2010 Volume 63 Issue 4 Pages 16-21
Many hillside landslides and debris flows occurred by typhoon T 0421 in Miyagawa village （the present Odai town） on September 29, 2004. These caused severe sediment-related disasters. In this study, several findings on the influence of precipitation, slope angle and forest practice on hillside landslides caused by typhoon T 0421 were shown in Miyagawa-dam basin as follows : 1） Correlation between precipitation and collapsed area ratio was not clear. 2） Angle of collapsed slope in deforestation sites was 5-10° lower than those of non-forestation and reforestation sites. 3） Collapsed area in deforestation sites occupied 57% in total collapsed area. 4） Most of landslides were occurred since 10 to 20 years after deforestation. 5） More hillside landslides observed in larger deforestation area. 6） A prediction model of slope stability changes considering forest practice （Kuroiwa and Hiramatsu, 2004） was applicable to Miyagawa-dam basin. As the result of this simulation, collapsed area is generally in proportion to collapsing possibility area ratio index （APhr（t））. 7） Angle of collapsed slope and APhr（t）and collapsed area ratio showed significant correlation.