Salt Intrusion and Secondary Flow in a Shallow, Well-mixed Estuary

Observations of an axial convergence front formed within the Conwy estuary, North Wales, U.K., during the flood tide were indicative of density-driven secondary circulations. It has been proposed that these were the result of lateral density gradients established by the interaction of the lateral shear of the longitudinal currents with the axial salinity gradients through a differential-advection mechanism. In the present study, the evolution of axial salinity gradients during a tidal cycle along the length of a shallow well-mixed estuary was examined using six recording instruments deployed over a 25-day period. The evolution of channel geometry and of the lateral shear of axial current speed was examined using simple models in conjunction with the observations of near-bed pressure and current speed. It is hypothesized that the complex interaction between these three parameters (channel geometry, lateral shear of axial current and axial salinity gradient) results in the observed evolution of secondary flow throughout the tide, which itself causes a redistribution of the density field.