Inundation analysis of the 2009 Chikusa River flood and comparison of evacuation criteria

The Chikusa River in Hyogo Prefecture suffered from extremely heavy rains by Typhoon Number 9 (Typhoon Etau) in 2009. In the Sayo-cho area, the inundated flood claimed more than 20 lives, many of whom were trying to evacuate from the inundated rising water. To investigate the cause of such a disaster, we paid attention to people's behaviour in two districts. In the Kuzaki district, most of the area was flooded because of a levee breach, but with no loss of life. On the other hand, the Makuyama River Basin yielded more than eight casualties. It was made clear from detailed inundation analyses and interviews with local people that the difference was caused by the previous knowledge about the flood and a sudden rise of fast-flowing water. In addition, a new criterion of walking difficulty based on a drag force was proposed to show its superiority to the other criteria from a physical viewpoint.