  We report on observations made on a run of transcritical flows over an
obstacle in a narrow channel. Downstream from the obstacle, the flows
decelerate from supercritical to subcritical, typically with an undulation on
the subcritical side (known in hydrodynamics as an undular hydraulic jump). In
the Analogue Gravity context, this transition corresponds to a white-hole
horizon. Free surface deformations are analyzed, mainly via the two-point
correlation function which shows the presence of a checkerboard pattern in the
vicinity of the undulation. In non-gated flows where the white-hole horizon
occurs far downstream from the obstacle, this checkerboard pattern is shown to
be due to low-frequency fluctuations associated with slow longitudinal movement
of the undulation. It can thus be considered as an artifact due to a
time-varying background. In gated flows, however, the undulation is typically
"attached" to the obstacle, and the fluctuations associated with its movement
are strongly suppressed. In this case, the observed correlation pattern is
likely due to a stochastic ensemble of surface waves, scattering on a
background that is essentially stationary.
