Tide-induced Long-term Oscillations in a Subterranean Estuary

Numerical simulations of coupled flow and solute transport processes in the intertidal zone of a subterranean estuary revealed significant oscillations of pore water salinity, velocities and fluxes over periods much longer than the primary tidal cycle. The occurrence of these long-term oscillations and their periods depend on aquifer properties and boundary forcing conditions. The critical conditions and extent of the long-term oscillations were found to be determined by three non-dimensional numbers: (1) a gravity number indicating if long-term oscillations can be initiated; (2) a critical Rayleigh number predicting the development of long-term oscillations; and (3) a perturbation parameter correlating with the oscillation period. In contrast with the quasi-steady condition observed previously, the long-term oscillations represent an important characteristic of tidally influenced subterranean estuaries. These oscillations intensify mixing of discharging freshwater with recirculating seawater and affect the fate of chemicals in the subterranean estuaries and subsequent discharge to the ocean.