Abstract
Flood disasters caused by torrential rainfalls have recently been increasing in Japan. Representative examples are Tochigi Prefecture's Yosasa River disaster in 1998. Far exceeding capacity of flow made river widths and channel shape change because of bank erosion. But, there are few observation data such as discharge or the water level in the middle and small-scale rivers. So, we could not have discussed about appropriate river width and depth required for large flood flows. In this study, at first, we applied two-dimensional flood flow analysis for the 1998 flood occurred in Yosasa River and examine an evaluation method of the water width from the analysis and inundation situation in the floodplain areas. Then, we clarified a decision method of appropriate river width for large floods in rivers flowing valley bottom plain.
