Abstract Diurnal and seasonal variations in particle size characteristics of suspended sediments of Dokriani glacier meltwater were studied during the 1994 ...
Hydrological Sciences—Journal—des Sciences Hydrologiques, 44(1) February 1999
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Particle size characteristics of suspended sediments and subglacial hydrology of Dokriani Glacier, Garhwal Himalaya, India
RENOJ J. THAYYEN, J. T. GERGAN & D. P. DOBHAL Wadia Institute of Himalayan Geology, Dehradun 248001, e-mail : wihg@giasd 101 .vsnl.net.in
India
Abstract Diurnal and seasonal variations in particle size characteristics of suspended sediments of Dokriani glacier meltwater were studied during the 1994 ablation period (May-October). Diurnal suspended sediment concentration curves exhibit two prominent peaks from May to mid-August and only one peak during the rest of the ablation period. The first peak resulting from dominance of fine and medium sand contributed to the rising limb of the diurnal hydrograph, whereas the second peak was dominated by fine and medium silt which coincides with the diurnal discharge peak. The trends observed on the appearance of silts and sand particles on diurnal scales at various stages of the ablation period suggest that the subglacial drainage system comprises a diurnally-reversing hydraulic gradient between the channels and distributed system along with translatory flow through the distributed system. This study suggests that the subglacial zone is the major source of sediments in the Dokriani glacier meltwaters. However, high intensity monsoonal rainfall has contributed supraglacial sediments and resulted in bimodal distributions. Fines (clay to medium silt) are the dominant classes in the suspended sediments except during the spring event; the supraglacial sediments had coarsely skewed distributions.
Caractéristiques granulométriques des matières en suspension et hydrologie du glacier Dokriani (Garhwal Himalaya, Inde) Résumé Les variations diurnes et saisonnières de la granulométrie des matières en suspension transportées par les eaux de fonte du glacier Dokriani ont été étudiées durant la période d'ablation de 1994 (mai-octobre). La courbe journalière de la concentration des matières en suspension présente deux extremums de mai à la miaoût et un seul extremum durant le reste de la période d'ablation. Le premier extremum résulte de la dominance des sables fins et moyens le long de la branche ascendante de l'hydrogramme journalier, alors que le second résulte de la présence de silts fins et moyens et coïncide avec le maximum journalier de débit. Les tendances observées concernant l'aspect des silts et des sables à l'échelle journalière pour différents moments de la période d'ablation suggèrent que le système de drainage sous-glaciaire subit tous les jours des renversements de gradient hydraulique entre ses chenaux allant de pair avec un flux d'eau transversal. Cette étude suggère que, en ce qui concerne les eaux de fonte du glacier Dokriani, le domaine sous-glaciaire est le principal fournisseur de sédiments. Les fortes intensités des pluies de mousson entraînent toutefois une contribution de sédiments du domaine supra-glaciaire et sont à l'origine de distributions bimodales. Les matières en suspension sont surtout constituées de fines (argiles à silts moyens) sauf au printemps et les sédiments supra-glaciaires sont distribués de façon très asymétrique.
INTRODUCTION Discharge and suspended sediment transfer characteristics of a glacierized catchment are regulated primarily by input conditions (climate) and routing conditions (Elliston, Open for discussion until I August 1999
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1973). The runoff characteristics of the Himalayan catchment are greatly influenced by the monsoonal rainfall from mid-June to mid-September (Thayyen & Hasnain, 1997). Precipitation over the extensive supraglacial debris cover and proglacial zone, especially over the large lateral moraines, enhances the erosion processes in the glaciated region of Himalaya. Within the glacial system, supraglacial and subglacial zones are the source areas of sediments (Gurnell & Fenn, 1984). Therefore the particle size characteristics of suspended sediments at the glacial portal carry the signatures of the supraglacial and subglacial contributions. Glacier channel characteristics play an important role in determining the particle size distribution of suspended sediments from the subglacial zone as well as in modifying the size distribution of suspended sediment produced from the supraglacial zone. Hydraulic characteristics of glacier drainage are determined by the amount of water input derived from snowmelt, icemelt, rainfall and groundwater which vary over the ablation period (Rothlisberger, 1972; Iken, 1981; Hooke et al, 1985; Iken & Bindschadler, 1986; Rothlisberger & Lang, 1987). Therefore, the variability in the hydraulic characteristics of glacier drainage systems and in the particle size characteristics of suspended sediments tend to show diurnal and seasonal variability. Particle size characteristics of suspended sediments in glacier meltwater and their variations through the ablation period are controlled by the size
78°47'
78°50 Fig. 1 Map showing location of Dokriani Glacier.
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characteristics of the source sediments, eroding mechanisms and hydraulic characteristics of the flow paths. In this work the diurnal and seasonal variations in particle size distribution of suspended sediments in Dokriani glacier meltwater during the 1994 ablation period (May-October) were studied. Samples representing various stages of suspended sediment transfer processes were analysed in relation to variability in routing conditions and climate. AREA OF STUDY Dokriani Glacier is a small Garhwal Himalayan glacier in the Bhagirathi basin, located in Uttarkashi district of Uttar Pradesh. It extends between latitude 30°50'30°52'N and longitude 78°47'-78°50'E (Fig. 1). The total length of the glacier is 5 km. It originates from Draupadi ka Danda (5716 m a.m.s.l.) and flows in a NNW direction for 2 km; then it turns in a WWN direction and flows another 3 km with an average gradient of 12° and terminates at a height of 3882 m a.m.s.l. The stream emerging from the Dokriani Bamak (Glacier) is known as Din Gad, a tributary of the River Bhagirathi. The total area of the glacier catchment is 9.58 km2, of which 5.76 km2 is covered with glacier ice. Three quarters of the glacier ablation area is covered by thick debris cover. The accumulation area of the glacier is nearly 2.5 km2. This glacier is formed by two cirque glaciers, one originating from Draupadi ka Danda and the other from the western slopes of Janoli peak (6632 m a.m.s.l.) joining near the equilibrium line (4960 m a.m.s.l.).
METHODOLOGY Observations were carried out from 8 May to 31 October 1994 covering the entire ablation period. A gauging station was established at Din Gad proglacial stream, 600 m downstream from the glacier portal. Discharge measurements and water sampling were carried out at this station. Discharge measurement was carried out by area-velocity method and by establishing a rating curve. Water samples were collected in narrow necked polyethylene bottles by the method described by Ostrem (1975). Meltwater samples were collected at intervals of three hours. Suspended sediments were filtered out from the samples by using pre-weighed 0.45 um Millipore membrane filter papers of 47 mm diameter with the help of a vacuum pump and filtration unit and the filtrate was dried in the open air. Before weighing the sediment, larger grains, if present, were removed manually from the samples. Such problems were encountered in few samples. After drying and weighing, sediments were scraped out from the filter paper. Ashing was not carried out, because of the low amount of sediment and to avoid textural variations of suspended sediments that would have occurred during the ashing. Particle size analyses of selected samples were carried out by Malvern Master Sizer E. The Master Sizer measures the volume percent of the particles by laser ensemble light scattering; it is programmed to convert the result into weight percent by using the density factor.
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The particle size of the samples analysed was in the range of 0.5-600 um. It is assumed that 600 um is the largest possible size in suspension, considering the hydrological characteristics of Din Gad proglacial stream. The Master Sizer E gives the results in 32 size classes. For the analyses, the results were grouped into six size classes (Table 1) based on the Woodden Worth scale. The statistical parameters were calculated using the method of moments in phi scale. Table 1 Particle size classes and mean values. Size class (um) 0.5-7.78 7.78-25.46 25.46-56.09 56.09-123.59 123.59-272.31 272.31-600
Mean (nm) 4.14 16.62 40.72 89.84 198.95 436.15
Size class () within the distributed system eroded first due to their low erosion velocity (Hjulstrom, 1939; Sly et al., 1983) by means of the translatory flow during the diurnal rise of discharge. This contributed to the first peak of suspended sediments which appeared between 14:00 and 17:00 h. Diurnal maximum discharges and associated translatory flow brought down maximum fines to the glacier portal and resulted in the second sediment peak. The second and subdued peak of fine and medium sand curves observed between 23:00 and 02:00 h was probably contributed by the reversed hydraulic gradient from the distributed system to the channels. These peaks were consistent throughout the ablation season. As ablation progressed, reduced availability of sediments within the subglacial system resulted in low suspended sediment concentration in meltwater and the disappearance of the first suspended sediment peak. Reduced efficiency of the distributed system during the later part of the ablation period is also evident from the early occurrence of a discharge peak through the fast system (Raymond et al., 1995).
CONCLUSIONS The relationships between discharge and suspended sediment concentrations on a diurnal scale at different stages of the ablation period show characteristic variations. During the first stage of ablation, to the last week of July, the suspended sediment concentration in meltwaters was controlled more by the internal channel development processes. During this period, variations in suspended sediment concentration occurred independent of discharge variations. After the sediment evacuation
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processes and the stabilization of glacial drainage systems, the relationship between discharge and suspended sediment concentration improved consistently. Diurnal suspended sediment concentration curves show two distinct peaks till mid August. The first peak is associated with the rising limb of the diurnal hydrograph and the second coincides with the peak discharge. The analysis of diurnal variations in particle size distribution shows that the first peak resulted from the larger contributions of fine and medium sand whereas the second peak was due to the fine and medium silt. The characteristics of diurnal variations in particle size and discharge at different stages of the ablation period suggest that the subglacial drainage system comprised a diurnally reversing hydraulic gradient between channels and the distributed system along with the translatory Darcy flow through the distributed system. The particle size distribution of Dokriani glacier meltwater was dominated by fine and medium silt. However, very high amounts of fine and medium sand were transported during the spring event. The surficial samples show coarse skewed distribution which is characteristically different from the fine skewed distributions of suspended sediments in meltwater and suggests that the subglacial zone is the major source of suspended sediments. However the bimodal distributions associated with the rainfall events suggest that the high intensity rainfall indeed contributes substantial amounts of suspended sediments from the surficial sources.
Acknowledgements The authors are grateful to the Director, Wadia Institute of Himalayan Geology, for assigning the project and providing the facilities. They also acknowledge the comments of reviewers and suggestions by Dr Mark Williams in improving the quality of this paper. Thanks are also due to Department of Science and Technology, Government of India, New Delhi, for funding the project under the Nationally Coordinated Project on Himalayan Glaciers. REFERENCES Elliston, G. R. (1973) Water movement through the Gornergletscher. In: Symp. an the Hydrology of Glaciers (Cambridge, UK, 7-13 September 1969). IAHS Publ. no. 95. Fountain, A. G. (1994) Borehole water level variations and implications for the subglacial hydraulics of South Cascade Glacier, Washington, USA. J. Glacial. 40(135), 293-304. Gergan, J. T. (1996) Report on scientific expedition to Dokriani Glacier. Submitted to Department of Science and Technology, Govt of India, New Delhi. Gergan, J. T., Dobhal, D. P. & Rambir Kaushik (in press) Ground penetrating radar ice thickness measurements of Dokriani Bamak (Glacier), Garhwal Himalaya. Current Sci. Gurneli, A. M. & Fenn, C. R. (1984) Flow separation sediment source areas and suspended sediment transport in a proglacial stream. Catena Suppl. 5, 109-119. Hjulstrom, F. (1939) Transportation of detritus by moving water. In: Recent Marine Sediments (ed. by P. D. Trask) (Symp. Am. Assoc. Petrol. Geol., Tulsa), 5-31. Hooke, R. L., Wold, B. & Hagen, J. O. (1985) Subglacial hydrology and sediment transport at Bondhusbreen, Southwest Norway. Geol. Soc. Am. Bull. 96, 388-397. Hubbard, B. P., Sharp, M. J., Willis, I. C , Nielsen, M. K. & Smart, C. C. (1995) Borehole water-level variations and the structure of the subglacial hydrological system of Haut Glacier d'Arolla, Valais, Switzerland. J. Glacial. 41(139), 572-583. Humphrey N. F. & Raymond, C. F. (1994) Hydrology, erosion and sediment production in a surging glacier: Variegated Glacier, Alaska, 1982-1983. /. Glacial. 40(136), 539-552. Iken, A. (1981) The effect of subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model. J. Glacial. 27, 407-421.
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