After reading this article you will learn about:- 1. Concept of Sand Dunes 2. Processes for Transportation of Sand Dunes 3. Extent 4. Types 5. Morphology 6. Sand Dunes System 7. Vegetation 8. Origin 9. Management 10. Utilization.
- Concept of Sand Dunes
- Processes for Transportation of Sand Dunes
- Extent of Sand Dunes
- Types of Sand Dunes
- Morphology of Sand Dunes
- Sand Dunes System
- Vegetation of Sand Dunes
- Origin of Sand Dunes
- Management of Sand Dunes
- Utilization of Sand Dunes
1. Concept of Sand Dunes:
The sand dunes are the most spectacular features of the hot Indian arid zone. The sand dunes are of different shape, size and magnitude and their intensity of occurrence increases from east to west. The sand dunes are stabilized and active and they create serious environmental problems.
The stabilized sand dunes due to intense biotic activities are disturbed and the sands from these dunes are transported by prevailing south west winds and are deposited on the adjoining cultivated fields which affect the production of food and fodder.
The active sand dunes and sheets cause more severe problems to the cultivated fields, railway tracks, roads and permanent civil and defence structures than those of stabilized sand dunes. Realising the seriousness of the environmental problems caused by the dune dynamics were carried out by Panday (1964), Vats (1976), Singh (1977) and Allchin and Goudie (1971).
2. Processes for Transportation of Sand Dunes:
The sands are transported by following three processes:
In this process the sediment particles below 0.06 mm diameter (dust) are generally carried by flow of air to long distances and are kept in suspension for longer period and ultimately dropped on the ground.
The movement of the particles by intermittence leaps and bounds is known as saltation. Fluid threshold velocity helps to set the sand grains into motion. Due to saltation, the sand grains are initially rolled along the surface in turbulent air stream, and after rolling a few meters, the bounding movement starts. Sand grains are lifted up into the airstreams. It is most important process for transporting the sand grains. About 90 per cent bulk transport of sand is due to saltation.
iii. Surface Creep:
The sand particles about 0.5 mm diameter are moved by the surface creep. In this process, the particles move along the ice and are deposited into various forms depending upon the nature of the surface topography.
3. Extent of Sand Dunes in Rajasthan:
Out of a total area of 342,000 (approximately) sq. km about 63 per cent is covered with blown sand of varying degrees. The sand dune and large hummocks are concentrated in the western section of arid zone in the districts of Barmer, Jaisalmer and Bikaner.
The degree of extend of sand dunes as calculated from the Survey of India toposheels in given as under:
In the districts of Barmer and Jaisalmer, over 80% of the area is covered with sand dunes. The area covered by dunes from 40 to 60% lies largely in the Barmer, Bikaner, Mohangarh tehsil of Jaisalmer, Suratgaili and Anupgarh tehsils of Sriganganagar and Bikaner tehsil. The degree and extent of sand dunes decreases towards east and north in the desert region.
4. Types of Sand Dunes:
On the basis of particle composition the dunes are of two types:
i. Sand dune:
The dune composed of sediment particles having the diameter of 0.06 to 2.0 mm i.e., sand is called Sand dune e.g., western arid Rajasthan.
ii. Gypsum dune:
The dune composed of gypsum particles of sand size is called gypsum or calcareous or time dune e.g. New Mexico and Death valley California.
One the basis of origin and shape the sand dunes are of following types:
a. Obstacle sand dunes:
The dunes formed due to abstraction in the path of sand laden winds. These are of two types,
(1) Major Obstacle sand dunes.
(2) Minor Obstacle sand dunes or Shrub – Coppice.
Fig. 16.1. Types of Sand Dunes
b. Barchan sand dunes:
The bare hail (Horse shoe) sand dunes are crescent shaped with a steep slope on the leeward concave side sand a relatively gentle slope on the windward convex side. These commonly occur warms” or “trains” and may reach heights of more than 20 metres.
c. Transverse sand dunes:
Transverse sand dunes are so named because they are elongated transverse to the direction of the effective sand moving winds. They generally occur in series of sub-parallel, asymmetric ridges, with an overall wavelike appearance. These are formed by winds that are essentially constant in direction.
d. Longitudinal sand dunes:
Longitudinal sand dunes originally wen so named for their supposed parallelism to prevailing winds, but ii now appears that they are formed by alternate convergent winds. They appeal as linear, more or less symmetrical sand ridges, much higher in proportion to width than the transverse type of dune. Ridge crests are sharp in profile and show minor crenulations lengthwise.
e. Parabolic sand dunes:
The long scoop shaped hollows or parabolas sand dunes with flanks tapering are called parabolic sand dunes. Compound or coalesced sand dunes are those in which many similar elemental forms are crowded or joined together to made up or to mantle a large dune mass of similar form and orientation, e.g., compound barchanminor crowded barchans. Superimposed on a major barchans form or coalesced parabolic dunes:
Complex sand dunes are sand masses made up of components dissimilar in form and/or discordant in their relations to one another or to the major form. This group includes a great variety of pyramidal, peaked, ridged, pitted, branching, radiating and intersecting assemblages.
The different forms of dunes have been caused by air streams which sweep across the arid zone predominantly from southwest to northwest. The dunes are subject to continuing displacement or modification, either progressive or retrogressive, as long as strong winds blow over bare sand.
5. Morphology of the Sand Dunes:
The winds are real sculptors that form the dunes. The orientation and the morphology of the dunes are directly related to the nature of the winds which have a free play on the vast structures of sands. The currents of the wind models and remodels the shape and size of dunes and determine their location.
I. Obstacle sand dunes:
(a) Major Obstacle sand dunes:
The dunes were formed on the wind ward and/or leeward sides of the isolated and continuous chains of the hills due to the obstruction in the path of sand laden winds during the prolonged arid phase and they are- known as wind ward and leeward obstacle dunes. The isolated hills form one obstacle dune each but the broad hills in chains create several obstacle dunes.
The obstacle dunes are of 10 to 40 m and 8 to 15 m in height and their sands are compact and highly calcareous. These dunes are well cemented and static. The mean diameter of their sand grains is 0.14 mm.
The obstacle dunes occur in Barmer, Jaislamer, Nagaur, Jalore and Jodhpur districts of Rajasthan desert.
(b) Minor obstacle sand dunes or shrub-coppice:
These dunes were formed on the wind ward and leeward sides of the minor obstacles like shrubs-Calligonum polygonoides, Leptadenia pyrotechnica and grasses like Lasiurus sindicus, Panicum turgidum. In other arid zone of the world, the small shrub-coppice dunes are known as nebkha and large one as rebdon.
The earlier formed dunes are old, stabilized, degraded and their sands are compact and calcareous. But recently formed dunes are active, their sands are loose and non-calcareous. They do not have any definite orientation and lie quite close to each other.
The fence line hummocks of 2 to 5 m in height occur at 120 to 250 m and 250 to 500 m apart from each other. The length and width of these fence line hummocks are 300 o 2000 m and 250 to 500 and respectively.
These dunes are widely distributed almost in all parts of the arid zone. They occur in the sandy undulating inter-dune and aggraded older alluvial plains and sandy buried pediments.
II. Barchan sand dunes:
The barchans dunes were formed by unidirectional uniform winds of moderate velocity on the hard surfaces with scanty vegetation and limited sand supply. The prevailing winds deposited the sand on the bare surfaces and they continued to deposit the sand in the lee of the crest till the angle of repose for loose sand was formed and the sand was moved down wind and created the steep face.
Due to the absence of vegetation, two elongated ridges on either side of the steep face advanced downwind up to a few kilometers and the crescent shaped barchans dune with concave leeward side and convex windward side was formed. These are generally formed on bare, surface mostly near the settlements.
The barchans sand dunes are active, devoid of vegetation and have non-calcareous, loose and single grained sands. The heights of the dunes vary from 2 to 5 m, 4 to 10 m or and 6 to 15 m. There are 5 to 15 dunes in the dune chains and the average spacing is 100 m in southwest and 300 m in north west.
The dunes are concentrated in Jodhpur, Bikaner and Churu districts.
III. Transverse sand dunes:
These sand dunes were originated due to prodigious sand cover on the ground, abundance sand supply, absence of vegetation, and presistant and widespread wind action under arid climatic conditions.
These dunes have the wave like ridges separated by the trough like I’m rows at right angles to the prevailing winds and the transverse dunes are formed Morphology. These dunes are of 5 to 10 m and 10 to 40 m in height and lie 500 to 1000 m apart from each other. The average length and width of these dunes are 750 m and 150 m respectively.
These occur in the south eastern part of Jodhpur district and Pugal area of Bikaner district (Fig. 16.2).
IV. Longitudinal sand dunes:
The longitudinal dunes had originated due to the strong prevailing winds during the prolonged arid phase of Pre-Holocene period. The longitudinal dunes have not resulted from the barchans dunes but they are the outcome of the parabolic dunes.
These dunes are also stabilized, well vegetated and sands are compact. They are 6 to 15 m and 15 to 60 m in height. These dunes are at a distance of 600 to 1200 m from each other. The average values of the length and width of these dunes are 3000 m and 260 m respectively. The dune sand are mixed with lime nodules of 2 to 25 mm length and they are highly calcareous and compact.
These dunes occur in Phalodi, Jodhpur and Shergarh tehsils of Jodhpur district and also in the western parts of Jaisalmer and Barmer districts (Fig. 16.2).
V. Parabolic sand dunes:
The sands were removed by the erosional process and seated the long scope-shaped hollows or parabolas with flanks tapering v to several kilometers towards the gentle windward slopes. The sands were subsequently re-deposited on the lee of the parabolas by deposional process, created the steep leeward slopes and the parabolic dunes are formed.
These dunes are steep. Parabolic dunes of 10 to 25 m heights are well stabilized and vegetated. The sands are calcareous and compact.
These are largely concentrated in Jodhpur, Barmer, Jaisalmer, Bikaner, Nagaur, Jalore, Shri Ganganagar, Churu and Jhunjhunu districts .of the Rajasthan arid zone (Fig. 16.2).
6. Sand Dunes System:
In Indian arid zone there are two types of sand dune systems viz.,
i. Old sand dune system:
The dunes of old system were formed by intensive dune building activities during the earlier prolonged arid climatic phases. These dunes are of high relief, well stabilized, vegetated and cemented. The obstacle, transverse, longitudinal and parabolic dunes are of high relief and maturity and belong to old sand dune system.
ii. New Sand dunes system:
The sand dunes of new system have been formed in the Recent arid climatic phase, due to the renewed Aeolian activities and are still in evolutionary stage. The sand dunes of this system are barchans and shrub-coppice. These dunes are active and smaller in size.
The sands are loose and calcareous. The sand dunes of this system are the greatest menace to the agricultural fields, settlements, roads, railway lines and other permanent structures. These dunes, therefore, need immediate attention for their rehabilitation.
7. Vegetation of Sand Dune:
The vegetation of sand dunes is poor, scanty, xerophytic, widely dispersed, leaving large areas bare. The stabilized types of sand dunes support two types of vegetation.
1. Psammophytic scrub desert.
2. Mixed xeromorphic thorn forest.
8. Origin of Dune Sand:
The origin of the dune sand has been a controversial subject. Wadia 1961 has reported that the dune sand of Indian Arid Zone has been transported by southwest winds from the rum of Kutch and in part from lower Indus, Ghosh (1952) found that dune sand of Rajasthan rather fine grained and rounded than beach sand.
The roundness in the sand grain has been caused during wind transportation by abrasion such as bouncing off obstructions and wearing off their irregularities and not due to the capacity of winds to pick up the rounded particles as reported by Pettijohn and Lundahl (1943).
The analysis of the size and shape of dune sand grains from different parts of the arid zone has indicated that the dune sand has originated due to the reworking of the lluvial sediments by the Aeolian action under prolonged dry climatic conditions.
9. Management of Sand Dunes:
The sand dunes could be stabilized by adopting the following phytoreclamation techniques:
i. Protection against biotic interference by fencing,
ii. Fixing brushwood barriers on the windward side of dunes in parallel lines;
iii. On stabilized dunes plantation of fast growing naturally and highly adapted species. Like Calligonum polygonoides, Acacia Senegal, Maytenus emarginatus, Prosopis cineraria, Panicum turgidum, Lasiurus sindicus and Zizyphus nummularia in a phased manner.
Highly unstable crests and upper flanks of the dunes should be managed by planting of suitable grass species like Panicumturgidum, Lasiurus sindicus, Cenchrus ciliaris and Aristida funiculata.
The plantation bricks (earthen bricks of 30 cm height with a cross section of 10 cm2 at top and 15 cm2 at bottom. These bricks are prepared from a mixture of sand, clay and farm yard (F.Y.) manure mixed in equal proportion) should be used for sowing of seeds and transplanting of seedlings.
10. Utilization of Sand Dunes:
Sand dunes are not universally sandy wastes. These are often cultivated during rainy season. The active sand dunes are often wastelands but the lower and middle flanks of these dunes are cultivated in years of very good rainfall, particularly in rainfall zone of over 200 mm.
Upper flank and crest of the dunes serve as grazing ground of the livestock. Hence in sand dunes, these land use units, viz., cultivated or cultivable (including short and long fallows), sandy waste and grazing grounds can be observed.