This work focuses on the dynamic behavior of bridge piers subjected to scour. Here, the paper is divided on two parts. The first part considers the model of bridge pier by assuming a rocking solid partially embedded in a Winkler soil with translational and rotational conditions at its base. Simple geometry and boundary conditions of bridge pier are represented because the aim of this work is is to show the feasibility of a new method based on free response analysis for bridge piers subjected to scour in general case. In fact, this physical model coupling solid mechanics for the structure and the continuum mechanics for the soil makes it possible for us to identify experimentally two rocking modes. In that way, the second part shows an experimental campaign in laboratory implemented on reduced pier models embedded in Fontainebleau sand with different geometries and inertias. From frequency decomposition of signals, natural frequencies and shape modes highlighted by the model are identified and compared from experiments. Analytical formulations and experiments show the interest to use vibration-based monitoring for scouring.