Developing Effective Flood Inundation Maps for Risk Communication and Evacuation Planning in Obuse, Japan

Obuse is a small town in the northern part of Nagano Prefecture in Japan surrounded by the Shinano and Matsukawa rivers. The typhoon Hagibis resulted in the flooding of the Shinano River and caused enormous damages to Obuse. In this study, we aim to devise a method based on which local government officials can use flood inundation maps to convey detailed flooding information to residents for small- and medium-sized rivers. To connect the information provided in the maps to actual evacuation actions, videos were created based on current flood inundation maps, and flood inundation simulation was performed based on the use of the International River Interface Cooperative (iRIC) software and Nays2DFlood solver. A terrain model was created for the simulations by using a digital elevation model (DEM). Two types of inundation analyses were performed for different terrain models in different drainage conditions. Videos were created from the data by using the geographical information system software (ArcGIS) when the levee broke at 58 sites along the Matsukawa River. In the iRIC simulation, the reproducibility was sufficient when a 10 × 10 m mesh was used, a simple correction was added to the levee parts of DEM, and a drainage pumping station was installed. In the simulations, it is possible to adjust the rainfall intensity and river discharge in an area; therefore, situationally rainfall and flood damages can be foreseen. New inundation information can be provided and can be used for evacuation planning because the information includes the spatiotemporal behavior of floodwater.