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

[ACG42-P01] Impact of a semi-enclosed shallow estuary on freshwater outflow from a small-scale river

*Shinichiro Kida¹, Kiyoshi Tanaka², Tomonori Isada³, Tomohiro Nakamura⁴ (1.Research Institute for Applied Mechanics, Kyushu University, 2.Atmosphere and Oceanic Research Institute, The University of Tokyo, 3.Akkeshi Marine Station, Hokkaido University, 4.Institute of Low Temperature Science, Hokkaido University)

Keywords:River runoff, Coast, Estuary

Shallow and large estuaries are often found at river mouths, limiting the direct input of freshwater from the river to the ocean. An idealized numerical model was used to understand how such a lagoon-type estuary may change the dynamics of freshwater outflows from rivers compared to a classical river plume established by large-scale rivers. Model experiment shows that the water mass exchange flow between the estuary and the ocean occurs with significant time variability across the narrow inlet channel by forming freshwater and oceanic water plumes. A freshwater plume forms in the ocean from the late-ebb to early-flood period while an oceanic plume forms in the estuary from the late-flood to early-ebb period. The area size of the estuary determines the strength of the oceanic plume that intrudes into the estuary and the shallow depth makes this plume barotropic and well-mixed in the vertical. If the estuary is deep, an oceanic plume will not form, and a classical 2-layer estuary circulation will establish. The tidally forced plumes are found to play an important role by exporting freshwater from the estuary to the ocean through eddy freshwater flux as the strength of the tidal exchange flow overwhelms that expected for the estuary circulation.