Annals of Arid Zone 26 (1&2) : 55-66, 1987

CHARACTERISTICS OF THE SHORT DISTANCETRANSPORTED FLUVIAL SEDIMENTS IN AN ARID

ENVIRONMENT

D. C. JOSHI AND K. D. SHARMA

Central Arid Zone Research Institute, Jodhpur

ABSTRACT

Characteristics of the short distance transported sediments in an arid environ-ment have been studied. Sediment derived from the sandy plain, dunecomplex and rocky gravelly catchments contained more clay and silt than thematrix soil. The sand fraction of these sediments, dominated by poorly sortedfiner grade, showed narrower size range than the soil proper indicatingrestricted velocity fluctuation during sedimentation.

INTRODUCTION

Aeolian sediments have drawn attention of many workers (Goudie, 1978; Vincent,1984; Khalaf and AI-Hashash, 1983; Behary et a!., 1985). Due to low rainfaIl, fluvialsediments are only localised, but they assume significance when regular sedimentationreduces the life span of structures designed to harvest rain water (Jones, 1981; Sharmaand Chatterji, 1982; Sharma and Joshi, 1982). The characteristics of the fluvial sedi-ments are directly related to the catchment attributes, particularly the type of soil.

In the Indian desert 'khadins' (Kolarkar et a\., 1983) and 'nadis' (Sharma andJoshi, 1981) are the principal water storage systems. The 'nadis' seldom overflow andprovide the excellent sites for the study of fluvial sedimentation processes. Grain sizeparameters have been used to separate the beach, dune, aeolian and fluvial sedimenta-tion (Folk and Ward, 1957; Mason and Folk, 1958; Friedman, 196\). Present study isintended to describe the composition and characteristics of the short distance« 100 m) transported fluvial sediments and to relate these with their environment.

STUDY AREA

The study was carried out in the Nagaur district of Rajasthan between 24°6 to26°0 'N latitude and 73°1' to 75°4 'E longitude. The northern and north-western partof the district is covered with dune complex and sandy plains, while southern part ismedium and fine textured all uvial plain. Steeply rising hills occur at the south-easternfringe and the rocky/gravelly pediments are scattered throughout the area but morecommonly in the south-western part. Average annual rain faIl for the lasl 80 years is384 mm - about 80% of which is received between June and September.

56 ; JOSHI & SHARMA

~iIATERIAL AND METHODS

Sediment samples were collected from the beds of the small 'nadis' or earthendug out/embankment ponds with catchment areas < 5 km2. The sampling sites wereselected by stratified random sampling technique representing the physiographieregions-sandy plain (Chirai, Bher, Kumparabas, Molasar and Jayal soils), duny com-plex (dune, Molasar hummocky and Molasar highly hummocky soils), medium andfine textured alluvial plain (Asop, Khajwan, Panchroli, Parbatsar and Palari pichk-iasoils) and rocky/gravelly pediments. The samples were collected during the dry monthswhen these ponds did not contain water. Soils, from the respective catchments, wereclassified as per the standard soil survey carried out for the district (Dhir et aI., 1979).Both the sediments and soil samples were ana lysed for primary particle composition(Piper, 1950). The sand fraction was shaken for ten minutes through a nest of sievesand separated in different grades (Griffith, 1967). Each fraction was weighed andreported as percentage. Particle size data were converted from mm to <1> scale andpresented as frequency curves. Through cumulative frequency curves the diameter ofsand fractions at 25 (Ql), 75 (Q3), 90 (P90) and ]0 (Plo) percentiles and median diame-ter (Md) at 50 percentile were calculated. The parameters of graia size dist.ibution suchas sorting coefficient (So), Skewness (Sk) and quartile kurtosis (k) were calculated byusing the following formulae (Twenhofe] and Tyler, 1941).

RESULTS AND DISCUSSION

The sediments, generated by torrential rainfall and after transported to a shortdistance (50 to 100 m) through runoff by saltation as well as surface creep, get deposi-ted in the ponds. Morphological characteristics and mechanical composition of thecatchment soils and respective sediments have been reported earlier (Sharma and Joshi1982). Relationships between sand fractions of the soils and sediments are discussed:

Sandy plain

The soils eontained 3 to 8% silt and 8 to 12% clay. The sand fractions of thesesoils showed bimodal distribution, with primary mode at 2. 5 to 3. 2 <1>, the secondarymode at 3. 8 to 5. 6 <1> (Fig. 1) and the median diameter varying from 0.117 to O. 154mm. The sand fraction of the~e soils was well sorted and skewed towards finer grade.The kurtosis values were in narrow range (Table 1).

In comparison with soils, the sediments contained higher amounts of silt (4 to20%) and clay (10 to 34%). The sand fraction showed bimodal distribution with theprimary mode to 3. 8 to 5. 6 cf> and secondary mode at 2. 0 to 3.09. About 80%grains were distributed in two grades. The median diameter had a range of O. 038 to

CHARACTERISTICS OF FLUVIAL SEDIMENTS: 57

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CHARACTERISTICS OF FLUVIAL SEDIMENTS: 65

maximum sorting from the median diameter (Mason and Folk, 1958). The sand frac-tion in sediments from the duny complex unit were sorted towards finer grade of themedian diameter (skewness> I); while in sandy plain, medium and fine texturedalluvial plain and rocky gravelly ped iments it \\ as sorted towards coarser side of themedian diameter (skewness < I).

Kurtosis less than unity indicates restricted velocity fluctuations during the sedi-mentation (Sahu, 1964). In the present study, the Kurtosis values were always lessthan unity and indicated a narrow size range of soils and sediments among the studiedenvironments.

These studies revealed that the sedimen ts derived in different environments havevariable amount of silt and clay. As regards sand, these are dominated by the finerfractions. A narrow range in the kurtosis and sorting coefficient values suggest thatthe sediment suit belong to the same population and evidently is a product of thesimilar processes \Visher, 1969). The dominance of finer grade of sand in differentenvironment can be attributed to its ubiquitous presence.

ACKNOWLEDGEMENTS

Thanks are due to Du. K. A. Shankarnarayan, Director, and to R. P. Dhir,Head, Division of Basic Resources Studies, Central Arid Zone Research Institute,Jodhpur for providing necessary facilities.

REFERENCES

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Chatterji, P. C. 1968. Some characteristics of the quaternary sediments from centralLuni basin, western Rajasthan. Geographical Outlook. Ranchi University, Ranchi.

Dhir, R. P. Joshi, D. C., Singh, N. & Bhola, S. N. 1979. Soil and Landuse CapabilityClassification of the Nagaur district. Sectional Report, Central Arid Zone ResearchInstitute, Jodhpur.

Folk, R. L. & Ward, W.C. 1957. Brazos river bar: a study in the significance of grainsize parameters. Journal of Sedimentary Petrology. 27: 3-26.

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Goudie, A. S. 1978. Dust storms and their geomorphological implications. Journal ofArid Environments. 4: 291-310.

66: JOSHI & SHARMA

Griffith, J. C. 1967. Scientific Methods in Analysis of Sediments. Mc Graw Hill,New York. 436 pp.

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Pandey, S., Singh, S. & Ghose, B. 1964. Orientation, distribution and origin of sanddunes in the Central Luni Basin. Proceedings of the Symposium on Problems ofIndian Arid Zone, New Delhi. : 84-91.

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Vincent, P.J. 1984. Particle size variation over a transverse dune in the Nafud as Sirr,Central Saudi Arabia. Journal of Arid Environments. 7: 329-336.

Vishar, G. S. 1969. Grain size distributions and the depositional proce.5ses. Journalof Sedimentary Petrology. 39: 1074-1106