Wind and Erosion

A lesser known problemGeo 18b

Source: Wind in the Environment, 1994

“Assess to what extent wind erosion is Assess to what extent wind erosion is a natural phenomenon, but the a natural phenomenon, but the

surfaces it acts upon may be made surfaces it acts upon may be made susceptible to active wind shaping and susceptible to active wind shaping and

transport by human actionstransport by human actions”

Definition: Wind erosion is the process of detachment,

transportation, and deposition of soil material by wind

The Mechanisms of Wind Erosion

There are 3 major processes involved in wind erosion. These are:

•Saltation•Suspension•Creep

Suspension is the process immediately after saltation where turbulent airflow keeps the (fine) sediment in the air. These grains travelling by suspension may remain airborne until rain washes them out of the air.

Creep occurs when larger grains of material cannot be picked up and are pushed downwind by the turbulence caused in suspension..

Soil grain motion or grain transport depend on particle size and rely on interactions between the grains and the passing airflow

Saltation involves small particles being lifted into the air by strong wind currents then blown back onto the surface.

Abrasion is the direct result of wind erosion. It is the process by which landforms in arid environments are shaped. Abrasion is the wearing away of (relatively) solid rocks by suspended particles being thrown against a surface at high velocity (aka sand blasting).

Here man copies nature by Here man copies nature by using wind abrasion in a using wind abrasion in a technique called technique called sandblastingsandblasting

Types of Erosion Damage

Loss of Soil – General Effects

Textural Change – Movement of fertile soilNutrient Losses – Loss of fertility

Productivity Losses – Nutrients and water holding capacities

Air Pollution – Discomforts caused.

Deposition - Sedimentation

Wind Erosion

• Wind erodes dry land much more effectively than it does moist land.

• As the wind erodes land it carries rock particles along with it, mostly sand, silt, and clay.

• Sand material is moved along by a number of jumps and bounces, much how a pebble is moved along the bottom of a stream bed.

• The grains do not rise higher than about 1 meter, and they move in the same direction the wind is blowing.

• Dust particles (silt and clay) can be carried along great distances and at greater heights than sand particles

Wind Erosion: Experience in the Great Plains

Wind Erosion: Experience in the Great Plains

• What is the Dust Bowl?What is the Dust Bowl?- The Dust Bowl extends from south-

western Great Plains including parts of new Mexico and Colorado. Its an area well known for the severe wind erosion that affects it from time to time.

• What Caused it?What Caused it?- It was an interaction between natural

and human factors but primarily due to human induced factors.

Effects of wind erosion

• Abrasion is the weathering of rock particles by the impact of other rock particles.

• In areas where there are strong, steady winds, large amounts of loose sand, and relatively soft rocks, abrasion causes a great amount of erosion.

• Pebbles and small stones exposed to wind abrasion show surfaces that are flattened and polished on two or three sides.

• Rocks smoothed this way are called ventifacts.

• Particles that have been moved by the wind are well rounded by the repeated impact of grain against grain, and when examined with a magnifier, their surfaces have a frosted appearance.

• Outcrops of rocks in arid regions undergo the greatest amount of erosion close to their base, because particles transported by the wind remain close to the ground.

Deflation

• Wind removes top layer of fine, dry particles, leaving larger rocks

• This is known as desert pavement

Deflation hollow

Dune Formation

• Need sand• Need wind• That’s it!

Sand

• The majority of the sand you'll see in temperate regions is composed of minerals eroded from bedrock. It is generally a combination of many mineral types. The primary component of rock-derived sand is silica, what both quartz and glass are made of, and is very resistant to erosion. It remains after other minerals have been broken down mechanically and altered chemically by their environments.

Sand

• Another mineral can also be found in sand - calcium carbonate, or CaCO3, in higher percentages the closer to the equator the sample comes from. Calcium carbonate is a biotic by-product - it is grown as a shell or skeleton by marine organisms suck as molluscs and corals. Many other minerals are also present in beach sand; among these black sand and green sand

Dunes

• Typically dunes have a low slope face, called the windward face, that faces the wind direction and a steeper face, called the slip face, which points downwind, as shown below. Dunes move by wind eroding sand from the windward face by pushing it and bouncing the sand along the windward face until it reaches the top of the dune. It is deposited along the slip face, which in effect moves the slip face forward in the direction of the wind

E. Star E. Star dunedune

Types of Dunes• Barchan dunes are crescent-

shaped dunes that are concave downwind, as shown below. Barchan dunes form in places where there is limited sand and a constant wind direction

Types of Dunes

• The parabolic dune is a crescent shaped dune that is concave upwind and forms in areas in which there is some vegetation and a good supply of sand.

Types of Dunes• The longitudinal dune is a linear dune

that is parallel to the direction of the wind and forms in areas in which the wind direction is not constant and the supply of sand is moderate to good. Longitudinal dunes can be kilometers in length, and the formation of them is not entirely understood. They are formed in areas in which there is a desert pavement and variable winds.

Types of Dunes• Transverse dunes are linear dunes

that are perpendicular to the direction of the wind and are not as long as longitudinal dunes. Transverse dunes form in areas with abundant sand and little vegetation.

Star Dunes• Star dunes, which are one of the

characteristics of The Namib desert, result where winds converge from several directions. Buffeted by the wind from all sides, this dune remains largely immobile. Its arms have sharp ridges, sometimes over 250 feet in height, that radiate from a central point.

Dune Migration• Dunes migrate by a eroding from the upwind

side and depositing on the downwind side, as illustrated below. Strong winds pick up sand grains from the slope of the dune (upwind or windward side) and blow them across the crest, where they fall onto the slip face. Protected from the wind, they accumulate on the upper slip face to a low, broad mound. Eventually the cornice deposit grows too steep and its sand begins to flow as an avalanche down the slipface. The process repeats over and over causing the dune to advance with the wind.

Loess

• Loess is a geologic term that refers to deposits of silt (sediment with particles 2-64 microns in diameter) that have been laid down by wind action (aeolian activity to geologists).

• Extensive, thick loess deposits generally formed in areas bordering large, continental glaciers. Large volumes of meltwater flowed from the edges of these glaciers during the summer. This meltwater carried large amounts of sediments that formed as the glacier ground the bedrock over which it moved.

• Much of this sediment was silt-sized material known as rock flour. During the winter, when the glacier did not melt, the area where the water flowed was primarily dry. The winter winds would pick up the rock flour from these dry areas and carry it long distances in huge dust storms.

• When the wind slowed, the silt would fall out and blanket the area. Frequently the resulting loess deposits are several meters thick (tens of feet). Because the source of the silt is the outwash from the glaciers, loess deposits are frequently most extensive and thickest downwind from large river valleys. Puget Sound glaciation led to the loessal deposits we know as the Palouse Hills.

The The PalousePalouse

Deposits Deposits were often were often very deep very deep and and obviously obviously well spread well spread outout

What factors control wind erosion?1. Heavy grazing of the original grass 1. Heavy grazing of the original grass

cover:cover:• Decreased the residual organic matter

present on the ground.2. 2. Cultivation:Cultivation:• Broke the soil aggregates of the land

increasing potential of wind erosion.3. Successive planting & harvesting 3. Successive planting & harvesting

of crops:of crops:• Affected soil moisture and left the

ground unprotected by vegetation.

Controlling wind erosion on agricultural land.

Plant cover to reduce wind speed, protecting soil.

Protecting Summer fallow fields.Livestock management to prevent overgrazing.Shelterbeds, Tree lines: reduce wind speed, trap soil moisture.

Maintain crop residue cover to protect Irrigated fields.

Emergency Control using manure and straw

Need for erosion control: