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.

This study investigates the basin scale sediment transport processes in the Sangu river in Bangladesh by using a rainfall-sediment runoff (RSR) model. One year numerical simulations are conducted with two different sediment transport formulas and the results were compared with observed sediment size distribution and suspended sediment discharge. As a result of computation, a clear longitudinal sediment sorting is shown in one case, while that is not shown in the other case, and the difference between two cases is explained by the difference in the erosion rate of suspended sediment formula. The sediment sorting significantly affects the results of the basin scale sediment transport analysis. Thus, we conclude that in a river dominated by suspended sediment such as the Sangu River, it is very important whether the erosion rate of suspended sediment and associated transport processes can evaluate the sorting process.

This study investigates the characteristics of suspended sediment transportation and associated morphological changes in the Meghna Estuary in Bangladesh, specifically focusing on sediment transport dynamics in the lower Meghna River and the sediment budget in the Noakhali islands area, which is located in the northeastern part of the Meghna Estuary. A depth-averaged 2-D model, spanning 350 km in length including the entire lower Meghna River and surrounding estuary area, is prepared for flow and sediment calculations, employing a uniform sediment size of 0.06 mm. The results of an entire year calculation, considering the wide seasonal variation, indicate that the Noakhali islands area experiences significant morphological changes during the ebb tide period of the spring tide time, particularly in the monsoon season. During this time, a strong current diverges from the main flow of the lower Meghna River, carrying a significant amount of suspended sediment, which leads to high sediment concentration particularly in the slack time, resulting in significant storage and deposition of fine sediment in this area. Therefore, this present study proposes a model which can evaluate the seasonal, daily, and hourly variation of sediment storage and deposition processes in an estuary dominated by fine sediment.

This study analyzes the effect of suspended sediment transport on the bed evolution of a suspended load dominated meandering river by using a two-dimensional (2D) numerical model. The numerical simulations were performed under steady and unsteady flow conditions using bathymetric data and in situ sediment samples. The erosion rate was calculated using a turbulent entrainment equation for fine particles. The results showed active changes in the riverbed when suspended sediment was considered in terms of changes in the riverbed profiles and cross sections. For the steady case, erosion was predominant in areas near the inflection points due to topographic features (narrower width and cross-sectional area) which led to active sediment erosion together with a phase-shifting in deposition patterns. Complex changes were observed for the case of unsteady flow that are caused partly by the non-equilibrium nature of suspended sediment.

Flooding of water, sediment, and driftwood and its associated landform changes often occurs downstream of debris-flow deposition zones in small, mountainous river basins. This type of flooding occurs due to a sudden decrease in the sediment transport capacity of river channels, resulting in considerable damage. Despite the significant need for appropriate countermeasures to mitigate damage, it has been a challenge to plan and implement these across all river basins at risk of these hazards. However, if a simple but reliable tool is available to identify hazardous areas in advance, this would be extremely useful for prioritizing such areas and designing and implementing the required measures. To this end, this paper examines the geomorphic characteristics of recent flood events in Japan and proposes a method for identifying hazardous areas due to inundation by flood flows with sediment. We first explain the occurrence of sediment inundation by focusing on longitudinal changes in sediment transport capacity using the geomorphic indicator defined as the product of the drainage area $A$ and bed slope $i$. In addition, we investigate the longitudinal profiles of the sediment transport capacity along the study channels using a one-dimensional governing equation for flood flows and, based on the results, discuss that finding that the computed longitudinal profiles of the sediment transport capacity are consistent with the longitudinal distribution of the $Ai$. Our findings indicate that the $Ai$ is an effective indicator for identifying areas prone to inundation by flood flows with sediment.

River flow, sediment transport and channel width influence one another, and channel width generally changes toward the equilibrium state in sediment transport. When the tidal range is large with the water level fluctuating periodically, the sediment transport capacity of the river channel increases in the river mouth due to the drawdown effect during low tide, whereas the backwater effect becomes dominant during high tide. Therefore, the channel width is determined by the flow discharge during low tide and tends to be wider toward the mouth. On the other hand, when the channel in the river mouth is submerged during the rainy season in tropical regions, the channel width is defined by the flow discharge during the transition period from the rainy season to the dry season. This presentation explains a mechanism that determines the channel width in the river mouth by taking rivers in Japan and Southeast Asia as examples.