In this study, the occurrence of vortices entering the intake of point sink character is explained by fluid mechanics principles. All types of boundaries (air, solid, fluid interface and flow-dividing subsurface) cause obstruction effects on the intake flow. A vortex basically originates from the boundary that has obstruction effects on the intake flow. All vortices originating from the free-surface, flow-dividing surface, fluid-interface and solid boundaries are "subsurface-vortices" of the same nature. Experiments included in this study have indicated that strainer-conic and a strainer-pipe type structures prevent the vortices effectively.