Laterally Divergent, Supercritical Flow Causes Intense Mixing

Intense mixing occurs in estuaries and river plumes where topographic forcing leads to hydraulic transitions to supercritical flow. Observations in the salt-wedge estuary of Connecticut River indicate that laterally divergent topography, both inside the estuary and at the mouth, provides the forcing for the transition to supercritical flow and intensified mixing. The zones of mixing are readily identifiable based on echo-sounding images of large-amplitude shear instabilities. The gradient Richardson number (Ri) averaged across the mixing layer decreases to a value very close to 0.25 during a most of the active mixing phase. The along-estuary variation in internal Froude number and interface elevation are consistent with a steady, two-layer hydraulic representation that includes a parameterization for interfacial stress based on adjustment to a critical value of Ri. The hydraulic calculation clearly indicates that the mixing results from lateral straining of the shear layer, and instabilities develop so rapidly that the overall flow is maintained at the mixing threshold value of Ri=0.25, even with continuous, active mixing. The amount of mixing is constrained by and can be predicted by the overall flow geometry, providing a means of estimating mixing and entrainment in hydraulically controlled flows in estuaries, river mouths and the ocean.