  Transcritical flows in free surface hydrodynamics emulate black hole horizons
and their timereversed versions known as white fountains. Both analogue
horizons have been shown to emit Hawking radiation, the amplification of waves
via scattering at the horizon. Here we report on an experimental validation of
the hydrodynamic laws that govern transcritical flows, for the first time in a
free surface water channel using an analogue space-time geometry controlled by
a bottom obstacle. A prospective study, both experimental and numerical, with a
second obstacle downstream of a first one is presented to test in the
near-future the analogous black hole laser instability, namely the
super-amplification of Hawking radiation by successive bounces on a pair of
black and white horizons within cavities which allow the presence of negative
energy modes necessary for the amplification process. Candidate hydrodynamic
regimes are discussed thanks to a phase diagram based on the scaled relative
heights of both obstacles and the ratio of flow to wave speed in the upstream
region.
