A computational method has been proposed to predict the flooded flows entering residential areas due to levee breaks and tsunamis. The focus is placed on the local free-surface flow whose momentum is so large that some buildings may be destroyed. The computational method is based on 3D MICS, a computational method for incompressible multiphase fields. In order to deal with complicated-shaped buildings in the areas, a tetrahedron sub-cell method has been newly introduced in 3D MICS. The arbitrarily-shaped bodies are represented as the assembly of multiple tetrahedron elements and their volumetric fraction in 3D computational cell is evaluated by the sub-cell method. A numerical model for the actual urban area near a river was set up and the artificial flooded flows were demonstrated with the present method.