Abstract
This report summarizes the results of the investigations on the sediment disasters that occurred in Yamaguchi Prefecture during the storm of Typhoon No. 14 on September 5 to 7, 2005. This typhoon caused serious damage to Kyushu, Shikoku and Chugoku areas. A number of sediment related disasters occurred in Yamaguchi Prefecture, especially in its eastern area, Iwakuni City. Most serious damages included flooding of the Nishikigawa River and a slope failure of the Sanyo Expressway bank, which caused three deaths. This report describes the morphology and scale surveyed for disasters at 59 sites, which consisted of 47 slope failures, landslides and debris flows, and 12 floods and inundation, and discusses their causes. The generation mechanisms involved in typical failures were also investigated by analyzing precipitation, topography, geology, and strength properties of collected soils. The analysis showed that the cases could be classified into six patterns of 1) failure of artificial slope (46%), 2) failure of natural slope (7%), 3) destruction of revetment (22%), 4) debris flow (15%), 5) landslide (8%), and 6) others (2%). These disasters were most likely attributable to the record-making storm, which resulted in accumulated rainfall of over 500 mm in this region. The disasters occurred in districts that were not affected by the Geiyo Earthquake in 2001 (M6.7) and the former storm on July 3.
