Majadur Rahman

This study aims to clarify the characteristics of the riverbed deformation, bank erosion, and channel changes in the lower Sangu River basin using a depth-averaged 2-D flow model where suspended sediment transport is dominant and its flow characteristics are influenced by active tides. A 45 km long area including the river mouth is computed using the 2-D model, and the results are compared with the observed channel changes. The computation results show that bed deformation and channel change are mainly caused during the ebb tide period of the spring tide particularly in an area of about 10 km from the river mouth. The detail study domain calculation results show that the flow concentration at the bend area causes the bed erosion there, and the eddy separated from the main stream causes the sediment deposition at the inner bank, while this eddy is not developed at the outer bank, resulting in the bank erosion there. Through these investigations, the characteristics and mechanisms of the morphodynamics in the lower Sangu River reach, particularly the potential of the combination of tides and floods to enhance riverbed deformations and the associated bank shifts, have been clarified.

The Sangu River, whose basin area is about 3600 km², conveys a huge volume of sediment laden water during the monsoon, which decreases the river's capacity, causing flooding, channel change, and navigability problems. In this study, a basin-scale rainfall-sediment runoff (RSR) model was used to analyse the sediment transport in the Sangu River. Simulations were conducted for the year 2011, and the results were validated in terms of suspended sediment concentration. The simulation results show deposition trends particularly in the lower and middle reaches, and that nearly 7.5 × 10⁵ m³ of sediment annually discharges to the Bay of Bengal. The simulation results also show the tributaries actively produce sediment, which results in sediment deposition in downstream channels. The analyses also find that the implementation of check dams in the upstream area significantly reduces sediment deposition in the downstream area. These findings offer valuable insights for the development of sustainable sediment management strategies.