Assessing bottom water movement during winter turnover in the deep Ogouchi Reservoir by particle tracking method

Turnover is a natural phenomenon in lakes and reservoirs that affects water quality. Previous research has evaluated the occurrence and non-occurrence of turnover through data analysis or simulations of dissolved oxygen and water temperature. This study implemented the three-dimensional (3D) environmental hydrodynamic model, Fantom-Refined, along with the application of buoyant particles in the Ogouchi Reservoir, to visualize individual particle movements in detail and gain insights into the turnover process. The comparison of observed water temperature data for two years with different bottom hypoxia conditions showed that the presence or absence of turnover was separated by a thermal gradient (TG) of 0.0035 °C m^-1. Furthermore, it was shown that strong winds exceeding 5 m s^-1 had an effect on the timing of turnover. The 3D hydrodynamic simulation showed that strong winds cause patchy positive and negative vertical velocities in the plane. The vertical velocities were several times larger than the horizontal velocities, which resulted in sharply angled descending and ascending currents. When particle tracking calculations were performed for two periods of comparable strong wind speeds, particles placed at the bottom only moved horizontally and did not ascend when TG was above the threshold. However, when TG was below the threshold, bottom particles were captured by the patchy vertical velocities, causing them to ascend to the surface before descending back to the bottom within a few hours. Although the turnover is well known, the detailed mixing structure has not been indicated by any previous studies. This study visually and statistically demonstrated the process of winter turnover through velocity calculations and particle tracking.