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 wordsFdeposits, initiation area of debris flow, pressure head, tensiometers