A simulation study on effects of suspended sediment through high riverine discharge on surface river plume and vertical water exchange
A simulation study on effects of suspended sediment through high riverine discharge on surface river plume and vertical water exchange
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DOI：
10.1016/j.ecss.2019.106352
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发表时间：
2019-11-15
影响因子：
2.8
通讯作者：
Suzuki, Keita W.
中科院分区：
文献类型：
--
作者：
Hoshiba, Yasuhiro;Matsumura, Yoshimasa;Suzuki, Keita W.
河流将淡水和悬浮沉积物从陆地输送到近海。在被称为淡水影响区（ROFIs）的近岸海域，当大量的SSM进入海洋时，SSM不仅被动地迁移，而且改变环境水体的密度，影响环境水体的物理特性，尤其是在洪水事件中。虽然SSM对ROFI中物理场的影响是显著的，但交互物理过程，如河流羽流和河口环流的动力学，几乎没有研究过低密度羽流（即，河流沉积物--淡水的密度不比海水大）。为了定量估计SSM的相互作用，我们采用了一个非静力学海洋模式与拉格朗日粒子，它代表SSM和影响周围水的密度和浮力。我们使用两个实验设置：（1）现实模拟的探戈湾，日本在2013年9月的洪水和（2）理想化的模拟一个开放的海湾ROFI。前者是为了评估模拟是否能在一定程度上再现实际事件。现实的模拟表明，选择参数，如SSM颗粒的大小和组成是重要的沿海模拟洪水事件。后者是进行了解基本的物理和研究的物理过程的定量敏感性河流通量，成分和颗粒大小的SSM。理想化的模拟表明，大量的河流SSM通过以下过程影响ROFI中的物理场：1）近岸与近海沃茨之间的水平密度差减小，河口附近的表观密度增加，2）水平密度差减小，垂直环流强度减弱，（3）地表与次表层之间的垂直水交换减少。在本研究的控制情况下的过程中增加了0.8%的总河流淡水输入的河流羽流中的地表淡水的相对量。通过改变SSM入河通量、SSM成分、SSM粒径等参数的敏感性实验表明，该比例可提高到2%。同时，上述过程不适用于小颗粒尺寸和巨大SSM输入的极端情况，其中同密度（即，河流沉积物-淡水和环境海水之间的相似密度）和超密度（即，河流沉积物-淡水混合物的密度超过环境海水）羽流发生。
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.