Sakurajima volcano in the center of Kagoshima Bay, southern Kyushu, Japan is characterized by explosive volcanic activities and mud flows at the time of rainfall. The slopes covered with older and younger volcanic deposits are very susceptible to dangerous erosions, slope failures and mup flows by the actions of surface and ground waters. However, all of the ravines in Sakurajima have dry beds at normal condition without rainfall. Observation stations of mud flows are established at five points of four ravines in the northern hillside slopes of Sakurajima, in order to investigate the occurrence and runoff mechanism of mud flows quantitatively. In this paper, the characteristics of mud flows in Sakurajima are described, and the influence of rainfall conditions on occurrence, peak discharge and total runoff of mud flows are analyzed, according to the video tapes of records of mud flows and the rainfall records observed from 1980 to 1983. The results are summarized as follows:
1) The materials that compose mud flows in Sakurajima are andesite, pumice, ash, wood and so on. The main materials composing mud flows depend on the geology and the degree of erosion in the upper reaches of the ravines.
2) In Sakurajima, the front velocity of mud flows does not always correspond to maximum velocity.
3) The activation of mud flows starts in the event of the amount of rainfall exceeding adout 10mm during a spell of rain and exceeding about 4 mm in rainfall per ten minutes.
4) It is ascertained that the relationships detween the peak discharge of mud flows and the amount of rainfall per ten minutes and between the total runoff of mud flows and the accumulated amount from the beginning of rainfall until time of mud flows have the correlation of high significant level from the statistical analyses.
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