Salt fluxes and volume transports in an estuary vary considerably over subtidal time scales of a few days to weeks in response to wind and neap–spring tidal forcings. Results from a numerical simulation of the Hudson–Raritan estuary are used to study subtidal variations of salt fluxes and the physical mechanisms for salt balance in the estuary. Simulated salt fluxes are compared with available observations. Observations support the model's finding that analysis of volume and salt fluxes based on short-length data records (<30 days) can lead to misleading conclusions.
“Tidal trapping” effects due to coastline irregularities contribute most to the salt balance at the Sandy Hook–Rockaway Point transect and at the Narrows. A two-week observational record is analyzed to support this finding. Simulated subtidal variation of the tidal trapping term at the Sandy Hook–Rockaway Point transect compares well with that observed. In Raritan Bay, where tidal currents are weak and effects of winds are significant, contributions to salt balance from vertical velocity and salinity gradients are comparable to transverse contributions. This occurs despite the fact that surface-to-bottom salinity differences during the simulation period—a period of low freshwater flow-never exceed 0.5‰ throughout most regions of the bay. A two-dimensional, depth-integrated xy–t model, in which the horizontal dispersion coefficients are modeled empirically, may not perform well in this case.