A numerical study of residual flow induced by eddy viscosity-shear covariance in a tidally energetic estuary

The inner regime of an estuary has unique tidal mixing processes but received relatively less attention. A numerical model was developed to investigate the tidal variability of vertical mixing and the residual flow induced by eddy viscosity-shear covariance (ESCO) in the inner regime of a tidally energetic estuary in Southeastern China. Because of migration of the saltwater/freshwater interface, the water column in the inner regime undergoes a saltwater-dominant high-water period and a freshwater-dominant low-water period during a tidal cycle. The different mixing processes of high- and low-water periods led to typical (reverse) internal tidal asymmetry, i.e. stronger (weaker) mixing during flood tides than ebb tides when the tidal range was large (small). Tidal straining was the main driver of internal tidal asymmetry during the high-water period, while the asymmetries of duration and current velocity between flood and ebb were the main drivers during the low-water period. For typical internal tidal asymmetry, the ESCO stress was negative and the ESCO flow had a two-layer structure with landward flow near the bottom and seaward flow near the surface. For reverse internal tidal asymmetry, the ESCO stress was positive and the vertical pattern of the ESCO flow was reversed. The magnitude of the ESCO flow was several times greater than that of the density-driven flow. The reverse internal tidal asymmetry occurred in the freshwater-dominant low-water period indicates that the ESCO stress could be an important driver of tidal rectification flow in homogeneous coastal waters.