JAUE2024-079: Feasibility of Horizontal Evacuation after Flooding in the Kinu River Flood

Abstract

During a flood, vertical evacuation is difficult to ensure the safety of life when the inundation depth is higher than the top floor of the house. Accordingly, horizontal evacuation is important when deep inundation is expected. Some residents, triggered by the visual sight of rising water, initiate evacuation during flooding. Therefore, considering evacuation behavior after the start of flooding, including horizontal evacuation, is necessary. In the September 2015 Kanto–Tohoku Heavy Rain (15KTHR), the Kinu River, the large river, breached its levees and flooded a wide area, but the Hachikenbori River, the smaller river, may have prevented residents from evacuating. In this case, numerous residents became isolated because they made poor evacuation decisions due to the inundation of the Hachikenbori River. This study aims to clarify whether horizontal evacuation after flooding was feasible and appropriate in the Mitsukaido District of Joso City during the flooding of the Kinu River on 15KTHR. The flooding situation at that time was reproduced by simulation, and analysis was performed assuming two modes of transportation: evacuation on foot and evacuation by car. The results showed that the time from start of flooding to difficulty walking in most of the locations affected by inland flooding was more than 180 minutes, indicating that evacuation on foot was highly feasible. Furthermore, the feasibility of evacuation by car was lower than that on foot in many districts, including 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 is slightly elevated and forms a closed floodplain