Zou, T.; Zhang, H.; Meng, Q. J., and Li, J., 2016. Seasonal hydrodynamics and salt exchange of a shallow estuary in Northern China.

Estuaries are important components of coastal ecosystem and function as dominant pathways of material exchange at the land-sea interface. The transport of terrestrial input through river inflow is controlled by physical process including tides, waves, and fresh water discharge. This study investigates hydrodynamic characteristic and salt flux within Xiaoqinghe River (XQR) estuary, which is a shallow estuarine system (water depth < 8 m) and exports substantial amount of nutrients and pollutants to the adjacent Laizhou Bay. Profile velocity and salinity are measured using ADCP and CTD through complete tidal cycles (25 hours) in April, July and September 2013. The instantaneous velocity and salinity data are decomposed into time-averaged means and time-varying components based on the improved Kjerfve (1986) method to quantify the contributions of various physical processes. The results showed that the XQR estuary had irregular semidiurnal tide with tidal form number of 0.5–0.7 and tidal range about 2 m. River discharge dominated the seasonal variation of tidal elevation, longitudinal velocity, and salinity. The river mouth was well mixed based on calculated layer Richardson number, and can be classified as Type 1b and 2b according to stratification-circulation diagram. The net salt flux was directed seaward during monsoon season and landward in non-monsoon seasons. Fluvial advective flow dominated the seaward salt flux throughout the year and contributed 78.9% of the net salt flux in monsoon. Stokes' drift and tidal sloshing dispersion flux dominated the landward salt flux, and contributed 72% and 57.5% of total salt flux in April and September, respectively. The cross-correlation between tide and salinity, and vertical shear, were only of marginal importance. Overall, tidal sloshing and Stokes' drift is the underlying process of salt transport while river discharge dominates its seasonal variation. This study revealed the inherent unsteadiness of the salt balance, and provided the scientific foundation for effective management of freshwater release from upstream during difference seasons.