[A-CG42] Water and sediment dynamics from land to coastal zones

[ACG42-10] Application of suspended sediment and physical process interaction to river-plume simulations in the microtidal coastal region during a flood

*Yasuhiro Hoshiba³, Yoshimasa Matsumura², Sachihiko Itoh³, Hiroyasu Hasumi¹ (1.Division of Climate System Research, Atmosphere and Ocean Research Institute, University of Tokyo, 2.Division of Marine Life Science, Atmosphere and Ocean Research Institute, University of Tokyo, 3.Center for Earth Surface System Dynamics, Atmosphere and Ocean Research Institute, University of Tokyo)

Keywords:Coastal region of river-water influence, River plume simulations, Interaction between suspended sediment and water density

Rivers transport freshwater, suspended sediment matter (SSM), and nutrients from land to coastal seas. In coastal seas termed as regions of freshwater influence (ROFIs), SSM is not only passively transported but also changes density of ambient water and influences physical characteristics especially in flood events, when a lot of SSM and nutrients are supplied to ROFIs. The interaction between SSM and water density in ROFIs can significantly change estuarine circulations and river plumes’ mode such as hypopycnal (i.e. the riverine sediment-freshwater is not denser than the seawater) or hyperpycnal (i.e., the density of the riverine sediment-freshwater mixture exceeds that of the ambient seawater) plumes. In this study here, we investigate the interactive effects on salinity, nutrient, phytoplankton distributions in river plumes, and so on. We also discuss whether the interactive effects should be considered or not in ROFIs’ simulations of various cases (e.g., changing riverine SSM-input flux, SSM particle diameter, and nutrient concentration). In order to quantitatively estimate the interactive effects of SSM, we employed a non-hydrostatic ocean model with Lagrangian particles, which represented SSM and affects the density and buoyancy of ambient water. A simple lower-trophic-level marine ecosystem model was also incorporated with the model. Many simulations (175 cases) reproducing river plumes during the developing stage initiated by a flood on a microtidal open bay were conducted to study the quantitative sensitivities of the interactive processes to the riverine SSM and nutrient flux, and SSM particle size. As a result, we strongly recommend that the interaction should be considered in hyperpycnal cases because the effects could make differences of the mean nutrient concentrations by 20 % or more in the river plumes. On the other hand, in hypopycnal cases, whether the interaction should be considered or not depends on the time scale (daily or not) and the substances (river originated or not) to focus on.