Sediment characteristics and transportation dynamics of the Ganga River

Abstract

Understanding of river systems that have experienced various forcing mechanisms such as climate, tectonics, sea level fluctuations and their linkages is a major concern for fluvial scientists. The 2525-km-long Ganga River derives its fluvial flux from northern part of the Indian subcontinent and drops in the Ganga–Brahmaputra delta and the Bengal fan regions. This paper presents a study of the Ganga River sediments for their textural properties, grainsize characteristics, and transportation dynamics. A suite of recently deposited sediments (189 bedload samples and 27 suspended load samples) of the river and its tributaries was collected from 63 locations. Dry and wet sieve methods of grainsize analysis were performed and Folk and Ward's parameters were calculated. Transportation dynamics of the sediment load was assessed by means of channel hydrology, flow/sediment rating curves, bedform mechanics, grainsize images, and cumulative curves. Textural properties of the bedload sediments of the Ganga River tributaries originating from the Himalaya orogenic belt, the northern Indian craton and the Ganga alluvial plain regions are characterised by the predominance of fine to very fine sand, medium to fine sand, and very fine sand to clay, respectively. Downstream textural variations in the bedload and suspended load sediments of the Ganga River are, therefore, complex and are strongly influenced by lateral sediment inputs by the tributaries and channel slope. At the base of the Himalaya, a very sharp gravel–sand transition is present in which median grainsize of bedload sediments decreases from over − 0.16 Φ to 2.46 Φ within a distance of 35 km. Downstream decline in mean grainsize of bedload sediments in the upper Ganga River within the alluvial plain can be expressed by an exponential formula as: mean grainsize (in Φ) = 0.0024 × Distance (in kilometres from the Himalayan front) + 1.29. It is a result of selective transport phenomena rather than of abrasion, the condition attributed to channel competence with low gradient of 70 to 25 cm/km. Coarsening of the bedload sediments in the middle segment of the river develops due to the coarse bedload sediments (mean grainsize: 1.8–2.2 Φ) supply from the tributaries of the northern Indian craton region and increased unit stream power. The Ganga River sediments show a wide range of grainsize parameters [Bedload sediments: mean grainsize (1.5–3.8 Φ), standard deviation (0.3–0.7 Φ), skewness (− 0.2–0.5), and kurtosis (0.8–2.4); suspended load sediments: mean grainsize (2.9–5.0 Φ), standard deviation (0.6–1.6 Φ), skewness (− 0.6–0.4), and kurtosis (0.7–4.6)]. Mean grainsize of the bedload sediments is 2.6 Φ, which is 1.3 Φ units coarser than the suspended load sediments. The cumulative grainsize curves of the bedload sediments show a distinct “break” in slope, representing nearly 80% of the bedload sediment transported as graded suspension. During the peak monsoon discharge, most of the bedload sediments are transported as the discontinuous, fine sandy, graded suspension by the unidirectional unsteady shooting flow at a high transport rate (about 8 × 10 6 tons/d). A model of sedimentation transportation at river channel is proposed. The Ganga River mobilises a total of 729 × 10 6 tons of sediments annually through a narrow zone within its river valley. Under the present hydrogeological conditions, the river sedimentation is climatically controlled and is predicted to produce a 2000-km-long, 2- to 40-km-wide and 25- to 50-m-thick ribbon-shaped, well-sorted symmetrically skewed fine sand body.