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INDIAN JOURNAL OF GEOMORPHOLOGY Volume 1 Number 2 Guly-December)1996 pp. 177-206
MORPHOLOGY AND EVOLUTION OF SAND DUNES IN THE THAR DESERT AS KEY TO SAND CONTROL MEASURES Amal Kar CentralArid Zone ResearchInstitute Jodhpur Abstract Sand control measures in any dune-covered area in the Thar desert are heavily dependent on the concept of vegetative propagation on the dunes. Mechanical and chemical control methods are not popular. The activities are often directed towards planting of trees and shrubs on the reactivated slopes of the high dunes, especially the downwind end of the dunes. While these practices have shown some results, it is more appropriate if the sites and the types of control are decided on the basis of a knowledge on the dune types, their morphology and history of development, their mobility, the air and sand flow patterns over and around them, the expected changes upon any interference to the sand body, as well as the environmental set up of the dunes, including vegetation status, land use practices, etc. A great variety of dunes exist in the Thar desert, which can be broadly grouped under the "old" and the "new" dunes. The new dunes, especially the barchans and the barchanoids, are most mobile and can at best be tackled by mechanical means.
The old dunes are naturally stabilized to a great extent, but have been reactivated in recent times due to popula::on pressure. Most stabilized aid dunes have been compartmentalised into private land holdings, and are put to dry farming practices. Restoration of natural vegetation on them is the best solution, but difficult to apply, unless such restoration adds to the economy of the land holders. Over-emphasis on tree and tall shrub plantation may not provide the best solutions, as the movement of saltating parJiiclesmay not be checked unless the basal plant cover is restored. The article discusses various aspects of dune morphology and dune dynamics and ther. makes a Illnited attempt to evaluate the sand control measures in the fight of those considerations.
INTRODUCTION Aeolian sand bodies in the desert often pose threat to human activities and man-made structures, burying settlements and canal networks, disrupting communication lines and affecting agricultural and other activities. Attempts are, therefore, made all over the world to control the
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AMAL KAR
menace of moving sand fhroqgh ,numerous mechanical and biological means, including plantation of suita'ble species. Since many of the menacing sand bodies are in the formdf sand dunes, the view of a sand dune conjures up in the mind of a ,common man an awa-inspiring vision of invading sand masses. Not all the dunes are, however, so menacing, unless their ecology is severely disturbed Many dunes are much stabilised and have a significantly low rate of movement, while some are so stable that these are caned the fossil dunes. There ,are, however, some dunes which have a higher rate of movement and pose the maximum threat to human activities. Therefore, for proper sand :control measures an understanding of the different sand dune types, their morphological set up and their genesis are a pre-requisite. This paper discusses the different dune types in the Thar desert, their possible genesis and their relevance to sand control measures.
SAND MOVING WIND AND SAND PARTICLES An understanding of the desert dune system requires some basic knowledge of the sand moving and the dune-forming winds, as well as of the sand particles prone to such movement. These are treated briefly and in turn. Sand 'Moving
Wind
Sar.d-Moving and dune forming winds are those which have sufficient velocity to move sand and create the dunes. Bagnold (1941) calculated the average impact threshold velocity of wind for dune sand as 16 Ian h-1 at 1 m height (or 4 m sec-l at 1 em height). The rate of sand movement varies approximately as the cube of the drag velocity. Thus, the sand transportation capacity per hour increases sharply as the wind velocity changes from slight to moderate or strong category. For example, the transportation by a 50 km h-1 wind for one hour is equivalent to that by a wind of 30 km h-1 for 14.3 hours and by a wind of 20 km h-1 for 611 hours (Mabbutt, 1977). Since the sand moving winds do not necessarily blow from all the compass directions, but from one or two specific directions and in some months of the year only, it is possible to narrow down the problems of sand movement to a few months and to find out the potential sand drift and the drift directions from wind records. In the Thar desert the aeolian activity is usually restricted to the period of strong summer south-west monsoon wind. Sand and dust raising winds start blowing from March onw:uds when the surface is sufficiently dry after the previous year's monsoon rainfall Guly -
MORPHOLOGY AND EVOLUTION:...
179
September)~-The mean maximum wind speed is reached at all the meteorological stations during June. May and July are also very windy. Since this is also the period when much of the ground flora is dry, the conditions are appropriate for aeolian activities. The wind dissipates with the arrival of monsoon rains and sand blowing activity ceases. The north-eastern wind of winter months is weak and is a poor agent for such activity. The spatial pattern of present day aeolian activity is reflected in the wind erosion index map that Was constructed (Kar, 1993)with the help of modified Chepil formula (Yaalon and Ganor, 1966) : 100 y3 C'= 2.9 (PE)2 where C' is climatic wind erosion index, V is mean wind speed at 10 m height, PE is the Thornthwaite's measure of precipitation effectiveness, and 2.9 is the annual average climatic index at Garden City, Kansas; values at other stations are expressed as a percentage of this Figure. A new scale of severity of wind erosion index was prepared to accommodate the observed patterns of sand mobility in the region. The values were classified into six groups, ranging from very low to extremely high (Table 1; Figure 1; Kar, 1993).
Grain Size The size and character of the particles are also important. The particles classified as sand range in size from 0.06 to 2.00 mm in diameter (4.0 to - 1.0 phi). Particles of smaller size are referred to as dust and have a platy structure with increased inter-particle cohesion. The sand particles are granular and cohesionless. Particles susceptible to movement by wind (or aeolian transportation) are the sand and the silt sized particles. With changing grain size and changing wind velocity the movement -: takes place as surface creep, saltation and suspension. Saltation accounts for about !J5percent of the bulk sand transport, consisting mostly of 0.15 to 0.25 mm size particles, and the suspension involves particles of smaller than 0.15 mm size. Much of the high level suspended load, moving out of the desert and settling along the desert margin and beyond as dust fall, is composed of particles smaller than 0.06 mm size.
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The Thar sand dunes have mean sand grain sizes of 0.15 mm (2.74 phi) to 0.20 mm (2.30 phi). Like in other deserts, quartz is the most common constituent, but feldspar and some heavy minerals are also found (Singh, 1977). The dominance of quartz is due to the resistance it provides to mechanical abrasion and chemical weathering. Nearer the rock outcrops many small and unaltered rock fragments are also noticed within the sand population. These are always related to the local geological set up. The other major source of sand is the alluvium in the pre-existing landforms. The earlier view that the Great Rann of Kachchh provided the sand has been proved wrong (Pandey et al., 1964). The average grain size characteristics of the major dune types in the desert are provided in Table 2. Plotting of the grain size sorting data against the mean grain size suggest that the sorting usually improves as the grain size decreases, although a wider scatter is noticed at 2.20 to 2.50 phi range. The dunes with stable crest (old dunes) have generally a finer grain size and better sorting than the dunes with mobile crest (new dunes and old dunes with reactivated crest). This latter category of dunes has a wider scatter of relationship between grain size and sorting (Figure 2). Table 1 Suggested Categories of Wind Erosion Index (March-July) For The Thar And Meteorological Stations in Them Wind erosion index (%)
Category
Station
480 and above
Extremely high
Jaisalmer, Chhor
Very high
Phalodi
- 479 60 - 119 120
- 59
High Moderate
Bikaner, Jodhpur, Pachpadra, Barmer, Hyderabad, Sukkur Khanpur
15-29
Low
Ganganagar, Chum, Nagaur, Jaipur, Badin
1-14
Very Lqw
Hisar, Sikar, Sambhar, Ajmer Erinpura, Bahawalpur
30
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MORPHOLOGY AND EVOLUTION.... Table 2 Average Grain Size Characteristics of The Crestal Sand of Thar Sand Dunes Type
Zone
Mean (phi)
Median (phi)
Sorting
Skewness Kurtosis
PARABOUC
South
2.82
2.78
0.360
+0.218
1.449
West
2.62
2.65
0.436
-0.128
1.381
Central
2.63
2.67
0.388
-0.270
1.573
East
2.60
2.64
0.333
-0.170
0.908
Western most
2.85
2.82
0.493
+0.130
1.983
West
2.57
2.62
0.486
-0.094
1.312
East
2.68
2.72
0.314
-0.212
1.185
West
2.57
2.62
0.388
-0.146
1.004
East
2.70
2.71
0.302
-0.133
0.180
SouthWest
2.33
2.57
0.811
-0.543
1.472
NorthWest
2.85
2.87
0.372
-0.085
1.327
West
2.54
2.57
0.402
-0.138
1.062
East
2.88
2.88
0.264
+0.140
1.529
ii) Parabolic form with networks
2.70
2.74
0.349
-0.182
1.086
iii)Longitudinal with transversals
2.56
2.60
0.602
-0.321
1.890
iv) Fish scali!
2.37
2.40
0.443
-0.121
1.740
BARCHAN
2.30
2.27
0.582
-0.231
0.993
BARCHANOID
2.25
2.31
0.524
-0.194
1.006
MEGABARCHANOID
2.35
2.42
0.548
-0.231
1.069
OBSTACLE
2.79
2.80
0.382
+0.242
1.535
LONGITUDINAL
TRANSVERSE
STAR
NETWORK i) Compound hooked
.... 00 N
THAR DESERT WIND EROSION
INDEX
a ..,
.. to. e ~.
'" t ..
~.
~ Ii
~ '" ..
~ ~
..
~ ~
!l 480 120
Extremely high Very high High
60 30 15 10 I
WIND EROSION INDEX(%>
UAVERAGE
ANNUAL
RAINFALL(mm)
I-IRESULTANT
Low Very low
Km/h
Flg.1
LEGEND
0
Moderote
WIND (JUNE)
t
~
l""
S
MEAN GRAIN SIZE vs. SORTING IN SAND DUNES OF THE THAR DESERT 1.4
a Transverse
. Parabolic '" Linear
1.2
.
. Mostly
1.0
III N.twork (W)'
Mobile
Crest Mostly
Stable
Crest
01)
- 0.8' c
.
.
(IJ
0.6
~
+
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+
IIIGL . lIIe+.111 ."-
a..+ . a fila ~
1 8
Fig.2.
.
2.0
2.2
III
.
.11\..
. Barchan
~
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8 §
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~ § § -
/' /" -- - -.....1"{j='i
