On July 23, 1982, 299 human lives were lost in Nagasaki prefecture due to the localized heavy rainfall, nearly 90% of which were due to sediment disaster. Especially, nearly half of them were lost due to debris flow which had not been experienced for about 100 years in Nagasaki city.
It is important not only to establish a risk evaluation method but also to develop a prediction method for debris flow occurrence more precisely than before.
In this paper, the authors spot only upon the debris flow disaster whose prevention is very difficult due to its unforeseenity and its gigantic power to destory checkdams and houses and so forth.
We first try to investigate a typical debris flow which occurred at Yamanokami basin in Kamitoisu-cho, Nagasaki on July 23, 1982 by some accomplished field approaches in order to clarify the actual flow feature as a contribution to the regional disaster planning.
We next describe how to evaluate the risk of debris flow occurrence by using both the mechanical model proposed by Takahashi et al and Hayashi's quantification theories class II and III. And we propose a ‘weighted risk evaluation method’ which can take account for each factor (items) 's degree of contribution to occurrence of debris flow. It can be used if an analytical area would have been suffered from debris flows before. We show some applications to two areas for study in Nagasaki city.
Finally, we describe an attempt to develop a prediction method for debris flow occurrence by considering the risk weight to the mean precipitation criterion in Nagasaki city based on each basin's risk value.

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In this paper, the relationship was analyzed between pipe flow and pressure potential measured on a surface failed hillslope, as a clue to elucidate the mechanism of moisture supply to pipe, from soils further away from the pipe.
First, values of storage were calculated from the interdiurnal variation of rainfall, and it became clear that values of storage were corresponded to measured pipe flow.
Secondly, a equation was derived, to calculate pipe flow from values of pressure potential measured with two tensiometers. The physical meaning of this equation is considered to be that pressure potential distribution curve in soil on hillslope be approximated by the curve of a steady state.
Thirdly, the applicability of the equation to calculate pipe flow from pressure potential, is closely related to values of storage. To explain this relationship, a conceptual model of “Moisture Supply Area to Pipe” was proposed.

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Besides the construction of sabo facilities such as sabo dams, it is necessary to inform the inhabitants of the presence of debris-flow hazard torrents and to evacuate them once debris flows are predicted to occur in order to prevent the loss of human lives due to debris flow.
Typhoon No. 19 has brought about a heavy rainfall to Uchinomi Town, Shozu-gun, Kagawa Prefecture in October, 1987. A questionnaire was carried out on the communication and application of the information on disasterprevention as well as the action taken by the inhabitants to evacuate from the debris flow. This is a report on the actual state of debris-flow warning and evacuation at that time for promoting the establishment of the system of debris-flow warning and evacuation.

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