Nobutomo OSANAI, Shin'ya KATSURA, Yoko TOMITA, Kiichiro OGAWA and Makoto NAKATA
Abstract
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 was1.2to7.6times 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.
Key wordsFSlope]failure area ratio, forest, sediment yield, sabo planning