  The flow of a two--layer stratified fluid over an abrupt topographic
obstacle, simulating relevant situations in oceanographic problems, is
investigated numerically and experimentally in a simplified two--dimensional
situation. Experimental results and numerical simulations are presented at low
Froude numbers in a two-layer stratified flow and for two abrupt obstacles,
semi--cylindrical and prismatic. We find four different regimes of the flow
immediately past the obstacles: sub-critical (I), internal hydraulic jump (II),
Kelvin-Helmholtz at the interface (III) and shedding of billows (IV). The
critical condition for delimiting the experiments is obtained using the
hydraulic theory. Moreover, the dependence of the critical Froude number on the
geometry of the obstacle are investigated. The transition from regime III to
regime IV is explained with a theoretical stability analysis. The results from
the stability analysis are confirmed with the DPIV measurements. In regime
(IV), when the velocity upstream is large enough, we find that Kelvin-Helmhotz
instability of the jet produces shedding of billows. Important differences with
flows like Von Karman's street are explained. Remarkable agreement between the
experimental results and numerical simulations are obtained.
