A simulation study on effects of suspended sediment through high riverine discharge on surface river plume and vertical water exchange

Abstract

Rivers transport freshwater and suspended sediment matter (SSM) from land to coastal seas. In coastal seas termed as regions of freshwater influence (ROFIs), SSM is not only passively transported but also changes the density of ambient water and influences the physical characteristics especially in flood events, when a lot of SSM is supplied to the sea. Although the influence of SSM on the physical field in ROFIs would be significant, interactive physical processes, such as dynamics of river plumes and estuarine circulations, have hardly been investigated for hypopycnal plumes (i.e., the riverine sediment-freshwater is not denser than the seawater). In order to quantitatively estimate the interactive effects of SSM, we employ a non-hydrostatic ocean model with Lagrangian particles, which represents SSM and affects the density and buoyancy of ambient water. We use two experimental settings: (1) realistic simulations of the Tango Bay, Japan under the flooding of September 2013 and (2) idealized simulations for an open-bay ROFI. The former is conducted to assess whether the simulations could reproduce an actual event to some extent. The realistic simulations demonstrate that the choice of parameters such as SSM-particle size and composition is important for coastal simulations of flood events. The latter is conducted to understand the basic physics and to study the quantitative sensitivities of the physical processes to the riverine flux, composition, and particle size of SSM. The idealized simulations demonstrate that a large amount of riverine SSM affects the physical field in ROFIs through the following process: 1) horizontal density differences between nearshore and offshore waters are reduced as apparent density is increased close to the river mouth by riverine SSM, 2) the strength of vertical circulation is weakened by the reduced horizontal density difference, and 3) vertical water exchange between the surface and the subsurface layers decreases. The process in the control case of this study increases the relative amounts of surface freshwater in the river plume by 0.8% of the total riverine freshwater input. Sensitivity experiments with changing the parameters of SSM flux into the river, SSM composition, and SSM-particle diameter show that the percentage can be raised up to 2%. Meanwhile, the above-mentioned processes do not apply to extreme cases of small particle size and enormous SSM input wherein homopycnal (i.e., a similar density between the riverine sediment-freshwater and the ambient seawater) and hyperpycnal (i.e., the density of the riverine sediment-freshwater mixture exceeds the ambient seawater) plumes take place.