Japan Geoscience Union Meeting 2022

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[J] Oral

A (Atmospheric and Hydrospheric Sciences ) » A-OS Ocean Sciences & Ocean Environment

[A-OS19] Coastal physical processes associated with mixing, eddies, internal waves

Tue. May 24, 2022 3:30 PM - 5:00 PM 106 (International Conference Hall, Makuhari Messe)

convener:Eiji Masunaga(Ibaraki University), convener:Taira Nagai(Japan Fisheries Research and Education Agency), Eisuke Tsutsumi(Atmosphere and Ocean Research Institute, The University of Tokyo), Chairperson:Eiji Masunaga(Ibaraki University), Taira Nagai(Japan Fisheries Research and Education Agency), Eisuke Tsutsumi(Atmosphere and Ocean Research Institute, The University of Tokyo)

4:45 PM - 5:00 PM

[AOS19-06] Circulation and water exchange in shallow lakes influenced by wind-induced mixing

*Eiji Masunaga1, Tatsumi Kitamura2 (1.Ibaraki University, 2.Ibaraki Kasumigaura Environmental Science Center)

Keywords:Mixing, Lakes, Coastal areas, Water exchnage

Understanding water transport processes is a critical issue to manage water environments in lakes and reservoirs. This study presents water circulation in a continuous lake system of Lake Kasumigaura and associated water exchange processes investigated with a high-resolution numerical simulator, SUNATNS. The study area of Lake Kasumiagura mainly consists of two lakes, West Lake (Nishiura) and North Lake (Kitaura). The surface area of West Lake (172 km2) is much larger than North Lake (36 km2). The model was forced by river discharges, surface height at the downstream end of the lake system and wind forcing. Model results showed good agreements with observed flow field observed by ADCPs at four locations. The correlation coefficient of the velocity magnitude ranged 0.34–0.75 (p < 0.01). Differences between time averaged velocity magnitude from the observations and model were less than 10%. Passive tracer was released with river discharges to investigate transport and water exchange. When water in a lake is completely mixed, the concentration of initially released tracer is explained by a continuously stirred tank reactor (CSTR) theory, C0exp(-t/Tf) (where C0 is the initial tracer concentration, t is time and Tf is the flushing time scale; Tf = V/Q; V is the lake volume and Q is the river discharge). The modeled tracer concentration was lower than the CSTR theory. The concentration was 90% and 68% of the CSTR theory at Tf in West and East Lakes, respectively, which indicates that mixing in larger West Lake is stronger than that in smaller North Lake. Large scale eddies/motions exhibited in West Lake due to wind forcing leading to enhanced mixing. This study suggest that mixing induced by winds in lakes significantly influence water exchanges in lakes and reservoirs.