A research on the sediment disasters caused by heavy rainfalls in the western part of Japan, in July 1997

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It rained heavily and recordically in western part of Japan on 6 to 15 July 1997. Landslides and debris flows which were caused by the torrential rainfalls, brought about serious damages with a heavy loss of lives in Kyushu and Hyogo prefecture. 21 persons were killed by debris flows at Harihara Izumi-shi Kagoshima pref. on 10 July. The purpose of this research is to investigate actual situations of these disasters and also to examine meteorological background of the heavy rainfalls, process and mechanism of the landslides and debris flows, warning and evacuation, disaster prevention and prediction and mental state of the sufferers. The results are summarized as follows.1.The heavy rainfall around Izumi-shi Kagoshima pref. on 9 to 10 July was created by stagnation of meso-scale of praecipitatio by topographical control of Koshiki-shima island under the stationary Baiu-front in the northern part of Kyushu.2.The debris flows at Harihara river on 10 July originated from a large-scale landslide caused by deep weathering of volcanic rocks and rising of ground water table under the heavy rainfall on a hillslope in the river catchment.3.The debris mass moved down at high speed along the river channel at the same time landsliding, pushing out the water in an irrigation pond and crashing against a check dam on the way, and attacked the Harihara residential area on the debris fan.4.A process of the debris flow from initiation through flow to terminal was reproduced by a simulation analysis.5.The warning and evacuation system of cities, towns and villages for the natural disasters was not completely prepared.6.According to the questionnaires to the sufferers at Harihara Izumi-shi, over 20 percents of them were screened as post-traumatic stress disorder.