  We numerically study the scattering coefficients of linear water waves on
stationary flows above a localized obstacle. We compare the scattering on
trans- and subcritical flows, and then focus on the latter which have been used
in recent analog gravity experiments. The main difference concerns the
magnitude of the mode amplification: whereas transcritical flows display a
large amplification (which is generally in good agreement with the Hawking
prediction), this effect is heavily suppressed in subcritical flows. This is
due to the transmission across the obstacle for frequencies less than some
critical value. As a result, subcritical flows display high- and low-frequency
behaviors separated by a narrow band around the critical frequency. In the
low-frequency regime, transmission of long wavelengths is accompanied by
non-adiabatic scattering into short wavelengths, whose spectrum is
approximately linear in frequency. By contrast, in the high-frequency regime,
no simple description seems to exist. In particular, for obstacles similar to
those recently used, we observe that the upstream slope still affects the
scattering on the downstream side because of some residual transmission.
