Three-dimensional hydrodynamic modelling of saltwater ingression and circulation in a Brackish Lake Shinji, Japan.

• This study examines lake Shinji, a mesohaline lake with intermittent saline water intrusion. • It employs a 3-D hydrodynamic model, EcoPARI, in hindcast mode for 2012 (Jan-Sep). • The focus was on optimizing the vertical turbulence diffusion model and identifying ideal parameters. • Differential model constants for temperature and salinity are explored, enhancing simulation results. • The tuned model successfully replicates a historical upwelling event in September, driven by strong westerly winds from a nearby typhoon. Lake Shinji is a mesohaline lake that has intermittent saline water intrusion from its east coast connection of Ohashi River. This intrusion is often in the shape of a very thin layer that moves along the Lake bottom and stayed at the center due to bowl shape bathymetry of the Lake having maximum depth at the center. To elucidate the intrusion mechanism a 3-D hydrodynamic model (EcoPARI) was employed in a hindcast mode covering the year 2012 from the beginning of January to the end of September. Initially, the core emphasis was given to the vertical turbulence diffusion model and optimum model parameters were determined to accurately simulate the hydrodynamics. It was highlighted by the numerical simulations that alteration of one model constant, which is the multiplying coefficient of Richardson number, "α", is influential instead of altering all the constants to tune the turbulence model. The possibility of using differential model constants for temperature and salinity was also tested and it helped in the improvement of simulations. Finally, the performance of the model was also assessed under extreme weather conditions and a historical upwelling event was simulated. The model was able to reproduce the intrusion in the summer months as well as the upwelling event in September due to strong westerly winds caused by a nearby passing typhoon. [ABSTRACT FROM AUTHOR]