A two-dimensional numerical model was developed for describing the solid-liquid-phase flow in an open channel where a permeable rubble mound groin was installed. The computational domain in the analysis was divided into two regions, i.e. (1) a single-phase open channel flow and (2) a liquid-solid-phase flow consisting of an upper layer over the rubble mound groin and a porous media flow inside the groin. The whole domain was considered to have a two-layer structure with an interfacial boundary at the same level as the rubble mound's top surface. Momentum and mass exchange through the interface was taken into account by using the concept of entrainment velocity between the upper and lower layers. Numerical solutions for flow velocity and water depth were obtained not only outside but also inside the groin. In addition, the drag force in the rubble mound and the internal shear stress acting on the groin's top surface can be computed by the model. The analysis was verified through comparison with laboratory data, and good agreement between them was confirmed.