2:15 PM - 2:30 PM
[HCG28-03] Numerical study on formation mechanism of shoal and cauldron topographies on the seafloor in the eastern Seto Inland Sea
The topography of the seafloor has an elemental importance to determining physical phenomena such as ocean currents, favorable habitats for marine organisms, optimal vessel navigation, exploration of undersea resources, etc. Prevailing currents and waves, as well as associated shear stresses acting on the ocean floor, are responsible for formation of typical topographic features including sea caldrons and sandbanks through erosion of bedrocks and sediments and their deposition processes. In the Seto Inland Sea (SIS), the most extensive semi-enclosed estuary in Japan, tidal currents are well known to serve pronouncedly for the formation of seafloor topographic features. However, due to the spatiotemporal complexity in 3-D flow field and resultant sediment transport, the formation processes of bathymetry and their underlying mechanisms have not been fully studied yet, in particular from a hydrodynamic viewpoint.
This study aims at understanding bathymetric formation under predominance of tidal currents in a semi-enclosed estuary, where the SIS is chosen for the study area. The simulation results from a 3-D high-resolution Seto Inland Sea circulation model using the Regional Ocean Modeling System (ROMS) in a double nested configuration embedded in an assimilative JCOPE2 oceanic reanalysis product were utilized to examine detailed hydrodynamic processes for the topography formations in the SIS. First, the bottom shear stress and residual flow were evaluated from the modeled flow field to assess the potential for erosion of the seafloor topography. A strong correlation between the bottom shear stress and the scour depth of the erosive areas is observed, suggesting that seafloor is most likely eroded by the erosion effect of the near-bed tidal currents. Second, a simple sediment budget model was developed based on sediment erosion fluxes for each particle size typical of the bottom sediments of the SIS, i.e., gravel, sand, silt and clay, using the modeled bottom shear stresses primarily due to tidal currents. This budget model presumes depth-independent sediment concentration in each water column determined by the erosion and deposition fluxes and considers horizontal suspended sediment transport to investigate where and how erosion and deposition occur. Horizontal divergence of the residual flows indicates consistency between divergence (convergence) and erosion (deposition); the divergent outflow occurs on the eroded terrains, while the convergent inflow is prevailed on the deposited terrains frequently appeared in the vicinity of the eroded terrains. The sediment budget model also exhibits that sediments are generally transported from deep to shallow areas in the eroded terrains to form the deposited terrains fringing the eroded terrains, whereas sedimentation tendency differs largely from location to location.
This study aims at understanding bathymetric formation under predominance of tidal currents in a semi-enclosed estuary, where the SIS is chosen for the study area. The simulation results from a 3-D high-resolution Seto Inland Sea circulation model using the Regional Ocean Modeling System (ROMS) in a double nested configuration embedded in an assimilative JCOPE2 oceanic reanalysis product were utilized to examine detailed hydrodynamic processes for the topography formations in the SIS. First, the bottom shear stress and residual flow were evaluated from the modeled flow field to assess the potential for erosion of the seafloor topography. A strong correlation between the bottom shear stress and the scour depth of the erosive areas is observed, suggesting that seafloor is most likely eroded by the erosion effect of the near-bed tidal currents. Second, a simple sediment budget model was developed based on sediment erosion fluxes for each particle size typical of the bottom sediments of the SIS, i.e., gravel, sand, silt and clay, using the modeled bottom shear stresses primarily due to tidal currents. This budget model presumes depth-independent sediment concentration in each water column determined by the erosion and deposition fluxes and considers horizontal suspended sediment transport to investigate where and how erosion and deposition occur. Horizontal divergence of the residual flows indicates consistency between divergence (convergence) and erosion (deposition); the divergent outflow occurs on the eroded terrains, while the convergent inflow is prevailed on the deposited terrains frequently appeared in the vicinity of the eroded terrains. The sediment budget model also exhibits that sediments are generally transported from deep to shallow areas in the eroded terrains to form the deposited terrains fringing the eroded terrains, whereas sedimentation tendency differs largely from location to location.