The dynamics of river outflows

2018 Fiscal Year Final Research Report

• Project/Area Number: 16K17808
• Research Category: Grant-in-Aid for Young Scientists (B)
• Allocation Type: Multi-year Fund
• Research Field: Meteorology/Physical oceanography/Hydrology
• Research Institution: Kyushu University
• Principal Investigator: Kida Shinichiro 九州大学, 応用力学研究所, 准教授 (50543229)
• Research Collaborator: YAMASHIKI yosuke ; YAMAZAKI dai
• Project Period (FY): 2016-04-01 – 2019-03-31
• Keywords: 河川プリューム / 水循環 / ガンジス・ブラマプトラ川 / デルタ域

Outline of Final Research Achievements

The dynamics of the Ganges Brahmaputra river outflow is investigated using a numerical ocean model. A significant amount of freshwater is observed to enter the Bay of Bengal through the Ganges mega delta with a maximum outflow from later summer to fall. Our focus was to understand how this river outflow behaves in the presence of a mega-delta and whether the presence of the ocean plays any role in its time-variability. The model showed that the majority of the outflow enters the ocean through the main branch with the seasonal cycle of the sea surface height analogous to observations. While the flow through the narrow branches within delta was weak, we found its time-variability to be strongly influenced by that of the main branch and the ocean. This was because the outflow establishes a geostrophic coastal current along the coast that alters the sea surface height.

Free Research Field

海洋物理学

Academic Significance and Societal Importance of the Research Achievements

河川と海洋の流れ場や変動場をコントロールする力学過程は、これまでそれぞれ独立なものとして理解が進んできた。本研究から河川と海洋の接続域であるデルタ域では、河川の本流と支流の流れが海洋の存在を通じて連動していることが明らかになった。ガンジス・ブラマプトラ川のようなデルタ域は洪水が数多く発生する地域であるとともに多くの人が生活する場でもあるため、海水と河川水の境界線の位置や海水の遡上のメカニズムは、土地の利用方法にも大きな影響をもつ。この研究成果は気候変動に伴う大規模河川の河口域における環境変動の予測精度の向上に繋がることが期待できる。