平成10年9月の高知県豪雨災害における降雨状況について

抄録

Heavy rain due to the activities of the front has attacked Kochi Prefecture on 24-25 September in 1998. One hour rainfall more than 70mm has continued almost 6 hours. Consequently, many slope failures and road and riverbank breakage have occurred. Collapse of natural slopes of weathered rock along the joint system, fall of fill slopes and fall down of slopes which have been utilized for the tombs or fruit garden were recognized. In this report, author has done the comparative investigation of the rainfall in 1998 with that in 1975 and 1990. The comparison of the rainfall indexes has proved the rain of last September to be very severe. The heavy rain of last September can be plotted on the right position in the disastrous zone on the diagram showing the relationship of the effective rainfall and one-hour rainfall which has been proposed by the author. The critical line by which the alarm for the evacuation can be given on the diagram was also effective. To express the time dependent reducing effect of the rainfall in use of a coefficient of the reduction of rainfall and a half-life of rainfall, one-hour rainfall was used as Yano has proposed. Considering the relationship of the snake-line and the critical line, the value of a half-life for the case of Kochi was proposed to be 36 hours.