  From geometry and conservation we derive two nonlinear evolution equations
for sand ripples. In the case of a strong wind leading to a net erosion of the
sand bed, ripples obey the Benney equation. This leads either to order or
disorder depending on whether dispersion is strong or weak. In the most
frequent case where erosion is counterbalanced by deposition, we derive a new
one-parameter nonlinear equation. It reveals ripple structures which then
undergo a coarsening process at long times, a process which then slows down
dramatically with the growth of the ripple wavelength.
