Planform changes and alterations of longitudinal connectivity caused by the 2019 flood event on the braided Brahmaputra River in Assam, India

Abstract

The longitudinal connectivity of rivers facilitates the downstream transfer of discharge and sediment. Any alterations to this component can potentially impair/block sediment movement, resulting in deposition and planform alterations. This effect is more enhanced in braided rivers, which usually convey large sediment volumes. In this study, we use Graph Theory to analyze the impact of a big flood event (July 2019 flood) on the channel planform configuration of the Brahmaputra River in the eastern Indian state of Assam. The in-betweenness centrality and structural sediment connectivity of the nodes (channel confluences/diffluences) and links were computed using different indices. The node accessibility was enumerated using the Shimbel Index. Sedimentographs were devised to simulate the likely sediment outflow volumes and its duration in the post-flood scenario. Our results reveal that significant changes in the channel planform were caused by this flood event, which repositioned mid-channel and bank-attached bar deposits and realigned the river's entwined multiple threads. These engendered alterations in the existing channel bifurcations and inter-node connections, thereby impacting on how the sediment was transferred and its pathways. The enumerated accessibility index and simulated sediment volumes reflected the above changes, with reach-wise variations apparent in these aspects. This study helps comprehend the flood-induced planform morphological changes in braided streams, which has implications for navigation. The framework used here can also be applied to assess other braided rivers.