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At the end of rainy season in 2009, an extraordinary torrential rain occurred at Hofu city in Yamaguchi prefecture. The rainfall caused severe sediment disasters in the mountainous region. Slope failures occurred at more than 670 sites at the heads of streams and giant debris flows swept away houses in the fans, so that 22 people died and many houses were damaged. The streams that carried the debris flows were distributed in the region of craterous granite along the Saba River flowing south through the city. Each stream has a lot of tributaries. In this study, we investigated geometrical and geomorphologic characteristics of slope failures and the resulting debris flows based on the results of airborne laser surveys before and after the disaster. As a result, tributaries of the main stream flowed through areas where slope failure occurred, and also where it did not. Also there was a correlation between the number of slope failure occurred at the head of the stream and the magnitude of debris flow. Finally we found the geomorphologic features of slope disaster peculiar to a granite region by analyzing the area of land affected by the debris flow.

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