  Periodic ripples generated from the off-normal incidence ion beam bombardment
of solid surfaces have been observed to propagate with a dispersion in the
velocity. We investigate this ripple behaviour by means of a Monte Carlo model
of the erosion process, in conjuction with one of two different surface
diffusion mechanisms, representative of two different classes of materials; one
is an Arrhenius-type Monte Carlo method including a term (possibly zero) that
accounts for the Schwoebel effect, the other a thermodynamic mechanism without
the Schwoebel effect. We find that the behavior of the ripple velocity and
wavelength depends on the sputtering timescale, qualitatively consistent with
experiments. Futhermore, we observe a strong temperature dependance of the
ripple velocity, calling for experiments at different temperatures. Also, we
observe that the ripple velocity vanishes ahead of the periodic ripple pattern.
