In recent years, since it has been confirmed that the piers of old bridges collapse and the entire bridge is washed away, it is necessary to quantitatively evaluate the fluid force acting on the piers and bridge girders during floods. In two-phase flow, the hydraulic force greatly depends on the water surface shape of the surface of the structural member, the river volume inhibition rate, and the degree of inundation. In this study, the finite volume method is analyzed using OpenFOAM, and while comparing single-phase flow and two-phase flow, inundation with respect to the water surface shape, river volume inhibition rate, and girder height of the pier surface is performed. It was found that the drag coefficient of the structur al member is constant in the region.
