Process-Based Sediment Transport Modeling to Study Fluvial Deposition in the Ganges-Brahmaputra River System of Bangladesh

The Ganges-Brahmaputra-Meghna delta faces growing risks from subsidence, sediment depletion, and sea-level rise. Sustaining delta elevation requires continued sediment supply, yet the spatial and seasonal dynamics of sediment retention across the fluvial system remain poorly resolved. This study employed a process-based two-dimensional model (Delft3D FM) to simulate suspended and bedload sediment transport across the Ganges, Brahmaputra and Padma Rivers in Bangladesh under contrasting monsoon conditions. Model results showed that over 35%–40% of annual suspended load was retained within the fluvial system, particularly in the upstream braided (Brahmaputra) and meandering (Ganges) reaches and their adjacent floodplains. The Padma reach also contributed notable retention, particularly under reduced Brahmaputra flow dominance. Seasonal variability governs sediment transport, with 80%–95% of annual suspended sediment delivered during the monsoon. Backwater effects in the Padma reach modulate sediment transport by altering flow gradients near fluvial-tidal transition zone. However, sediment transport and retention within and upstream of this zone remain spatially variable, shaped by discharge intensity and channel morphology. Bedload transport remains active in the Brahmaputra reach but becomes increasingly variable downstream, especially in the Ganges-Padma corridor. These findings clarify spatial contrasts in fluvial sediment routing and deposition, providing a system-scale basis for mass balance assessments and sediment management. Identifying key zones of sediment retention and bypass is essential for maintaining sediment connectivity and supporting delta resilience under increasing environmental and anthropogenic stress.