Image analysis and reconstruction of the 2008 Toga River Flash Flood in an urbanised area

In the afternoon of 28 July 2008, a flash flood occurred in the water-friendly reach of the Toga River in Kobe City, tragically drowning five people that included three children. They were among about 50 people enjoying the river environment. The flash flood was caused by sudden localised torrential rain in urbanised area of the small river basin. The onset of the flash flood was captured by a river monitoring camera as consecutive still images, while the surface flow just after the peak flow was videotaped by a TV cameraman without using a tripod. In order to estimate the peak discharge of the flash flood, the space-time image velocimetry technique developed by the authors was applied to the video images after applying image stabilisation. The estimated discharge was used as an input hydraulic parameter of the 2D numerical simulation, with a success of reproducing the transient flow pattern observed by the monitoring camera. In addition, the distribution of hydrodynamic force the people in the river were exposed to was calculated to reveal the difficulty of evacuation in flash flood condition, even when the depth of water is less than knee high.