The design rainfall for flood control planning is assumed to have 24 or 48 hours duration and spatially uniform distribution. However, the heavy rainfall which has short duration and small area seems to put small rivers at risk. The purpose of this study is to reveal the effects of Depth-Area-Duration (DAD) of heavy rainfall on the flood risk assessment. DAD relationship is calculated using Radar-AMeDAS rainfall data and the methods of making rainfall models holding DAD relationship are proposed. Finally, the flood risk is assessed by runoff analysis. The result says that peak flow at points on river channel tends to be maximum value when the spatial scale of heavy rain equals to the catchment area of each point. It suggests that the flood risk along the tributaries located at the upper stream of the river may be underestimated when the target rainfall is assumed to have the basin scale.
