Erosion by wind and developed features – Abrasion and Deflation

The earth is surrounded by an envelope of gases called the atmosphere. The movement of the atmosphere in a direction parallel to the earth surface is wind.i.e. the air in motion is called wind whereas the vertical movements of the atmosphere are termed as air currents.

Erosion by wind and developed features:

Wind erosion is generally caused by two erosion processes:

i) Deflation 

ii) Abrasion.

Deflation:

Deflation is the process of simply removing the loose sand and dust sized particles from as area, by fast moving winds. Wind deflation can successfully operate in comparatively dry regions with little or no rainfall and where the mantle is unprotected due to absence of vegetation.

Such a removal of loose fine particles may at certain places leave a denuded surface consisting mostly of hard rocks or coarse materials like gravel and is called lag gravel. This gravel layer prevents further deflation.

Abrasion:

The wind loaded with such particles attains a considerable erosive power which helps a considerable er4osive power which helps in eroding the rock surfaces by rubbing and grinding actions and produce many changes. This type of wind erosion is known as abrasion.

Vertical column of rocks are thus more readily worm out towards their lower portions and a result pedestal rocks are formed which wider tops have supported on comparatively narrower bases. Such type of rock formations is called Pedestal or Mushroom rocks.

Transportation by wind:

The total sediment load carried by a wind can be divided into two parts. a) Bed load

b)  Suspended load

The larger and heavier particles such as sands or gravels, which are moved by the winds but not lifted more than 30 to 60 cm of the earth surface constitute the bed load. Whereas the finer clay

or dust particles which are lifted by the moving winds by a distance of hundreds of meters above the earths surface constitute the suspended load.

Deposition of sediment by wind and the developed features:

The sediments get dropped and deposited forming what are known as Aeolian deposits. There are two types of Aeolian deposits;

a) Sand dunes b) Loess

Sand dunes:

Sand dunes are huge heaps of sand formed by the natural deposition of wind blown sand sometimes of characteristics and recognizable shape. Such deposits are often found to migrate from one place to another due to change in the direction and velocity of wind.

The active dunes can be divided into three types:

a) Barchans or Crescent shaped dunes b) Transverse dunes

c)  Longitudinal dunes

Barchans:

These dunes that look like a new moon in plan are of most common occurrence. They are triangular in section with the steep side facing away from the wind direction and inclined

at an angle of about 30⁰ to 33⁰ to the horizontal.

The gently sloping side lies on the windward side, and makes an angle of about 10 to 15⁰

with the horizontal. They may have variable sizes, with a generally maximum height of about 335 meters and horn to horn width of say 350 meters.

Transverse Dunes:

A transverse dune is similar to a barchans in section but in plan it is not curved like barchans such that its longer axis is broadly transverse to the direction of the prevailing winds.

Longitudinal dunes:

Longitudinal dunes are the elongated ridges of sand with their longer axis broadly

parallel to the direction of the prevailing wind. When seen in the side view they will appear to be triangular on an average. They may be 3 m height and 200 m long.

Loess:

The finest particles of dust travelling in suspension with the wind are transported to a considerable distance. When dropped down under favorable conditions these have been found to accumulate in the different constituents the form of paper-thin laminae, which have aggregated together to form a massive deposit known as Loess.

Engineering considerations:

In general no site is selected for any type of important work on the moving dunes . Because such dunes are always a source of trouble to an engineer. It has been experienced that sometimes the moving dunes damage certain important works. But if an engineer is compelled to select such a site, special methods should be adopted to check the motion of the moving dunes. For ex:

Either to construct wind breaks or growing vegetation on the surrounding areas.