Stratified flows observed in nature and the laboratory often separate from the lee of topography, creating a stagnant low-flow pool of water beneath the swifter flow aloft. Existing theories and numerical models of stratified flow over topography do not exhibit this behavior because it is usually assumed that the topography is a streamline of the flow. This produces predictions of flow velocities near the topography that are often far in excess of those observed. We present observations of the flow separation of tidal flow over the Knight Inlet B.C. sill during ebb tides. Previously, it has been thought that the flow separation in Knight Inlet arose from boundary layer separation at the sharp sill crest, a process which is quite complicated to model since it depends on the accurate simulation of boundary layer processes. We show, however, that the mechanism mediating the flow separation in Knight Inlet is a dense pool of water trapped seaward of the sill that the lighter fluid simply passes over. Using a dense pool of water in a numerical model, we can simulate the development of the flow quite well without resorting to complicated boundary-layer physics.
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