Open-channel flows with a relatively large-scale side cavity zone exhibits water surface oscillations in spanwise or streamwise directions. Although the side cavity zone is installed for improving the water-friendly environment, such oscillations, once occurred in the actual river in the urban areas, may cause flooding, resulting in a large damage in the residential area. In order to investigate the flow features associated with such surface oscillation, the authors have developed a 3D LES model that utilizes a density function for detecting air-water interface. With the developed model, it becomes possible to simulate water surface oscillation observed in the experiments. In this paper, the flow features related to the water surface oscillation as well as the local flow features along the interface between the main channel and the side cavity zone are investigated for the subcritical flows with relatively large Froude number.