Gravity current flow over topography with a two-layer stratified ambient*

We report upon a laboratory experimental study of dense lock-released gravity currents propagating through a two-layer ambient and over sinusoidal topography of amplitude $A$ and wavelength $\lambda$ or $2\lambda$. Particular emphasis is placed on a Boussinesq flow regime and the initial (or ``slumping'') stage of motion. Because of the presence of the topography, the height of both the lower layer and the channel varies in the downstream direction. In contrast to the flat-bottom case, the gravity current front therefore accelerates and decelerates as it respectively flows up- and downhill. Overall, the topography has a retarding effect on the average front speed, $U_{\mathrm{avg}}$, whose variation with $A$, the layer densities and the interface height is described. The topography also alters the structure of the gravity current head by inducing large-scale vortices in regions characterized by a substantial shear flow. As in the flat-bottom case, the forward advance of the gravity current can excite a downstream-propagating interfacial wave. We identify the parametric region corresponding to wave generation.