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The flood risk of the Naka River is much higher than the other rivers in the Fukuoka Metropolitan area. The Naka River watershed has the dense downtown along the lower river reach. If the Naka River and its tributaries would overflow in the lower river reach, significant economic loss would be caused. The urban flood simulation detects areas of higher flood-risk in the downtown. The urban flood simulation is divided into two parts; one is one-dimensional flood flow simulation along the lower Naka River reach and the other is two-dimensional flood flow simulation on the downtown area. Furthermore, the latter is subdivided into two sub-parts; one is two-dimensional flood flow simulation for the ground level and the other is that for the underground space. The land use in the downtown is complicated. The land use in the downtown is incorporated into the simulation.