Spatio-temporal study of morpho-dynamics of the Brahmaputra River along its Majuli Island reach

Abstract

The River Brahmaputra is one of the largest rivers, ranking seventh in terms of mean annual discharge (19, 800 m3s-1). The river has a multi-channel, braided/anastomosed planform that changes frequently and exhibits rapid transformations in channel morphology. This research was set in the context of the River Brahmaputra near Majuli Island, Assam, India. Morphologically, this part of the river is very volatile; increased braiding intensity of the river is believed by many to be responsible for severe erosion in the southern part of Majuli Island. In 2016, Newton-Bhaba funding facilitated the authors to study the existing braid parameters for developing new ones to quantitatively define changes in the braided channel. The authors then employed these new parameters to evaluate historical and current trends of morpho-dynamic changes in the Brahmaputra River along Majuli Island. Our quantitative analysis establishes on a rational foundation the extent to which braiding intensity has increased. Trends in our new parameters indicate that the frequencies with which braid bars form, deform and erode are increasing, accelerating the rate at which channel morphology evolves near Majuli Island. Alarmingly, our analysis brings out that the deepest channel of the Brahmaputra (i.e., ‘thalweg’) is moving northwards – bringing it ever closer to Majuli Island. Engineered structures were constructed in 2004 to provide protection to the southern bank of Majuli Island and its flood embankments from erosion. To scientifically establish the effect of these river training measures on braiding forms and processes, we applied our newly developed braid indexes to the study reach before and after 2004. We could find that river channel braiding was relatively less intense, and the river was less unstable prior to 2004. However, following the construction of bank protection and river training works at Majuli Island in 2004, braiding intensity registered a sharp increase and the river waterway widened. Also, the deepest sub-channel started to migrate northwards, and through time it has become deeply entrenched along the southern bank line of Majuli Island around Ahatguri posing a major streambank erosion threat to it.