A numerical study on salinity stratification at the Oujiang River Estuary, China

The Oujiang River Estuary(ORE) is a macrotidal estuary with drastic variation of river discharge and large tidal range. Numerical simulations based on the unstructured grid, Finite-Volume, primitive equation Community Ocean Model(FVCOM) are conducted to investigate the intratidal and intertidal variations of salinity with an extremely upstream river boundary and large computational domain. The dynamic equation of potential energy anomaly is adopted to evaluate the stratification and mixing processes from model results. Meanwhile, the stability of estuarine stratification on different timescales and its spatial variation are studied using estuarine Richardson number and stratification parameter. The critical values of tidal range and river discharge that determine the stratification state are obtained. The critical values exhibit distinct spatial difference. The north branch of the ORE exhibits well-mixed conditions when the tidal range exceeds 3.8, 4.0 and 4.6 m at upper inlet,middle segment and the river mouth, respectively. When river discharge is below 280 m3/s or exceeds 510 m3/s,the upper part of the north branch is well-mixed sustainably. Near the river mouth, river discharge of 280 m3/s is a rough critical value that separates well-mixed and stratified states. It is also concluded that periodic stratification exists in the North Channel. The lower estuary appears to be partially stratified at early ebb or early flood tide, and well-mixed in other tidal stages. The stratification only develops during early ebb in the upper segment. The enhancement of stratification is mainly caused by longitudinal advection and lateral velocity shear, while turbulent mixing and longitudinal tidal strain are the main factors of stratification attenuation.