抄録
Although forests are expected to reduce sediment discharge during heavy storms by preventing slope failure, current sabo planning uses the ratio of slope failure area to the total watershed area (i.e., slope-failure area ratio) already set for diverse geology type and does not take into account the effect of vegetation conditions. This study analyzed the effects of surface vegetation conditions, forest conditions, geology, and slope gradient on the slope-failure area ratio observed in the past three sediment-related disasters caused by heavy storms. Together with the summary of the previous studies, we discuss how the effect of vegetation and forest conditions can be reflected in the sabo planning. The slope-failure area ratio analyzed in this study was 1.2 to 7.6 times higher for the grassland and new planting areas than for the mature and old forest areas, supporting the result of previous studies that mature and old forests effectively reduced slope failure. On the other hand, the results of this study and previous studies demonstrated that taking into account a distinction between coniferous and broad-leaved forests, tree height/age, and tree density is not effective for increasing the prediction accuracy of slope-failure area ratio. Hence it is concluded that setting the different standard value of the slope-failure area ratio for each geology type and surface condition (grassland/new planting area and mature/old forest) is desirable.
