  We are interested in a model of rotating fluids, describing the motion of the
ocean in the equatorial zone. This model is known as the Saint-Venant, or
shallow-water type system, to which a rotation term is added whose amplitude is
linear with respect to the latitude; in particular it vanishes at the equator.
After a physical introduction to the model, we describe the various waves
involved and study in detail the resonances associated with those waves. We
then exhibit the formal limit system (as the rotation becomes large), obtained
as usual by filtering out the waves, and prove its wellposedness. Finally we
prove three types of convergence results: a weak convergence result towards a
linear, geostrophic equation, a strong convergence result of the filtered
solutions towards the unique strong solution to the limit system, and finally a
"hybrid" strong convergence result of the filtered solutions towards a weak
solution to the limit system. In particular we obtain that there are no
confined equatorial waves in the mean motion as the rotation becomes large.
