Estimating the feasibility of horizontal evacuation after flooding using inundation analysis: a case study of the Kinu River floods during the September 2015 Kanto–Tohoku heavy rain

During a flood, it is difficult to ensure safety in a vertical evacuation when the inundation depth is above the top floor level of a specific building. Thus, horizontal evacuation is important when deep inundation is expected. Some residents, triggered by the sight of rising water, begin evacuation during flooding. It is, therefore, necessary to consider evacuation behavior after the beginning of flooding, including horizontal evacuation. In the September 2015 Kanto–Tohoku Heavy Rain (15KTHR), the large Kinu River breached its levees and flooded a wide area, but flooding from the smaller Hachikenbori River may have prevented residents from evacuating. This study seeks to clarify whether horizontal evacuation following flooding is feasible in the Mitsukaido District of Joso City during the flooding of the Kinu River in the 15KTHR. The flooding was reproduced with a simulation, and analysis was performed assuming two modes of evacuation: on foot and by car. The results show that the time from the start of flooding to difficulty walking in most of the locations affected by inland flooding lasted more than 180 min, which indicates that evacuation on foot was feasible. Furthermore, the feasibility of evacuation by car was lower than that on foot in many districts, including the closed floodplains, where the time available for evacuation after inundation was shorter. Notably, evacuation by car was particularly difficult near rivers and sluice pipes, as well as in locations where the surrounding area was slightly elevated and formed a closed floodplain.