Barchan

Also called: Barchan dune, Crescent-shaped dune · Topic: Landforms and their Evolution: Rivers, Groundwater, Glaciers, Waves and Wind · NCERT: Class 7, Ch 1 "Geographical Diversity of India"; Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 2 "Structure and Physiography"; Class 11, Ch 6 "Landforms and their Evolution"

Meaning

A barchan is a crescent-shaped (half-moon-shaped) sand dune whose two horns point downwind. It forms where a steady, moderate wind blows from one direction over flat, even ground that has limited sand.

It is the dune type UPSC tests most often. Its shape shows the wind direction, and it is a common trap when set against the parabolic dune. In India it is linked to the Thar Desert around Jaisalmer (Rajasthan).

Explanation

How a barchan forms (cause → process → result)

• What a dune needs:
• Loose sand, from dry riverbeds, weathered rock or beaches.
• A steady wind that blows mostly from one direction.

• An obstacle, such as a bush, rock or small bump.

• How the heap starts:

• The wind hits the obstacle and slows down.
• Slower wind cannot carry as much sand, so it drops sand behind the obstacle.

• The small heap now becomes an obstacle itself. More sand collects and the dune grows.

• How the sand moves: most sand moves by saltation (grains make short hops and bounces, usually within about 1 cm of the ground) [2].

• Why it becomes a crescent:
• Sand is limited, so the dune is a single heap on bare, flat ground.
• The edges of the heap are thinner and lower than the middle, so the wind moves them forward faster.

• The two edges get ahead of the centre and form horns that point downwind.

• Result: sand keeps moving from the windward side to the leeward side. So the whole barchan creeps forward downwind.

Parts of a barchan

Part	What it looks like
Windward side (the side facing the wind)	The outer, convex (bulging) side. It has a gentle slope, and sand is pushed up it.
Slip face (on the leeward, or sheltered, side)	The steep slope inside the curve. Sand slides down it.
Horns	The two tips of the crescent. They point downwind.

Conditions it needs, and how it changes

• Wind: steady and moderate, from one direction.
• Ground: flat and even.
• Sand supply: limited.
• If conditions change, the dune changes:
• Wind changes direction → one wing grows very long and high, forming a seif ("sword" in Arabic) [2].
• Plants cover the sand → the arms are held back and the dune turns into a parabolic dune (a reversed barchan).

• Plenty of sand → dunes merge (coalesce), lose their separate shapes and form a sand sea.

• Moving or fixed: a bare barchan is a shifting dune. It can bury fields, roads and railway tracks. Grasses, shrubs and trees can turn it into a fixed (stabilised) dune.

• Classification: the US Geological Survey lists the barchan as one of six main dune types: transverse, oblique, longitudinal, barchan, star and parabolic [6].

Where it occurs

• India: the Thar Desert (Great Indian Desert), especially around Jaisalmer in western Rajasthan. The Thar covers 196,150 sq km [3].
• World:
• Sahara Desert (North Africa).

• Peru coastal desert (west coast of South America).

• Wider setting: dunes are also found in the Arabian (Rub' al Khali), Namib and Australian deserts, and on low sandy coasts. These are general dune regions, not barchan sites named in the study note.

Don't confuse with

• Parabolic dune: its arms point upwind because plants hold them back. It is a reversed barchan. A barchan's horns point downwind on bare sand.
• Seif: it has only one wing, which grows very long and high, and it forms when the wind changes direction. A barchan has two horns and forms under a steady wind.
• Longitudinal dune: a long, low ridge parallel to the wind. It also forms with little sand and a steady wind, but it is a ridge, not a crescent.
• Transverse dune: a ridge at right angles to the wind. It forms where the sand source is a long feature, such as a beach or riverbed, lying across the wind. A barchan forms where sand is limited.

Prelims Hooks

• Barchan: horns point DOWNWIND. Parabolic dune: arms point UPWIND (plants hold them). This is the classic "which direction" trap.
• Barchan + Jaisalmer (Thar, Rajasthan) is a standard feature–location match. Barchans are also seen in the Sahara and the Peru coastal desert.
• Barchan needs a steady, moderate wind from one direction, flat and even ground, and limited sand.
• Barchan slopes: the windward side is gentle and convex; the slip face is steep and lies inside the curve.
• Seif = a barchan with one wing, formed by changing wind. "Seif" is Arabic for "sword" [2].
• Aeolian deposits (laid down by wind), such as dunes, are well sorted (grains of about the same size). Glacial till is unsorted.

Mains Points

• Dune shape reflects local climate (GS-I):
• One steady wind with little sand → barchan, or a longitudinal dune.
• Changing wind → seif. Winds from many directions → star dune.
• Plant cover → parabolic dune. Plenty of sand → a merged sand sea.

• So the shape of a barchan tells you the main wind direction and shows that sand is scarce.

• Shifting barchans are a hazard in western Rajasthan (GS-III):

• Bare dunes creep downwind and bury farmland, the Indira Gandhi Canal, roads and railways.

• Removing plants re-activates dunes and speeds up desertification (good dryland slowly turning desert-like). Wind erosion is one of the eight land degradation processes mapped in the ISRO-SAC Atlas, which puts India's degraded land at 97.84 Mha in 2018-19 [4].

• Fixing dunes works (GS-III, policy):

• CAZRI, Jodhpur (started in 1952) researches shelterbelts (rows of trees planted across the wind) and ways to fix drifting sand [5]. It recommended roadside and shelterbelt planting [3].
• Other methods are fencing out grazing animals, brushwood micro-windbreaks, and planting Calligonum, Prosopis and Acacia.
• Result: about 4 lakh ha of dunes stabilised, and dune area down by about 12% since 1980 [3]. This supports India's goals under the UNCCD (UN Convention to Combat Desertification).

Related concepts

• Saltation
• Sand dune
• Parabolic dune
• Seif
• Longitudinal dune
• Transverse dune

Read more

• Wind deposition: sand dunes