Numerical experiments are performed to investigate the oscillatory flow over a two-dimensional wavy wall characterized by a large amplitude, such as to induce flow separation. Even though the Reynolds number is moderate, a three-dimensional turbulent flow is observed. The turbulence dynamics is characterized by the presence of coherent ribs superimposed on the main spanwise vortices generated by the roll-up of the free vortex sheets shed at the crests of the wall waviness. The ribs are formed by the stretching of vorticity patches which are generated by the instability of the two-dimensional flow at two different locations. The first are the saddle points of the flow field created, far from the wall, by the vortex pairs generated every half-cycle. The second are the saddle points created, close to the upstream side of the wavy wall, by the combined action of the free-stream flow and of the vortex structures shed by the ripple crests. Later, the ribs wrap around the main spanwise vortices and cause the distortion of these vortices and the alignement of the vortex lines with the free-stream flow, thus inducing large contributions to the coherent helicity. Simultaneously, regions of high dissipation appear which tend to be separated from those characterized by large values of helicity.