  An experimental study of a granular surface submitted to a circular fluid
motion is presented. The appearance of an instability along the sand-water
interface is observed beyond a critical radius $r_c$. This creates ripples with
a spiral shape on the granular surface. A phase diagram of such patterns is
constructed and discussed as a function of the rotation speed $\omega$ of the
flow and as a function of the height of water $h$ above the surface. The study
of $r_c$ as a function of $h$, $\omega$ and $r$ parameters is reported.
Thereafter, $r_c$ is shown to depend on the rotation speed according to a power
law. The ripple wavelength is found to decrease when the rotation speed
increases and is proportional to the radial distance $r$. The azimuthal angle
\az of the spiral arms is studied. It is found that \az scales with $h\omega
r$. This lead to the conclusion that \az depends on the fluid momentum.
Comparison with experiments performed with fluids allows us to state that the
spiral patterns are not the signature of an instability of the boundary layer.
