This paper proposes a method for structural monitoring using operational modal analysis (OMA). Frequency domain decomposition (FDD) of the output-only system is developed to define new modal parameters to be monitored. Parameters from OMA make it possible to characterize the structure using experimental modal analysis EMA). The method is applied to a bridge pier subjected to scour. Indeed, a prestressed concrete, motorway bridge on A71 in France has been equipped with sensors to follow in real time the vibrational behavior and external events such as temperature, hydraulic flow velocity and scour depth. This measuring chain is connected to a central acquisition system. Collected data are post-processed in the frequency domain combining a standard method for modal parameter estimation and statistic methods using averaging and Principal Component Analysis (PCA). A fully automated method is developed by correlating operational modal parameters with data of external event. Finally, an assessment of the evolution of the structural health condition of the bridge in real time is carried out.