Channel planform dynamics of the Ganga-Padma system, India

Abstract

The Landsat programme, which started in 1972, initiated an era of space-based Earth observation relevant to the study of large river systems through the provision of spatially continuous, synoptic and temporally repetitive multi-spectral data. Free access to the Landsat archive from mid-2008 has enabled the scientific community to reconstruct the Earth’s changing surface and, in particular, to reconstruct the planform dynamics of the world’s largest rivers. The present research reconstructs the planform dynamics in the lower reaches of one of the Asian mega-rivers, the River Ganga-Padma (Ganges), from 1972-2010 using the Landsat archive. The research based on sequential river planform maps generated from the time series revealed a periodic pattern of evolution of the river system over the study period which began by means of meandering at four locations. The meander bends increased in sinuosity until chute cut-offs were triggered, returning the river to a state similar to that at the beginning of the sequence. This periodic pattern is constrained by natural and artificial hard points, and by the Farakka Barrage, meaning that the observed cyclic pulsing is likely to continue into the future.

The characteristics and dynamics of meandering rivers have been the subject of extensive research, though the mechanisms involved are still not completely understood. Presently, availability of archival satellite sensor data at regular and frequent intervals for almost four decades presents a great potential for increasing our understanding of the natural processes of meander growth. Though early research indicates that meander growth can be explained by instability of alternate bars in a straight channel, but research based on field data and simulation models have shown that instability of river meanders is an inherent property and the meanders reach a critical value of sinuosity when cut-offs occur and then the complex system undergoes an self-adjusting process. The meander dynamics of the lower reaches of the Ganga-Padma system has been studied in the context of threshold response of a complex system. A conceptual model was developed based on spatial information from the sensor data and quantitative information on river metrics to explain the behaviour of the river system including evidence for self-organising criticality and the attempts of the river to reach dynamic equilibrium. The meandering channel pattern with a tendency of braiding of the river Ganga-Padma were explained based on existing empirical models as well as models based on mobility number and channel stabilization criterion. The threshold for chute cut-off was explored and subsequently the conditions for soft avulsion / branching were studied which showed that the condition for chute cut-off in the Ganga-Padma system is not due to bankfull flow velocity and the super elevation of flow at the centreline of channel but may be due to lack of vegetation stabilization on the Ganga-Padma floodplain.

The effect of tectonics and meandering in the moderately paced avulsion of the Ganga-Bhagirathi system to the present Ganga-Padma system was modelled in the present research. It was found that gradient advantage and bend upstream of bifurcation does not result in modelled avulsion as observed in small and medium rivers and large rivers in tectonically inactive regions. A tectonic uplift results in a modelled avulsion period consistent with historical observations. It was found that backwater effect and high sediment mobility keep both bifurcated channels active to attain an anabranching pattern. The backwater effect was found to play an important role for sustaining the anabranch planform of many of the largest rivers of the world.

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ORCID for P.M. Atkinson: orcid.org/0000-0002-5489-6880

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