The Structure and Dynamics of an Estuarine Tidal Intrusion Front

Tidal intrusion fronts are surface convergences that occur at constrictions in estuaries during the flood tide, separating incoming higher-salinity water from lower-salinity, stratified estuarine water. Previous observations of tidal intrusion fronts describe a V-shaped planform, with the apex of the V pointing into the estuary, however the significance of this structure has not been previously explained. Observations near the mouth of the James River estuary during the flood tide reveal the development of a quasi-steady, V-shaped front. Considering a reference frame oriented normal to the front, the velocity and density structure are consistent with gravity-current dynamics, but the oblique orientation of the front relative to the impinging flow indicates strong, along-front shear, which results from vorticity produced by flow separation at the lateral boundaries as well as topographic torque from upstream. The combination of convergence and along-front shear leads to enhanced mixing, as revealed by acoustic backscatter images of shear instability and persistent subcritical gradient Richardson number in the frontal zone. Oblique fronts such as this tidal intrusion front are common features of estuaries, and they play an important role in vertical exchange due to subduction and mixing of surface water. Surface fronts are among the most notable features of estuarine and coastal environments. They have important consequences for the physics as well as the biology, due to the strong gradients, aggregation of particles and organisms, and mixing that occur at fronts. One of the most conspicuous types of estuarine fronts is the tidal intrusion front, which forms near the mouth during the incoming tide and usually exhibits a V shape pointing into the estuary. This study explains the processes responsible for the V-shaped configuration of the front and their implications. The density contrast across the front is one important ingredient of the dynamics, resulting in a strong convergence of surface flow. The other key ingredient is the shear across the front, with faster inflow on the inside of the V and weaker flow on the outside. This shear is due in large part to "flow separation" at the sides of the estuary-the same process that causes stall of airplane wings or the formation of eddies behind rocks at the edges of a stream. The paper shows that this combination of processes leads to strong mixing in the frontal zone-much stronger than would occur in the absence of the cross-front shear. The "V"-shaped tidal intrusion front exhibits a combination of density-driven, convergent flow and along-front shear resulting from lateral flow separation The along-estuary flow at the front is supercritical (with respect to gravity current propagation), but the velocity in the front-normal direction is critical The along-front shear significantly augments vertical mixing within the frontal zone relative to the mixing in normal gravity currents