This work deals with the dynamics of sand ripples which occur on a flat sandy sea bed excited with an oscillatory flow in a wave flume. The bathymetry of the bed is reported since the first stages of ripple formation with an optical technique. A statistical approach is developed to study the dynamical evolution of ripples. With an identical mobility number and different Reynolds number, two tests leading to 2D and 3D pattern formation are compared. Two stages are detected during ripple formation independently of pattern morphology. However, pattern tridimensionality can be detected with statistical tools. 2D and 3D equilibrium ripple geometric data are compared with the literature.