Characterization of Suspended Sediment in Meltwater from Gangotri Glacier

Abstract

Glacierised basins are significant sources of sediments generated by glacial retreat. In Himalaya, most of the glaciers are covered by thick debris, especially in the ablation zone. Supraglacial debris cover might play an important role for sediment budget of the glaciated area or for the ablation of ice masses mantled in debris. The glacier system includes sedimentation from different parts of the glacier such as the accumulation zone, ablation zone, snout and the lateral moraines, whereas bedrock system deals with glacier bottom ice and bedrock. During summer season, proglacial meltwater carries considerable amount of suspended sediment. The deglaciated area provides a ready source of sediment during monsoon. Estimation of suspended sediment transfer from glacierised basins is very important in reservoir planning for hydropower projects in Himalaya. Assessment of these sediments transported by the melt stream is important because it has direct influence on the capacity of the reservoirs. Suspended sediment yields in particular are viewed as a sensitive parameter of environmental change, since suspended sediment is broadly supply-controlled, while bed load is broadly hydraulically controlled; therefore, it is expected that suspended sediment fluxes are more responsive than bed load fluxes to climate-driven environmental change, other factors being equal. An assessment of suspended sediment concentration (SSC), load, yield and erosion rate has been undertaken for the Gangotri Glacier drainage basin (nearly 50% glaciated) located in the Garhwal Himalayas. . The proglacial melt water stream, known as Bhagirathi River, emerges out from the snout of the Gangotri Glacier at an elevation of 4000 m. The Gangotri Glacier system most commonly known as Gangotri Glacier, is a cluster of many glaciers comprising of main Gangotri Glacier (length: 30.20 km; width: 0.20 - 2.35 km; area: 86.32 km2) as trunk part of the system. The flow data was collected from field observations near the snout of the glacier. To determine the mean suspended sediment concentration, load, yield and particle size in the Gangotri Glacier melt stream, two water samples at 0830 and 1730 hours were directly scooped from the channel at the gauging site in a cleaned polyethylene bottle (500 ml). Data were collected for four ablation seasons (2011–2013). Mean monthly SSCs, for May, June, July, August and September during the study period was 1,435, 1,677, 2,086, 1,499 and 528 ppm, respectively, indicating highest SSC in July, followed by August. For the entire melt season, the mean daily SSC was computed to be 1,445 ppm. Similar trends were also found for the sediment load and about 67% of the total suspended sediment load of the melt period was transported during the months of July and August. There is a wide variation in the daily concentration of suspended sediment and SSLs. Both sediment concentration and load were found to be highest in the monsoon months and nearly two-thirds of the total sediment load was transported in these 2 months.