岡山県倉敷市北村地域で発生した土石流災害の特徴と要因

The debris avalanche occurred in Kitamura area, Kurashiki City, Okayama Prefecture caused by the hard rain of Typhoon 12 of September, 2011. Furthermore, 11 small landslides occurred in the study area. In this area, the altitude of mountainous are not so high (150m to 250m) and the amount of precipitation which producted the debris avalanche was not very serious quantity with 230mm. However the debris avalanche occurred. One private house collapsed, and the first floor part of the Kitamura Elementary School was buried due to the debris flow. The purpose of this study is to examine causes of the debris avalanche disaster occurred in Kitamura area using applied geological technique. The length of the debris avalanche scar is about 250m, and alluvial fan is formed as a debris slope. The ground is composed of weathered granite and the bottom of the valley was filled by soils derived from disintegrated granite. There are three small landslides as head scarps with steep slopes at approximately 30°-35°. The debris avalanche was inferred to have started as a shallow soil slip at the head by rainfall. 3 major causes are inferred which originated the debris avalanche in the study area; steep slope, ground composed of weathered granite, existence of sandy soil which occupied at the bottom of valley.