Heavy rain attacked northern part of Tochigi prefecture at the end of August, 1998. The rain caused a serious flood disaster, which killed 5 people, missing 2 people, flushed bridges and damaged tons of agricultural products along Yosasa River. The flood eroded the surrounding riparian forest and produced a large amount of woody debris. Some bridges that cross over the Yosasa River trapped woody debris and the flood water flushed their abutments and piers. To mitigate these kinds of disasters, it is important to understand the characteristics of woody debris, its source, yield, deposit positions, etc.. However, data are deficient in spite of the efforts of researchers. The following information were obtained after executing a field survey and studying the aerial photos.
1) Much of the woody debris was produced from riparian forest.
2) Emergence types of woody debris were by slope failure, by bank erosion and by riverbed erosion.
3) Woody debris was deposited at the bridges, at the upstream of riparian forest and on the riverbed.
4) A cluster of woody debris moved downward along the river channel by repeating the cycle of “Clogging→Dam up →Failure→Moving down”.
5) Bridges were damaged by the dense woody debris, so the girder heights and the pier spans were not large enough for the debris to flow down.
A dense riparian forest which developed in and around the river made it difficult to distinguish the actual boundary of the river till the disaster. It is important to determine the land use near rivers with consideration of the previous flood data, the topography, the location and the priority of land use.
