Sediment Sorting Processes Affected by Tidal Currents in the Meghna Estuary

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This study investigates the temporal and spatial characteristics of bed-material sediment sorting in the Meghna Estuary, focusing on both riverine and estuarine regions using a depth-averaged two-dimensional numerical model. A 350 km stretch, including the Lower Meghna River, its mouth, and the surrounding estuarine areas dominated by suspended sediment transport, is simulated to understand the non-uniform sediment transport mechanism and the associated morphological changes that significantly influence the grain size distribution of bed materials. The computational results indicate that sediment sorting in deposition-dominated areas is primarily influenced by the size distribution of deposited materials, which is determined by the deposition rate of each particle size class. In erosion-prone regions, increased erosion rates enhance the downstream transport of finer particles, leading to bed material coarsening. Within the estuarine environment, stronger tidal currents during the ebb phase of spring tides generate increased hydrodynamic energy throughout the tidal cycle, enhancing sediment entrainment from the bedload layer and facilitating the downstream transport of fine particles, thereby contributing to sediment coarsening. In contrast, during the flood phase of neap tides, this process is substantially weakened, leading to reduced sediment mobility and a trend toward sediment fining. Therefore, tidal effects are particularly predominant in erosion-dominated areas of the Meghna Estuary.