The western part of Hiroshima Prefecture suffered from a large scale disaster with heavy rainfall on June 29, 1999. This area suffered from a heavy damage in this disaster in which 24 lives were lost and 64 houses completely destroyed. In this area, there is something in common between the geographical and geological features. And if the same condition is met, it is assumed to suffer the same disaster. However, in this kind of disaster, the situations at Hiroshima City and Kure City are very different though both cities are in the same western part of Hiroshima Prefecture. Against the 155 debris flows which occurred at the western part of Hiroshima City ; Saeki, Asakita, Asaminami Ward, only few debris flow occurred at Kure City, regardless of being a neighboring city to Hiroshima City. On the other hand, more slope failures occurred mainly at Kure City while only few occurred at the western part of Hiroshima City. These differences of occurring disaster are considered to be caused by the difference of rainfall or geographical and geological features between the western part of Hiroshima City and Kure City. In this study, we analyzed the differences of these factors between the 2 cities through the statistical method, Willcoxon test, in which we tried to make clear relationship between occurring disaster (debris flow and slope failure) and rainfall as well as the geographical and geological features.
We quantified the woody debris on the flood plain of the Appetsu River, watershed area is 287.9 km2, by Typhoon 0310, Etau. This typhoon caused heavy damage, especially in the Hidaka and Tokachi districts of Hokkaido. We established 61 study sites in the river basin and investigated the size, species, and freshness of woody debris in a sampling plot in each study site. We used aerial photographs to estimate the volume of woody debris outside the study sites. The volume of woody debris totaled 46, 260 m3. The deposits were greatest in the upper river basin in the northern area. Of the total volume, 76% originated from landslides on mountain slopes or bank erosion, and the remaining 24% was from riparian forests. Of the woody debris, 15% were conifers and the remaining 85% were broadleaf trees. An estimated 74% of the volume came from trees that were living just before Typhoon 0310 struck, and the remainder was old fallen timber. Of the woody debris, 12% of the trunks had diameters under 10 cm, 57% were 10 - 30 cm, 28% were 30 - 50 cm, and 3% were over 50 cm.