Stratified Flow Over Topography and the Generation of Internal Solitary Waves

Internal solitary waves, which form such a prominent feature in remote sensing images of the coastal zone, are the far field expression of stratified flow over topography. Nonlinear evolution of large amplitude internal tides propagating over the continental shelf is one means by which they might be generated, but detailed observations of wave generation over topography in an inlet illustrate other mechanisms that occur when the topography is abrupt. Tidally forced flow over abrupt topography can lead to streamline splitting. The streamline bifurcation leads to topographic control of the flow beneath the bifurcated layer, even though a simple calculation of the densimetric Froude number using tidal velocities and the unperturbed density field would imply that the flow was subcritical. Its evolution over the changing tidal current is intimately linked to the formation and suppression of boundary layer separation. The split streamlines enclose a slowly moving and weakly stratified layer accompanied by shear flow instability which itself may be a source of nonlinear internal waves. Moreover the collapse of this slowly moving layer as the tidal current slackens generates an internal undular bore. Observations are required to show the generality of these different mechanisms, especially in areas where solitary waves are commonly observed over the continental shelf.