Integrating Floodplain Processes and Floodwaves Influence on Sediment Dynamics in the Brahmaputra Basin

Abstract

Limited studies exist on the downstream variation of grain size in sand‐bed rivers, particularly in natural sediment regimes. The impact of local grain‐size variability, lateral tributaries, and bedrock confined zones on the downstream trend remains under explored. The work of first objective of this thesis examined 54 sediment samples from the sandbars of the Brahmaputra main trunk, analyzing grain‐size and bulk mineralogy, along with selected samples for clay mineralogy. End‐member modeling was applied to the grain‐size data, and existing depth profile data were used to propose a transport mechanism. Results corroborate previous studies indicating limited chemical weathering in the Brahmaputra basin. Additionally, diverse median grain sizes ranging from 17 μm to 356 μm was observed, with a notable scarcity of samples having dominant mode in intermediate range of approximately 63‐172 μm. This study reveals that local flow conditions and selective transportation and deposition of sediments in specific size classes drive the grain‐size variability. The grain‐size gap is explained by proposed transport mechanism, with sediments within the gap selectively transported during peak monsoon periods, resulting in coarser deposits on riverbed and depleting sediments within the 63‐172 μm range over repeated annual floods. Sediments below 63 μm are consistently present in the water column and can be deposited in low‐flow zones within floodplains. Evolution of downstream grain‐size variation is influenced by low width bedrock confined zones, characterized by increased grain‐size. The findings contribute to understanding sediment transport and deposition in river floodplains, with implications for downstream fining and grain‐size gap formation in sand‐bed rivers.