Taro MIZUTANI, Yoshifumi SATOFUKA, Daizo TSUTSUMI and Takahisa MIZUYAMA
One of the causes of debris flow is fluidization of deposits. Although it is
generally estimated that the deposits of riverbed are saturated when the debris
flow occurs, no observations have been conducted of moisture changes of debris
flows. Thus, in this study, we observed moisture changes of deposits in the
field just before the occurrence of debris flow. The study site was located
in uppermost part of the Hirudani Valley of Mount Yakedake, where thick deposits
have formed on a steep stream(34). We installed nine tensiometers at three
depths in three plots across the deposits. The results are summarized as follows.
1) The initial responses were about 20 minutes faster at the deep tensiometers
than at the middle|layer tensiometers. This suggests that water flow progressed
quickly along the base of the deposit. 2) Although the upper and lower layers
were saturated by the heaviest rainfall (24.5 mm/h), the middle layer was unsaturated.
This suggests that water flowed not only vertically downward but also parallel
to the stream.
Key wordsFdeposits, initiation area of debris flow, pressure head, tensiometers