Sediment Dynamics of the Lower Ganga River: Sediment Budget and ‘Hotspots’ of Siltation using an Integrated Approach of Hydrology and Remote Sensing

Abstract:

The Ganga river is one of the largest river systems in the world that has built extensive alluvial plains in northern India. The stretch of the Lower Ganga is very dynamic and is particularly vulnerable to siltation for three main reasons: (a) a naturally low slope being the most downstream stretch in the alluvial plains; (b) confluence of several highly sediment-charged rivers such as Ghaghra, Gandak, and Kosi in this stretch, and (c) reduction of non-monsoon flows because of upstream abstractions of both surface and groundwater. As a consequence, large-scale siltation in several reaches has reduced the channel capacity leading to catastrophic floods in this region even at low discharges. This work is aimed at developing a first-order sediment budget and computing sediment volumes accumulated in the channel belt along the Lower Ganga river using hydrological data and planform maps prepared from historical remote sensing images for the last 52 years (1965-2017). The work was carried out by dividing the total length of the river into four continuous stretches: (a) Buxar-Gandhighat (160 km), (b) Gandhighat-Hathidah (106 km), (c) Hathidah-Azmabad (182 km) and (d) Azmabad-Farakka (132 km). We document that major ‘hotspots’ of siltation have developed in several reaches of the Lower Ganga river during the last 4-5 decades. The Farakka barrage has affected the morphology of the Ganga river significantly both upstream and downstream of the barrage since its construction as reflected from our mapping work. Sediment budgeting using planform maps suggests that a total of 3094 Mm³ of sediments have accumulated in this stretch during the last 52 years out of which 1665 Mm³ are ‘extractable’. The Hathidah-Azmabad is documented as the most problematic stretch in terms of siltation where ∼1240 Mm³ (∼3300 million tons) of sediment may have accumulated in the past 52 years at a rate of 6-7 cm/yr. Due to continued siltation, alluvial islands and bars have developed and stabilized through time in several reaches. It may no longer be possible to remove these hotspots ‘naturally’ through ‘regular’ flows. The perception that the monsoonal flows may be sufficient to remove them may not be correct as these high flows also bring additional sediments, and there has been an incremental effect over the years in terms of excessive siltation in several reaches.