Variations of longitudinal diffusivity in the Hudson Estuary

Abstract

The coefficient of longitudinal diffusion for salt has been calculated from the distribution of salinity observed in the Hudson Estuary at nine different times during 1974. The salinity distribution appears to be quasi steady-state, and the diffusion coefficient is spatially constant between the Upper Bay and Verplanck. The diffusion coefficient varied in time by a factor of three. It was not well correlated with the stratification parameter. It was slightly less dependent on the freshwater discharge in the Estuary than on the tidal amplitude, which varies by a factor of nearly two between spring and neap tides. Salinities predicted by a model are slightly less accurate if the diffusion coefficient depends on the stratification parameter, than if the diffusion coefficient is kept constant. If the diffusion coefficient is a power function of both fresh water discharge and tidal amplitude, salinity predictions are significantly improved. These results suggest that density-induced, gravitational, vertical circulation does not dominate the longitudinal diffusion of salt in the Hudson Estuary. Transverse circulation may be at least as significant a salt transport mechanism as vertical circulation. The predictive reliability of a one-dimensional, advective-diffusive model of the salinity distribution in the Hudson Estuary depends on a realistic, variable coefficient of longitudinal diffusion for salt. Furthermore, such a model cannot use the same coefficient to predict the distribution of other properties unless the combination of transport mechanisms for these other properties is the same as that for salt.