Stratified flow and turbulence over an abrupt sill

This thesis presents measurement made to study flow and turbulence over a sill in Knight Inlet B.C. The sill presents a large obstacle to the barotropic tidal currents, and large internal waves are created. We made repeated velocity and density surveys during a 17 day experiment in the late summer of 1995, augmented by surveys using a microstructure profiler to study these internal waves, in particular large, non-linear waves in the lee of the sill. These observations provide a number of new insights about stratified flows over abrupt topography.; First, we examine three-dimensional flow near the sill and demonstrate the importance of topographic steering of dense water and recirculations within the lee-waves. These vortices originate from flow-separation at headlands which bracket the sill, and are first-order features to volume budgets within the wave.; We next discuss the time dependence of the lee-wave growth. We find that the flood lee-wave grows almost in phase with the barotropic tidal forcing, but that the ebb leewave is delayed until quite late in the tide. We demonstrate that this asymmetry is due to a mean density gradient across the sill, and that there is a pool of dense water trapped downstream that induces flow separation that suppresses wave growth. We also demonstrate that there is an internal wave response upstream of the sill that displaces at least as much fluid as accumulates in the lee-wave.; Last, we examine the turbulence we measured near the sill and find that the strongest turbulence is associated with the lee-waves, with diapycnal diffusivities four orders of magnitude higher than found in the open ocean. We compare the dissipation rates observed with the energy removed from the barotropic tide and find that there is a missing sink of energy. Either our sampling misses energy being dissipated near the sill, or energy is carried away by internal tides.