms radiating from a central peak. They develop where winds blow from multiple directions and sand is plentiful. The Grand Erg Oriental of Algeria contains striking examples.

Other Aeolian Depositional and Erosional Features

• Loess deposits are extensive blankets of wind-blown silt that accumulate into thick, relatively homogeneous layers. China's Loess Plateau covers over 640,000 square kilometers and reaches thicknesses exceeding 300 meters in places, making it one of the most significant aeolian deposits on Earth.
• Ventifacts are rocks sculpted and polished by wind abrasion. A classic form is the dreikanter, a three-sided ventifact shaped by wind-driven sand striking the rock from a prevailing direction. The rock may be periodically reoriented (by undermining or settling), exposing new faces to abrasion.
• Yardangs are streamlined, elongated ridges carved from bedrock by sustained wind erosion. They form where bedrock is relatively soft and wind direction is consistent. The Lut Desert of Iran contains some of the most spectacular examples, with yardangs reaching tens of meters in height.

Grain Properties in Aeolian Processes

Influence of Grain Characteristics

Grain size is the primary control on how a particle behaves in wind. It determines both the transport mode and the threshold wind velocity needed for entrainment. The Udden-Wentworth scale classifies sediment sizes; aeolian processes primarily affect the range from clay through coarse sand.

Particle shape influences aerodynamic behavior in ways that aren't always intuitive. Spherical grains actually require higher wind velocities for entrainment than angular or platy particles, because flat or irregular shapes generate more aerodynamic lift. Shape is quantified using metrics like the flatness ratio and sphericity index.

Grain density affects the threshold shear velocity for movement. Denser particles need stronger winds. For comparison, quartz sand has a density of ~2.65 g/cm³, while volcanic ash ranges from ~2.0 to 2.4 g/cm³, meaning ash becomes airborne more readily at lower wind speeds.

Sediment Behavior and Analysis

The equivalent diameter concept accounts for variations in shape and density when predicting transport behavior. It converts irregularly shaped or variably dense particles into a hypothetical sphere that would behave the same way aerodynamically, allowing meaningful comparison across different sediment compositions.

Sorting is a hallmark of aeolian deposits. Wind is highly effective at separating particles by size, shape, and density during transport. This is why many desert sands are notably well-sorted compared to fluvial or glacial sediments.

The Shields parameter ($\theta$) relates the fluid shear stress acting on a particle to the gravitational force resisting motion. Originally developed for water flow, it has been adapted for aeolian systems to predict when particle motion begins. The critical Shields parameter varies with the particle Reynolds number, meaning the threshold for motion depends on both grain size and the flow conditions near the surface.