Tide-Induced Variation of the Dynamics of a Salt Wedge Estuary

The time variations of the salt wedge intrusion in the Fraser estuary, British Columbia, were monitored during a variety of tidal and run-off conditions using instruments and sampling methods that provided high resolution of the velocity and water properties in space and time. The salt wedge was found to vary considerably in position and vertical structure through the tidal cycle due to the interaction of the tidal flow with the density-driven motion of the salt wedge. During the flood, the salt wedge progressed up the estuary as a gravity current, while during the ebb the salinity structure was eroded by shear instability. The difference in character of the flow between flood and ebb is attributed to the transition between a subcritical and supercritical internal Froude number. During flood tide, the internal state is subcritical, and the estuarine shear flow is maintained with a small amount of vertical mixing, while in the supercritical ebb flow, vertical shears become so large that the pycnocline becomes unstable, and the salt wedge structure breaks down.