Mechanisms of salt transport by the advective and tidal oscillatory flux in the macro-tidal estuary, Gyeonggi Bay in South Korea

Many estuaries have been damaged by material movements like marine debris, suspended sediment, and pollutants. Understanding the estuarine circulation system is necessary to solve such problems. The salt transport analysis provides hydrodynamic processes about material circulation in the estuary. In this study, to understand the mechanisms of salt transport, a three-dimensional hydrodynamic model was applied in the hyper-tidal estuary system, Yeomha Channel, Gyeonggi Bay. The simulation period of the model was a total of 245 days (January 20 to September 20, 2020), including the dry and wet seasons. The model results for the temporal variation of tide, current velocity, and salinity were validated by comparing them with the observed in-situ data. The salt transport (F_S) was calculated in three cross-sections of the Yeomha Channel and was decomposed into three components (Q_fS₀: advective salt transport; F_E: steady shear dispersion; F_T: tidal oscillatory salt transport). During the dry season with strong tidal forcing, the salt transport patterns were mainly dominated by Q_fS₀. During the wet season with large river discharge, the salt transport patterns were determined by the balance between Q_fS₀, F_E, and F_T. The long-period tidal constituents (MS_f and M_m) were the main mechanisms causing Q_fS₀ with the spring-neap variation during the dry season. The tidal trapping effect, caused by the phase difference of less than 90° between tidal current and salinity, generated landward F_T in the dry and wet seasons. In addition, the high river discharge during the wet season decreased the phase difference between tidal current and salinity to less than 70°, resulting in a much strong landward F_T. This study suggests that the long-period tidal constituents and tidal trapping effect are unique characteristics that contribute to material circulation in the hyper-tidal estuary.