Local scour around the river bridge pier foundation during flood and heavy rains often causes critical tilting and settlement of the river bridge. In this study, therefore, a series of hydraulic model tests using middle size channels was performed, focusing on the destabilization of river bridge piers due to local scour. The subsoil reaction forces acting on the bottom surface of the pier model was measured at four positions, the horizontal displacement at the top of the pier and the natural frequencies of the pier model were measured during the tests. The experiments revealed that the natural frequency was the most sensitive index in detecting instability of the pier model due to local scour, among the horizontal displacement, ground reaction force distribution and natural frequency. Next, changes in the subsoil reaction forces distribution were detected, followed by changes in the horizontal displacement of the pier tops.