CHANNEL VARIATION OF THE YOSASA RIVER TRIBUTARY OF THE NAKA RIVER DUE TO 1998 FLOOD

Abstract

A severe flood disaster was occurred at the Yosasa River, a tributary of the Naka River, located in the northeast of Tochigi Prefecture due to heavy rainfall on August 27, 1998. The amount of precipitation of 1, 229 mm was recorded at the Nasu meteorological observatory, the Meteorological Agency, from Aug. 26 to Aug. 31. Owing to the discharge of a large amount of driftwood as well as bank erosion and river bed variation, many houses and bridges were damaged. In this paper, a channel variation of the Yosasa River was discussed by comparing the aerial photos of before and after the disaster. A Flood runoff analysis by using a tank model was done and one dimensional numerical simulation was carried out to examine the characteristics of the river bed variation of the Yosasa River.