A Numerical Study of Salt Fluxes in Delaware Bay Estuary

摘要

The results of a numerical study of Delaware Bay using the Regional Ocean Modeling System (ROMS) are presented. The simulations are run over a range of steady river inputs and used M_2 and S_2 tidal components to capture the spring-neap variability. Results provide a description of the spatial and temporal structure of the estuarine exchange flow and the salinity field, as well the along-channel salt flux in the estuary. The along-channel salt flux is decomposed into an advective term associated with the river flow, a steady shear dispersion F_e associated with the estuarine exchange flow, and a tidal oscillatory salt flux F_t. Time series of F_e and F_t show that both are larger during neap tide than during spring. This time variability of F_t, which is contrary to existing scalings, is caused by the lateral flows that bring velocity and salinity out of quadrature and the stronger stratification during neap tide, which causes F_t to be enhanced relative to spring tide. A fit for the salt intrusion length L with river discharge Q for a number of isohalines is performed. The functional dependences of L with Q are significantly weaker than Q~(-1/3) scaling. It is concluded that the response of the salt field with river discharge is due to the dependence of F_e and F_t with Q and the relative importance of F_t to the total upstream salt flux: as river discharge increases, F_e becomes the dominant mechanism. Once F_e dominates, the salt field stiffens because of a reduction of the vertical eddy viscosity with increasing Q.